With the arrival of football season, I decided to add a second ceramic filter in series with the first to increase selectivity in the receiver.
The two stations I absolutely needed to receive were only 4kw ERP and 7kw ERP sandwiched in the middle of the band, with blowtorches a couple of channel slots away. On some ocassions, the receiver would try to re-capture on one of the adjacent channels when receiving the low power stations. This did not happen often, but was annoying when it did. The second filter seems to have fixed this. Senstitivity is probably diminished from the extra insertion losss, but this seems to be a non problem. It hears a station on almost every assigned channel. Fidelity seems unaffected.
Interestingly, the AFC works differently with the addition of the second filter. Before, with only one filter, the radio could be set within twenty or thirty KHz of center channel, and the AFC would walk the station in and hold it within a few KHz of center channel. Now the AFC just holds whatever the frequency the radio is tuned to. When tuned to center channel, the AFC will hold it within a KHz of center channel, even on the weak stations.
Thanks for posting. I now have search optimazation correct for google seraches for this board. You'll be able to find articles easier. Even Bing AI finds them now.
Once the receiver is determined to be working properly, alignment is required.
To properly align the discriminator coil, two pieces of test equipment are essential - an accurate frequency counter with resolution to 1 KHz and a high impedance voltmeter, either a digital VOM, or a VTVM.
To align the discriminator:
1) place the voltmeter across the 5K6 resistor between pins 7 and 10 of the LM3089;
2) Using the frequency counter to measure local oscillator frequency, tune the radio to exactly the center frequency of a strong broadcast signal;
3) With the receiver tuned to exact center frequency, adjust the discriminator coil so that the meter reads exactly zero volts. The AFC will fight this process, so it will be necessary to repeatedly retune to center frequency and readjust the discriminator to zero volts until zero volts is realized at exactly center frequency of the broadcast channel.
Unfortunately, I could not properly adjust the discriminator coil by ear. I tried it several times by adjusting the discriminator coil for what I perceived to be the correct response, and it was off when I checked with the voltmeter. The IF transformer I am using for a coil has more than one response peak, and I picked the wrong one every time by ear. The receiver will work with an alignment by ear, but it won't work as well as it is capable of.
The commercial front end is pre aligned, and the coils should not be touched. The IF transformer might benefit from adjustment to the load presented to it, but it probably won’t make much difference. Align for maximum received signal at exactly the center of the IF channel with a scope or spectrum analyzer. It might be possible to align by ear by tuning to center channel of a noisy broadcast station and tuning for maximum quieting.
When properly aligned, the radio should be very sensitive, tune cleanly, and have very clear and undistorted recovered audio.
I printed an adapter to allow installation of the new module in the existing chassis. This avoids the inconvenience and expense of having to immediately print a new chassis, The adapter has mounting points for the 50 x 80mm RF board, and a second set of 50 x 70mm mounts for a perfboard to contain the stereo decoder if, or when, I get around to that. At present, I do not have a need for stereo reception. The expense of printing a new chassis is insignificant, but the inconvenience (time involved in printing) is not. It only took a few minutes to design the adapter and an hour or so to print it out.
I did not bother with installing connectors on the RF board, I simply soldered the existing wire directly to the relevant pads on the board to speed re assembly. I did reconnect the AFC with a 1M resistor on pin 7 of the LM3089 and the other end of the resistor to the tune pin on the tuner.
I ran into one bit of difficulty - on re assembly, there was a low-level audio tone on the audio output even with the volume control set to full attenuation. When volume was at a soft to normal listening level, the tone was masked, but I still considered this unacceptable. I first thought this to be a ground loop between the RF board and the LM386 audio board, but pursuing that theory resulted only in a reduction of the amplitude of the tone, not elimination. Ultimately, the tone was found to be coming in on the DC supply line to the frequency counter. The fix turned out to be simple - a 15 ohm 0805 chip resistor in the supply line to the counter, but it took me a while to solve the issue. It is now silent at any frequency I can hear with the volume control at max attenuation.
Interestingly, as part of that trouble shooting, I found that the new board had markedly more audio output than the prior board, enough to allow me to remove the gain set capacitor on the LM386, and turn its gain down to 20 from 200. On inspection of the removed board, I found that I had carelessly installed a 27K resistor in the RC de emphasis network instead of the correct 2K7 value.
This board works well; at first impression I believe the combination of better quality / smaller parts, and a true RF ground plane has made a worthwhile improvement. It seems to have better selectivity with one filter than the dual filter arrangement of the previous board, where there was potentially considerable blow by around the filters.
The KEC tuner is apparently a Far East market tuner as it tunes from about 75 MHz to 110 MHz. I don't need that kind of range, so the end point resistors on the tuning potentiometer will need to be adjusted. Although the data sheets specify that the KEC tuner is typically about 5 dB less sensitive than the KSE tuner, I cannot tell any difference in apparent sensitivity, The new tuner may be more sensitive, as it does not have the loss of a second filter. The KEC is very stable, it seems to easily hold + - 1 KHz tuning and has good capture when tuning to a station. The tuner module is secured to the ground plane of the pcb at four points - two at the center rear of the tuner, and one at each front corner. It is rigidly mounted and well grounded.
If there is any variance between the most recent schematic sheet and the prior sheets, the most recent should control.
Here is the simple FM broadcast band tuner rebuilt unto a 50 x 80 mm prototype board by BusBoard Systems. I anticipate the pc board format with a true ground plane and smaller components / physical size will give improved RF performance over the original build.
Besides the obvious SMD construction, a few differences from the original: LM3089 rather than CA3089; a KEC (Korea Electronics Corp.) KCF201H tuner instead of the Kwang Sung tuner; one ceramic filter rather than two in series (for now).
Generally, I think sockets for RF IC’s are a bad idea because of the increased lead length, but I used a socket here to make it easier to swap out for an LM3189 at some point in the future. The socket legs are bent to a 90 degree angle and then soldered directly to the bus board.
The tuner pictured is fully functional, although AFC is not installed - I’m still studying how it is implemented on the LM3189. The delayed AGC of the LM3189 is also wired slightly different from the LM3089, although the chips are pin compatible.
The parts are mostly 0805 SMD, although there are a few 0603 10nF bypass caps. The resistors with flying leads are 1/8 watt. I put 10 nF disc ceramic bypass caps on the tuning and AGC line at the tuner, which is likely unneeded as the schematic shows internal caps, but does not list a value. I buy caps in bags of 1000 pieces, so I have decided not to be stingy with them after the power supply debacle. I used SMD electrolytics, although leaded caps are easier to install on these boards and actually have a significantly smaller footprint.
The output resistance of the LM3089 (and CA3089) is 5K. A 2K7 resistor and 10nF capacitor set the proper de emphasis for US broadcast standard and then the audio is taken from a 10 uF electrolytic.
The other picture shows a simple tuner built from a Phillips TBA120U (the suffix is important on these - there are a lot of variants). This chip is intended for television sound systems but is rated to 12 MHz. I did not pursue use of this chip because the preliminary circuit demonstrated that at 10.7 MHz, it is clearly inferior in terms of sensitivity and audio output relative to the CA/LM3089. To compete on equal terms with the ‘3089, it needs one, possibly two, IF stages, and an audio preamplifier. It does not have an AGC output, nor any provision for AFC.
Although CA3089 is approaching fifty years of age now, it still appears to me to be the chip of choice for homebrew FM receivers.
I think I am at a pause point with this radio, after some incremental improvements.
After using it the last week, just as when I first built it, I became undecided as to whether or not the extra gain of the IF amplifier ahead of the CA3089 was needed. RCA and National Semiconductor say that it typically is not needed. After using the device in two separate locations, I concur.
The CA3089 features a delayed AGC for the tuner front end, the voltage of which corresponds to signal amplitude at the input to the CA3089, and which is graphed out in the data sheets. I measured the delayed AGC voltages at pin 15 of the CA3089, and was seeing some in the 3.5 vdc range and many in the 4 volt range with only a few feet of wire for an antenna. After thinking on this, and temporarily bypassing the IF amp for real world comparison/analysis, I decided that the consequent gain reduction from AGC in the front end outweighed whatever benefit (if any) I was getting from the IF amplifier, and that actual sensitivity might be degraded by the excess IF gain, when the increase in front end noise figure from excess delayed AGC is considered. More gain is not always better. Sometimes it is just more gain.
I then permanently bypassed the IF amplifier by connecting the output of the first ceramic filter to the input of the second ceramic filter, and since 10.7 MHz IF transformers are now scarce, I removed it and put it back in my parts inventory. Selectivity has improved noticeably, and apparent sensitivity is not affected. Additionally, some intermod artifacts are no longer present, and overall the radio seems better behaved. I put a note on the board indicating the circuit modification.
With regard to local oscillator stability (which operates at received frequency plus 10.7 MHz), the KSE tuner I used does not provide a specification, but a similar varactor diode tuned device, the KEC KCF201, does.
Two specifications are provided: ordinary oscillator stability, and frequency drift against strong signal input, which I take to mean frequency drift when AFC is active. The former spec is typically +- 200 KHz, and the latter is typically +- 5 KHz.
Ordinary oscillator stability is observed by just removing the antenna input. My tuner module will wander around over about a +- 50 KHz span under these conditions. After careful adjustment of the quadrature coil, and when tuned to a signal sufficient to quiet the receiver, my receiver exhibits stability under AFC control consistently +- 3 KHz, and on a strong station it will hold +- 1 KHz. On cold startup, the oscillator may be as much as 40 KHz from center channel as last tuned, but within a minute the AFC will begin moving the oscillator to center channel and won’t stop until it gets there. It’s rather fascinating to watch, in the first minute or so the frequency change can be dramatic, and as it approaches center channel, the rate of change slows. Most FM tuners only display to the nearest 100 KHz, so AFC action on received frequency is never seen. AFC also aids manual tuning, as it makes the radio try to lock unto a station frequency.
In short, I think the oscillator stability is satisfactory, and AFC action is also satisfactory, and working properly.
I removed the cheap generic LM386 from the premade board and replaced it with a genuine 1980's vintage LM386-4. This made a noticeable improvement in audio quality.
I removed the capacitor to set the gain to 20 and decided this was inadequate. So, I reinstalled the capacitor and reset the gain to 200. For now, this provides a satisfactory level of audio output.
I installed a phono socket for an antenna plug (I anticipate changing this to BNC) and a barrel connector for DC input. I printed a big hole for a snap in on / off switch identical to the switch used in the power supply, but right now I just unplug the radio to turn it off. Or turn off the power supply.
Many of the holes in the rear panel are not presently used and were printed in anticipation of possible future needs
Cosmetically, I printed a fairly large tuning knob for the radio.
I printed a bezel to go around the panel display to give it a little more pleasing appearance.
I printed a “lens” to cover the LED display from translucent blue plastic. The trick to doing this is to use a translucent or semi translucent plastic, make the lens thin, and use minimal infill. This lens is 1mm thick with 10% infill. The difference between an unfiltered display and filtered can be easily seen in the side-by-side pictures with the power supply. The lens makes the display more attractive, at least in my opinion, and when powered on the display has a bit of the same appearance as the old Noritake or Futaba vacuum fluorescent displays on imported late 70's, early 80's HF ham rigs.
That’s it for now. The radio is satisfactory for its intended purpose. Future improvements (if any) are anticipated to be using the top deck of the chassis for better / different audio amplifiers. That will likely be the subject of a separate thread.
I built the tuner back in August 2017, and haven’t powered it up since, so it seemed prudent to make a functional test before proceeding further. When originally constructed, it was designed for and planned that this receiver would stay in my ham shack, but five years later I am spending the majority of my time at our second house up in the mountains at a local lake, so it will stay here, at least temporarily.
I considered replacement of the terminal blocks with plug in connectors similar to what I used in my general coverage receiver and SSB transceiver. After thinking about it, other than aesthetics (this board will never be a beauty queen) I decided that as almost all of the terminal blocks are just carrying DC or audio, there was no material improvement to be had in replacing them, and there is always the risk of breaking something in the process of fixing something that was not broken, not to mention the time involved. So I just left them alone.
The radio powered up and performs satisfactorily from an RF perspective. For testing, I moved the CA3089 audio output from the LM1310 multiplex chip and connected it to a de-emphasis network for mono reception. For quick testing purposes, a store-bought module using an LM386 audio chip with integral volume control drives the same 4 inch homebrew speaker used with my other gear up here. Scraps of leftover wire are being used as an antenna.
Interestingly, one of the reasons it was built in the first place was DX reception of sports broadcasts. Here at the lake, my Yamaha AV receiver with a pair of rabbit ears (behind a big screen TV) cannot quiet on the sports station I need to receive, and portable FM radios are very position sensitive and hand capacity sensitive. The ugly homebrew fully quiets on the target station with about a meter long piece of scrap wire held up by a small vase.
At my ham shack, with the receiver board out on the work bench and my A/C blowing across it, the AFC struggled, and I had planned on spending some time on it to improve AFC action. Here, with the board under the chassis, and sitting on the kitchen table, it seems to be better. It’s been tuned to the needed sports station at 99.500 MHz, and over the last three hours, it has wandered around about +- 4 KHz, usually within about +-2 KHz. In an FM broadcast bandwidth, this seems completely inconsequential, and I would not be aware of it at all but for having an actual frequency counter on the LO that displays down to the KHz level. The LO is 10.7 MHz above the receive frequency, so this seems okay for a varactor tuned oscillator at 110 MHz, but I have no experience with this, and would appreciate any other comments on the matter.
The LM386 audio module is from AliExpress, presumably with the -1 variant of the chip, and in an unusual abundance of caution, I initially powered it from the 8 volt 78L08 source on the tuner board. This confirmed that everything was working, but it left a lot to be desired - on voice peaks the counter would dim, the oscillator would pull, and the audio would distort. In short, unacceptable. I reconnected the module from the 8 volt source to the 12 volt input to the tuner board (leads were barely long enough), and all of the above was solved.
The LM386 is set for 200 gain, which is way too much given the strong audio from the CA3089 and will have to be set to 20 when I get a chance. I could cut a lead but will probably wait and unsolder the gain set capacitor. For listening to football, stereo does not seem immediately necessary, although I do require enough clean audio to fill the upper floor of the house here. The LM386 board may or may not be satisfactory on a temporary basis. I have some -4 chips that might be a better choice than the supplied -1 chip.
A few pics are attached. My cheap phone has a real tough time seeing the blue LED’s in the counter display, hence the odd angle pics. I might change it to a different color.
Football season is coming up, so I decided to case the FM Receiver and put it in service. I’ve always liked the CO series of cabinets from LMB-Heeger: https://lmbheeger.com/coseries.aspx. The downside is that they are industrially priced, and somewhat expensive for homebrew.
So, I thought I would take some baby steps toward 3D printing a knock off chassis similar to one of the slide in chassis series in the CO line. The chassis pictured here is about 7.1"d x 6.3"w x 3.75"h inches, similar to a CO-3 which is a $65 chassis / cabinet combination: LMB Heeger Inc. - Slide In Chassis. Mine lacks the nice, rounded corners of the LMB, and of course is plastic, rather than aluminum. The LMB CO cabinets are perforated; a perforated cabinet should not be difficult to print, but I have not tried that yet.
With some exceptions, you generally can’t print a surface that would otherwise be hanging in mid air, unless it is supported underneath, otherwise the extruded plastic will just sag under the force of gravity until it cools enough to harden. It therefore becomes necessary to print this type of chassis with its edge on the print bed and continue vertically until it completes at the other edge, otherwise there will be a waste of both plastic and considerable time as supports are printed underneath the horizontal part of the chassis.
Because of this prohibition against printing in midair, I have been making breadboard style chassis, as I prefer to print the supports for circuit boards and other parts directly on the chassis, which are floating in midair if the chassis is printed from the edge. I’ve been experimenting with print head temps, fan speed, and print speed and on this print, the standoffs are quite good considering they started in midair. They are completely satisfactory for holding circuit boards.
I’m not really sure what I want to do with the receiver, so I created a couple of potential chassis - one is a smaller chassis suitable for solid state only; the other is deeper to potentially allow for vacuum tube audio amplification. There is only about 40 grams of additional plastic between the two, so I printed the larger of the two.
I’ve added some of the parts to test fitment but haven’t done anything else. Larry says there is unlimited storage on the site, so I’ve attached too many pictures and a zip file of the .stl’s.
It's amazing what can be done with 3D printing nowadays. That panel and chassis are beautiful work. This technology gives us experimenters the ability to make 'finished' one-offs. If only these techniques had been available back when I was in high school, the perfect girlfriend would have been no problem.
SIMPLE SUPERHET FM BROADCAST RECEIVER STEREO DECODER
A stereo decoder is optional. I did construct one for this receiver.
There are many linear IC’s developed especially for FM stereo decoding. As usual, I used what I had in stock - LM/MC1310 and LM/MC1800. I do not think there is much difference between the two, other than that LM1800 has a couple of extra pins that would make wiring a bit less crowded. Both are well documented in the National Semiconductor Audio Radio Handbooks, and in other literature.
I used a surplus, house marked, 1310 decoder. I used an LED with a current limiting resistor, instead of an incandescent lamp to indicate stereo decoding.
Adjustment of the circuit is simple - adjust the trimmer connected to pin 14 until a 19 Khz square wave is visible at pin 10 as viewed on an oscilloscope, and then center the trimmer in the range that produces the 19 KHz square wave. If a scope is unavailable, follow the same procedure on a known FM stereo broadcast - adjust the trimmer until the stereo lamp lights, and center the trimmer in the lit range.
I think the horizontal ones are not too common - when I was trying to replace the horizontal mount tuner I robbed from that kit, all I could source were vertical mount modules.
For homebrew it makes little difference - just lay the module down flat and solder to the leads and case, or use it vertical to save space. It won't make any difference to the module.
That is an extremely good deal on a useful, difficult to source, part. I would strongly encourage anyone who even thinks they might want to make an FM band tuner now, or in the future, to get in touch with Jim and take that company up on their generous offer of being willing to surplus those modules out to DIY'ers.
Also, don't get in the rut of not looking outside the box - these modules don't have to be used solely for tuning the FM broadcast band - put a converter in front of them and use it to tune any 20 MHz swath of the HF / VHF / UHF spectrum. A great basic part to have on hand for homebrewing.
The picture shows one of these modules next to an ordinary paper clip for scale. Imagine trying to scratch build that at any price.
W5JAG: I can confirm that my company does indeed have excess surplus stock of those Kwang Sung FM tuner modules, and would be delighted to supply small quantities at a price of $7.20 each, plus shipping; doubtless would offer a discount on quantities.
I can't immediately confirm that ours are the same model number as these are in obsolete stock at another location. They may depart slightly as they mount vertically on the board rather than laying flat... I think. I do recall that the last ones we bought needed a higher tuning voltage than the +9V in our product to tune to the top of the band. We bootlegged-in +12V from a second regulator and that worked just fine. They are a good tuner and work well with any of the available ceramic filters and monolithic IF chips like the obsolete RCA one you used.
Let me know if you have interest and I can supply further instructions. -Jim
Simple Superheterodyne FM Tuner - IF and FM Demodulator June 2022
The preceding post did not make it clear that the commercial tuner module is tuned to frequency by application of a variable DC voltage to the appropriate pin on the module.
In my receiver, I use a 10 turn precision pot that I obtained at a Hamfest for the tuning. When it goes into service, it will likely be replaced with a more compact 10 turn pot. It is also possible to channelize this type of tuner module simply by switching preset voltages, corresponding to the desired frequency, to the appropriate pin on the module.
Likewise, use of a DC voltage for tuning makes application of AFC ( automatic frequency control ) easy (easier?).
Almost all of the commercial FM tuner modules that ever were used a high side LO injection to a 10.7 MHz Intermediate Frequency. If one is scratch building a receiver, I would suggest 10.7 MHz, 21.4 MHz or 4.5 MHz as a suitable IF as parts are readily available for these frequencies.
There are probably as many ways to convert the FM modulated IF to baseband audio as there are builders, but in this particular receiver, I chose a CA3089 linear IC to accomplish this function.
I chose CA3089 for multiple reasons: I had some in stock; it was the dominant industry workhorse in both narrowband VHF communications transceivers, and wideband entertainment receivers for decades; it is readily available as NOS or NOS surplus; and best of all it is incredibly well described in both amateur and commercial literature. It was originated by RCA, second sourced at some point by National Semiconductor as LM3089, and then a tweaked version as LM3189, and RCA’s successors, Harris RF and Intersil, even made SMD versions of the chip.
The 1976 National Semiconductor Audio Handbook, at chapter 3, and the 1980 National Semiconductor Audio and Radio Handbook, also at chapter 3, describe the chip and it’s application in considerable detail. Both are under copyright, so I can’t put them here, but probably can be found on the internet as a download if one does not have the published paper version.
There are other chip choices, or it could be done the hard way with discrete components.
The CA3089 chip is sensitive and most applications do not require any additional IF amplification between the tuner output and the chip, but I found that DX reception of my target station here in the mountains of west Arkansas was spotty without additional IF amplification.
At first I used an MC1350 linear IF amplifier chip, but that provided too much gain. I then used a common emitter single stage amplifier using a PN2222A and that provided about the right amount of gain, in my subjective judgment. On the Old Radioboard, there was a discussion within a thread about the relative merits of LC loads versus ceramic filter loads, and I tried it both ways here, settling on a simple green core IF transformer as the load.
A single tuned circuit does not provide much selectivity at 10.7 MHz, so here two ceramic filters are used - one at the output of the tuner module, and another at the IF input to the CA3089.
A second green core IF transformer is used at the demodulator of the CA3089 for convenience, any resonant circuit at the IF should be fine here. The literature indicates that lower distortion can be obtained with two L/C circuits at the demodulator, but I did not try that and would have no way of measuring detector distortion in any event.
Most, but not all of the functions of the CA3089 are used here - the AFC output is applied as feedback to the tuner module. Signal strength metering is available but I have not used those - as long as it limits and quiets, I am okay with the signal strength. The interstation mute function is used here, but can be overrode if desired.
I have two deemphasis networks - one for mono and one for stereo - the mono output is taken to screw terminals for connection to an external audio amplifier. As shown, a jumper wire connects the deemphasis network to an optional stereo decoder.
The optional stereo decoder will be described in a subsequent post.
Thanks for posting all that. Should make it easy for anyone to duplicate what's probably a quite good receiver. I know that Korean front-end; my company used it in an FM Modulation Monitor for broadcasters. That monitor product is being superseded by an SDR-based design this year, and we'll probably have a surplus of those tuners if anyone's interested... I believe we bought them in lots of 500 for a few bucks each. The mod-monitor was a dual-conversion design with a first IF at 10.7MHz and a second at 700kHz. FM demodulation was accomplished with a 'pulse-counting' scheme adapted from a circuit in the British RSGB's equivalent of the ARRL Handbook. ±75kHz deviation at 700kHz yields an impressive S/N figure. Being an old timer I have my suspicions of 'them newfangled digital chips,' not the least of which is that they are single-sourced and often hard to find in stock. That analog monitor will have had almost a 20-year run. In the beginning we used a Telefunken tuner, but they became unavailable and the Korean ones actually constituted an improvement at a much lower cost.
SIMPLE FM BROADCAST BAND RECEIVER - OVERVIEW / TUNER
About five years ago, the broadcast network for the local sports teams I follow, abruptly moved from a local AM station to a considerably more distant FM station.
I hadn’t built anything using RF in quite a while, so a simple broadcast receiver seemed like a good place to reenter the RF homebrew hobby. This thread will describe the receiver I actually constructed, which is not necessarily the receiver I would build if I were starting this project from scratch today. Those possible differences will be explored in a subsequent post.
The receiver shown here is functional, but not entirely complete. I have “designed” a breadboard chassis to match my homebrew SSB/CW transceiver and general coverage receiver, and also some other enclosures, but none have been printed out, and the receiver is still just a bare board right now. The large area of empty space on the perf board was (is?) to install some 10 turn pots that can be switched to channelize the receiver.
Probably the first critical decision a prospective builder of an FM broadcast tuner will confront is the receiver architecture, i.e., software defined radio ( “SDR” ) or superheterodyne, and, if the latter, how you actually get the FM broadcast band down to the chosen Intermediate frequency ( “IF” ). I did not consider any other architectures, such as regenerative.
Back in the day when SCA was a thing ( maybe it still is? ) Radio - Electronics ( now defunct ) ran a two part article by William Sheets and Rudolf Graf describing a simple FM broadcast receiver with a very simple tracking FET front end. The articles are appended here, extracted from the complete magazine issue that is available for download on world radio history. After carefully considering using the Sheets / Graf simple front end, I rejected it for the practical concern that it might not be quiet enough to receive the dx station I needed to receive.
Rather than design my own front end, I elected to use a commercial product. FM broadcast band tuners down converting to a 10.7 MHz IF, were once ubiquitous, in a wide variety of formats, as new production and surplus, and then virtually disappeared overnight. They are not unobtainium ( yet ), but they are awful darn close.
Searching every nook and cranny of my ham shack yielded two prospects - a really nice permeability tuned solid state AM/FM tuner module, likely for a car radio, probably out of a Radio Shack “Surprise Box” decades ago, and a solid state “three gang” modern tuner laying idle in a Velleman MK118 discontinued FM raio kit. After considering the pros and cons of each, I robbed the three gang tuner from the kit. Later, I found some identical tuner modules as surplus on ebay, so I was able to restore the kit.
I’ve attached the data sheet for the tuner module I used. I expect that all of the last production three gang tuner modules were a simple commodity item with virtually identical specifications regardless of the OEM, differing only in the vertical or horizontal mounting format.
As noted, these once common parts are now near unobtainium, with surplus or a friends stash being the only source as far as I know. They occasionally turn up on ebay. Some surplus houses may have a few if one wanted to call around. There are surplus vertical mount Mitsumi FE-352 three gang tuners being offered for sale on AliExpress at present. They look to be of good quality.
Good deal! Looks intriguing; sharpen that pencil and grab the yellow, lined pad! It does look to use a commercial front-end. Are those still available? I wonder what it would take to roll one's own.
It's a real radio. I guess - depends on how you define real radio ...
It's not SDR, although I've played with the SDR chips some and they work surprisingly well.
The radio is up and working - I'll try to start writing it up in installments over the weekend or next week, I built it about five years ago, and then got sidetracked on an SSB transceiver and the general coverage receiver.
I've had experience with the 'one-chip-wonder' radios for FM; they are remarkable in performance and give you signal metrics that would be hard to derive otherwise. But these still require some peripheral chips and, as you remarked, Larry, are near-impossible to even see, let alone solder-down to a board or otherwise make connection to. Moreover, these are typical in-demand consumer parts, usually single-sourced, and many are either out of stock with suppliers or already have been superseded by an 'improved' successor that hasn't been released quite yet.
I, personally, like to work with time-tested, always-available discrete components that, if I drop them on the floor, I can find, pick up and keep on truckin'.
W5JAG, how about some particulars on the design? Is this a bare-bones-minimum radio or is it the performance equivalent of a home hi-fi? MPX-out for stereo decoding? Fairly sensitive? More info, please.
I've tried the surface mount chips. Bought everything I needed, solder equipment, paste, breakout boards, Skyworks SI4835-B30-GU, you name it. Anyone need a necklace made out of over soldered electronic discharge dead chips ?
When I switched to SMD I also tried to use solder paste and found it to be a huge PITA and not at all homebrew friendly. I quickly abandoned it.
Now, I just use my regular temperature controlled soldering iron, ordinary tip, and radio shack fine gauge silver solder. I use my ordinary tip down to 0603, and 0402 in a pinch.
I think 0805 is the sweet spot for homebrew. 1206 is too big, and 0603 and 0402 are just showing off.
@w5jag Thanks for letting me know what to use. I have an adjustable soldering iron, and a bunch more chips I can burn. I'll get the correct solder. The past was fun to watch , btw.
Are we talking a 'performance' receiver, or one using these ubiquitous SDR chips? If it's a real radio, with coils and caps, I'm definitely interested.
With the arrival of football season, I decided to add a second ceramic filter in series with the first to increase selectivity in the receiver.
The two stations I absolutely needed to receive were only 4kw ERP and 7kw ERP sandwiched in the middle of the band, with blowtorches a couple of channel slots away. On some ocassions, the receiver would try to re-capture on one of the adjacent channels when receiving the low power stations. This did not happen often, but was annoying when it did. The second filter seems to have fixed this. Senstitivity is probably diminished from the extra insertion losss, but this seems to be a non problem. It hears a station on almost every assigned channel. Fidelity seems unaffected.
Interestingly, the AFC works differently with the addition of the second filter. Before, with only one filter, the radio could be set within twenty or thirty KHz of center channel, and the AFC would walk the station in and hold it within a few KHz of center channel. Now the AFC just holds whatever the frequency the radio is tuned to. When tuned to center channel, the AFC will hold it within a KHz of center channel, even on the weak stations.
Win W5JAG
Alignment
Once the receiver is determined to be working properly, alignment is required.
To properly align the discriminator coil, two pieces of test equipment are essential - an accurate frequency counter with resolution to 1 KHz and a high impedance voltmeter, either a digital VOM, or a VTVM.
To align the discriminator:
1) place the voltmeter across the 5K6 resistor between pins 7 and 10 of the LM3089;
2) Using the frequency counter to measure local oscillator frequency, tune the radio to exactly the center frequency of a strong broadcast signal;
3) With the receiver tuned to exact center frequency, adjust the discriminator coil so that the meter reads exactly zero volts. The AFC will fight this process, so it will be necessary to repeatedly retune to center frequency and readjust the discriminator to zero volts until zero volts is realized at exactly center frequency of the broadcast channel.
Unfortunately, I could not properly adjust the discriminator coil by ear. I tried it several times by adjusting the discriminator coil for what I perceived to be the correct response, and it was off when I checked with the voltmeter. The IF transformer I am using for a coil has more than one response peak, and I picked the wrong one every time by ear. The receiver will work with an alignment by ear, but it won't work as well as it is capable of.
The commercial front end is pre aligned, and the coils should not be touched. The IF transformer might benefit from adjustment to the load presented to it, but it probably won’t make much difference. Align for maximum received signal at exactly the center of the IF channel with a scope or spectrum analyzer. It might be possible to align by ear by tuning to center channel of a noisy broadcast station and tuning for maximum quieting.
When properly aligned, the radio should be very sensitive, tune cleanly, and have very clear and undistorted recovered audio.
73,
Win W5JAG
I printed an adapter to allow installation of the new module in the existing chassis. This avoids the inconvenience and expense of having to immediately print a new chassis, The adapter has mounting points for the 50 x 80mm RF board, and a second set of 50 x 70mm mounts for a perfboard to contain the stereo decoder if, or when, I get around to that. At present, I do not have a need for stereo reception. The expense of printing a new chassis is insignificant, but the inconvenience (time involved in printing) is not. It only took a few minutes to design the adapter and an hour or so to print it out.
I did not bother with installing connectors on the RF board, I simply soldered the existing wire directly to the relevant pads on the board to speed re assembly. I did reconnect the AFC with a 1M resistor on pin 7 of the LM3089 and the other end of the resistor to the tune pin on the tuner.
I ran into one bit of difficulty - on re assembly, there was a low-level audio tone on the audio output even with the volume control set to full attenuation. When volume was at a soft to normal listening level, the tone was masked, but I still considered this unacceptable. I first thought this to be a ground loop between the RF board and the LM386 audio board, but pursuing that theory resulted only in a reduction of the amplitude of the tone, not elimination. Ultimately, the tone was found to be coming in on the DC supply line to the frequency counter. The fix turned out to be simple - a 15 ohm 0805 chip resistor in the supply line to the counter, but it took me a while to solve the issue. It is now silent at any frequency I can hear with the volume control at max attenuation.
Interestingly, as part of that trouble shooting, I found that the new board had markedly more audio output than the prior board, enough to allow me to remove the gain set capacitor on the LM386, and turn its gain down to 20 from 200. On inspection of the removed board, I found that I had carelessly installed a 27K resistor in the RC de emphasis network instead of the correct 2K7 value.
This board works well; at first impression I believe the combination of better quality / smaller parts, and a true RF ground plane has made a worthwhile improvement. It seems to have better selectivity with one filter than the dual filter arrangement of the previous board, where there was potentially considerable blow by around the filters.
The KEC tuner is apparently a Far East market tuner as it tunes from about 75 MHz to 110 MHz. I don't need that kind of range, so the end point resistors on the tuning potentiometer will need to be adjusted. Although the data sheets specify that the KEC tuner is typically about 5 dB less sensitive than the KSE tuner, I cannot tell any difference in apparent sensitivity, The new tuner may be more sensitive, as it does not have the loss of a second filter. The KEC is very stable, it seems to easily hold + - 1 KHz tuning and has good capture when tuning to a station. The tuner module is secured to the ground plane of the pcb at four points - two at the center rear of the tuner, and one at each front corner. It is rigidly mounted and well grounded.
If there is any variance between the most recent schematic sheet and the prior sheets, the most recent should control.
73 and good DIY,
Win W5JAG
Apparently, the pause was short lived.
Here is the simple FM broadcast band tuner rebuilt unto a 50 x 80 mm prototype board by BusBoard Systems. I anticipate the pc board format with a true ground plane and smaller components / physical size will give improved RF performance over the original build.
Besides the obvious SMD construction, a few differences from the original: LM3089 rather than CA3089; a KEC (Korea Electronics Corp.) KCF201H tuner instead of the Kwang Sung tuner; one ceramic filter rather than two in series (for now).
Generally, I think sockets for RF IC’s are a bad idea because of the increased lead length, but I used a socket here to make it easier to swap out for an LM3189 at some point in the future. The socket legs are bent to a 90 degree angle and then soldered directly to the bus board.
The tuner pictured is fully functional, although AFC is not installed - I’m still studying how it is implemented on the LM3189. The delayed AGC of the LM3189 is also wired slightly different from the LM3089, although the chips are pin compatible.
The parts are mostly 0805 SMD, although there are a few 0603 10nF bypass caps. The resistors with flying leads are 1/8 watt. I put 10 nF disc ceramic bypass caps on the tuning and AGC line at the tuner, which is likely unneeded as the schematic shows internal caps, but does not list a value. I buy caps in bags of 1000 pieces, so I have decided not to be stingy with them after the power supply debacle. I used SMD electrolytics, although leaded caps are easier to install on these boards and actually have a significantly smaller footprint.
The output resistance of the LM3089 (and CA3089) is 5K. A 2K7 resistor and 10nF capacitor set the proper de emphasis for US broadcast standard and then the audio is taken from a 10 uF electrolytic.
The other picture shows a simple tuner built from a Phillips TBA120U (the suffix is important on these - there are a lot of variants). This chip is intended for television sound systems but is rated to 12 MHz. I did not pursue use of this chip because the preliminary circuit demonstrated that at 10.7 MHz, it is clearly inferior in terms of sensitivity and audio output relative to the CA/LM3089. To compete on equal terms with the ‘3089, it needs one, possibly two, IF stages, and an audio preamplifier. It does not have an AGC output, nor any provision for AFC.
Although CA3089 is approaching fifty years of age now, it still appears to me to be the chip of choice for homebrew FM receivers.
73,
Win W5JAG
Epilogue August 2022
I think I am at a pause point with this radio, after some incremental improvements.
After using it the last week, just as when I first built it, I became undecided as to whether or not the extra gain of the IF amplifier ahead of the CA3089 was needed. RCA and National Semiconductor say that it typically is not needed. After using the device in two separate locations, I concur.
The CA3089 features a delayed AGC for the tuner front end, the voltage of which corresponds to signal amplitude at the input to the CA3089, and which is graphed out in the data sheets. I measured the delayed AGC voltages at pin 15 of the CA3089, and was seeing some in the 3.5 vdc range and many in the 4 volt range with only a few feet of wire for an antenna. After thinking on this, and temporarily bypassing the IF amp for real world comparison/analysis, I decided that the consequent gain reduction from AGC in the front end outweighed whatever benefit (if any) I was getting from the IF amplifier, and that actual sensitivity might be degraded by the excess IF gain, when the increase in front end noise figure from excess delayed AGC is considered. More gain is not always better. Sometimes it is just more gain.
I then permanently bypassed the IF amplifier by connecting the output of the first ceramic filter to the input of the second ceramic filter, and since 10.7 MHz IF transformers are now scarce, I removed it and put it back in my parts inventory. Selectivity has improved noticeably, and apparent sensitivity is not affected. Additionally, some intermod artifacts are no longer present, and overall the radio seems better behaved. I put a note on the board indicating the circuit modification.
With regard to local oscillator stability (which operates at received frequency plus 10.7 MHz), the KSE tuner I used does not provide a specification, but a similar varactor diode tuned device, the KEC KCF201, does.
Two specifications are provided: ordinary oscillator stability, and frequency drift against strong signal input, which I take to mean frequency drift when AFC is active. The former spec is typically +- 200 KHz, and the latter is typically +- 5 KHz.
Ordinary oscillator stability is observed by just removing the antenna input. My tuner module will wander around over about a +- 50 KHz span under these conditions. After careful adjustment of the quadrature coil, and when tuned to a signal sufficient to quiet the receiver, my receiver exhibits stability under AFC control consistently +- 3 KHz, and on a strong station it will hold +- 1 KHz. On cold startup, the oscillator may be as much as 40 KHz from center channel as last tuned, but within a minute the AFC will begin moving the oscillator to center channel and won’t stop until it gets there. It’s rather fascinating to watch, in the first minute or so the frequency change can be dramatic, and as it approaches center channel, the rate of change slows. Most FM tuners only display to the nearest 100 KHz, so AFC action on received frequency is never seen. AFC also aids manual tuning, as it makes the radio try to lock unto a station frequency.
In short, I think the oscillator stability is satisfactory, and AFC action is also satisfactory, and working properly.
I removed the cheap generic LM386 from the premade board and replaced it with a genuine 1980's vintage LM386-4. This made a noticeable improvement in audio quality.
I removed the capacitor to set the gain to 20 and decided this was inadequate. So, I reinstalled the capacitor and reset the gain to 200. For now, this provides a satisfactory level of audio output.
I installed a phono socket for an antenna plug (I anticipate changing this to BNC) and a barrel connector for DC input. I printed a big hole for a snap in on / off switch identical to the switch used in the power supply, but right now I just unplug the radio to turn it off. Or turn off the power supply.
Many of the holes in the rear panel are not presently used and were printed in anticipation of possible future needs
Cosmetically, I printed a fairly large tuning knob for the radio.
I printed a bezel to go around the panel display to give it a little more pleasing appearance.
I printed a “lens” to cover the LED display from translucent blue plastic. The trick to doing this is to use a translucent or semi translucent plastic, make the lens thin, and use minimal infill. This lens is 1mm thick with 10% infill. The difference between an unfiltered display and filtered can be easily seen in the side-by-side pictures with the power supply. The lens makes the display more attractive, at least in my opinion, and when powered on the display has a bit of the same appearance as the old Noritake or Futaba vacuum fluorescent displays on imported late 70's, early 80's HF ham rigs.
That’s it for now. The radio is satisfactory for its intended purpose. Future improvements (if any) are anticipated to be using the top deck of the chassis for better / different audio amplifiers. That will likely be the subject of a separate thread.
73,
Win W5JAG
I built the tuner back in August 2017, and haven’t powered it up since, so it seemed prudent to make a functional test before proceeding further. When originally constructed, it was designed for and planned that this receiver would stay in my ham shack, but five years later I am spending the majority of my time at our second house up in the mountains at a local lake, so it will stay here, at least temporarily.
I considered replacement of the terminal blocks with plug in connectors similar to what I used in my general coverage receiver and SSB transceiver. After thinking about it, other than aesthetics (this board will never be a beauty queen) I decided that as almost all of the terminal blocks are just carrying DC or audio, there was no material improvement to be had in replacing them, and there is always the risk of breaking something in the process of fixing something that was not broken, not to mention the time involved. So I just left them alone.
The radio powered up and performs satisfactorily from an RF perspective. For testing, I moved the CA3089 audio output from the LM1310 multiplex chip and connected it to a de-emphasis network for mono reception. For quick testing purposes, a store-bought module using an LM386 audio chip with integral volume control drives the same 4 inch homebrew speaker used with my other gear up here. Scraps of leftover wire are being used as an antenna.
Interestingly, one of the reasons it was built in the first place was DX reception of sports broadcasts. Here at the lake, my Yamaha AV receiver with a pair of rabbit ears (behind a big screen TV) cannot quiet on the sports station I need to receive, and portable FM radios are very position sensitive and hand capacity sensitive. The ugly homebrew fully quiets on the target station with about a meter long piece of scrap wire held up by a small vase.
At my ham shack, with the receiver board out on the work bench and my A/C blowing across it, the AFC struggled, and I had planned on spending some time on it to improve AFC action. Here, with the board under the chassis, and sitting on the kitchen table, it seems to be better. It’s been tuned to the needed sports station at 99.500 MHz, and over the last three hours, it has wandered around about +- 4 KHz, usually within about +-2 KHz. In an FM broadcast bandwidth, this seems completely inconsequential, and I would not be aware of it at all but for having an actual frequency counter on the LO that displays down to the KHz level. The LO is 10.7 MHz above the receive frequency, so this seems okay for a varactor tuned oscillator at 110 MHz, but I have no experience with this, and would appreciate any other comments on the matter.
The LM386 audio module is from AliExpress, presumably with the -1 variant of the chip, and in an unusual abundance of caution, I initially powered it from the 8 volt 78L08 source on the tuner board. This confirmed that everything was working, but it left a lot to be desired - on voice peaks the counter would dim, the oscillator would pull, and the audio would distort. In short, unacceptable. I reconnected the module from the 8 volt source to the 12 volt input to the tuner board (leads were barely long enough), and all of the above was solved.
The LM386 is set for 200 gain, which is way too much given the strong audio from the CA3089 and will have to be set to 20 when I get a chance. I could cut a lead but will probably wait and unsolder the gain set capacitor. For listening to football, stereo does not seem immediately necessary, although I do require enough clean audio to fill the upper floor of the house here. The LM386 board may or may not be satisfactory on a temporary basis. I have some -4 chips that might be a better choice than the supplied -1 chip.
A few pics are attached. My cheap phone has a real tough time seeing the blue LED’s in the counter display, hence the odd angle pics. I might change it to a different color.
73,
Win W5JAG
Football season is coming up, so I decided to case the FM Receiver and put it in service. I’ve always liked the CO series of cabinets from LMB-Heeger: https://lmbheeger.com/coseries.aspx. The downside is that they are industrially priced, and somewhat expensive for homebrew.
So, I thought I would take some baby steps toward 3D printing a knock off chassis similar to one of the slide in chassis series in the CO line. The chassis pictured here is about 7.1"d x 6.3"w x 3.75"h inches, similar to a CO-3 which is a $65 chassis / cabinet combination: LMB Heeger Inc. - Slide In Chassis. Mine lacks the nice, rounded corners of the LMB, and of course is plastic, rather than aluminum. The LMB CO cabinets are perforated; a perforated cabinet should not be difficult to print, but I have not tried that yet.
With some exceptions, you generally can’t print a surface that would otherwise be hanging in mid air, unless it is supported underneath, otherwise the extruded plastic will just sag under the force of gravity until it cools enough to harden. It therefore becomes necessary to print this type of chassis with its edge on the print bed and continue vertically until it completes at the other edge, otherwise there will be a waste of both plastic and considerable time as supports are printed underneath the horizontal part of the chassis.
Because of this prohibition against printing in midair, I have been making breadboard style chassis, as I prefer to print the supports for circuit boards and other parts directly on the chassis, which are floating in midair if the chassis is printed from the edge. I’ve been experimenting with print head temps, fan speed, and print speed and on this print, the standoffs are quite good considering they started in midair. They are completely satisfactory for holding circuit boards.
I’m not really sure what I want to do with the receiver, so I created a couple of potential chassis - one is a smaller chassis suitable for solid state only; the other is deeper to potentially allow for vacuum tube audio amplification. There is only about 40 grams of additional plastic between the two, so I printed the larger of the two.
I’ve added some of the parts to test fitment but haven’t done anything else. Larry says there is unlimited storage on the site, so I’ve attached too many pictures and a zip file of the .stl’s.
73,
Win W5JAG
SIMPLE SUPERHET FM BROADCAST RECEIVER STEREO DECODER
A stereo decoder is optional. I did construct one for this receiver.
There are many linear IC’s developed especially for FM stereo decoding. As usual, I used what I had in stock - LM/MC1310 and LM/MC1800. I do not think there is much difference between the two, other than that LM1800 has a couple of extra pins that would make wiring a bit less crowded. Both are well documented in the National Semiconductor Audio Radio Handbooks, and in other literature.
I used a surplus, house marked, 1310 decoder. I used an LED with a current limiting resistor, instead of an incandescent lamp to indicate stereo decoding.
Adjustment of the circuit is simple - adjust the trimmer connected to pin 14 until a 19 Khz square wave is visible at pin 10 as viewed on an oscilloscope, and then center the trimmer in the range that produces the 19 KHz square wave. If a scope is unavailable, follow the same procedure on a known FM stereo broadcast - adjust the trimmer until the stereo lamp lights, and center the trimmer in the lit range.
73,
Win W5JAG
I think the horizontal ones are not too common - when I was trying to replace the horizontal mount tuner I robbed from that kit, all I could source were vertical mount modules.
For homebrew it makes little difference - just lay the module down flat and solder to the leads and case, or use it vertical to save space. It won't make any difference to the module.
That is an extremely good deal on a useful, difficult to source, part. I would strongly encourage anyone who even thinks they might want to make an FM band tuner now, or in the future, to get in touch with Jim and take that company up on their generous offer of being willing to surplus those modules out to DIY'ers.
Also, don't get in the rut of not looking outside the box - these modules don't have to be used solely for tuning the FM broadcast band - put a converter in front of them and use it to tune any 20 MHz swath of the HF / VHF / UHF spectrum. A great basic part to have on hand for homebrewing.
The picture shows one of these modules next to an ordinary paper clip for scale. Imagine trying to scratch build that at any price.
73,
Win W5JAG
W5JAG: I can confirm that my company does indeed have excess surplus stock of those Kwang Sung FM tuner modules, and would be delighted to supply small quantities at a price of $7.20 each, plus shipping; doubtless would offer a discount on quantities.
I can't immediately confirm that ours are the same model number as these are in obsolete stock at another location. They may depart slightly as they mount vertically on the board rather than laying flat... I think. I do recall that the last ones we bought needed a higher tuning voltage than the +9V in our product to tune to the top of the band. We bootlegged-in +12V from a second regulator and that worked just fine. They are a good tuner and work well with any of the available ceramic filters and monolithic IF chips like the obsolete RCA one you used.
Let me know if you have interest and I can supply further instructions. -Jim
Simple Superheterodyne FM Tuner - IF and FM Demodulator June 2022
The preceding post did not make it clear that the commercial tuner module is tuned to frequency by application of a variable DC voltage to the appropriate pin on the module.
In my receiver, I use a 10 turn precision pot that I obtained at a Hamfest for the tuning. When it goes into service, it will likely be replaced with a more compact 10 turn pot. It is also possible to channelize this type of tuner module simply by switching preset voltages, corresponding to the desired frequency, to the appropriate pin on the module.
Likewise, use of a DC voltage for tuning makes application of AFC ( automatic frequency control ) easy (easier?).
Almost all of the commercial FM tuner modules that ever were used a high side LO injection to a 10.7 MHz Intermediate Frequency. If one is scratch building a receiver, I would suggest 10.7 MHz, 21.4 MHz or 4.5 MHz as a suitable IF as parts are readily available for these frequencies.
There are probably as many ways to convert the FM modulated IF to baseband audio as there are builders, but in this particular receiver, I chose a CA3089 linear IC to accomplish this function.
I chose CA3089 for multiple reasons: I had some in stock; it was the dominant industry workhorse in both narrowband VHF communications transceivers, and wideband entertainment receivers for decades; it is readily available as NOS or NOS surplus; and best of all it is incredibly well described in both amateur and commercial literature. It was originated by RCA, second sourced at some point by National Semiconductor as LM3089, and then a tweaked version as LM3189, and RCA’s successors, Harris RF and Intersil, even made SMD versions of the chip.
The 1976 National Semiconductor Audio Handbook, at chapter 3, and the 1980 National Semiconductor Audio and Radio Handbook, also at chapter 3, describe the chip and it’s application in considerable detail. Both are under copyright, so I can’t put them here, but probably can be found on the internet as a download if one does not have the published paper version.
There are other chip choices, or it could be done the hard way with discrete components.
The CA3089 chip is sensitive and most applications do not require any additional IF amplification between the tuner output and the chip, but I found that DX reception of my target station here in the mountains of west Arkansas was spotty without additional IF amplification.
At first I used an MC1350 linear IF amplifier chip, but that provided too much gain. I then used a common emitter single stage amplifier using a PN2222A and that provided about the right amount of gain, in my subjective judgment. On the Old Radioboard, there was a discussion within a thread about the relative merits of LC loads versus ceramic filter loads, and I tried it both ways here, settling on a simple green core IF transformer as the load.
A single tuned circuit does not provide much selectivity at 10.7 MHz, so here two ceramic filters are used - one at the output of the tuner module, and another at the IF input to the CA3089.
A second green core IF transformer is used at the demodulator of the CA3089 for convenience, any resonant circuit at the IF should be fine here. The literature indicates that lower distortion can be obtained with two L/C circuits at the demodulator, but I did not try that and would have no way of measuring detector distortion in any event.
Most, but not all of the functions of the CA3089 are used here - the AFC output is applied as feedback to the tuner module. Signal strength metering is available but I have not used those - as long as it limits and quiets, I am okay with the signal strength. The interstation mute function is used here, but can be overrode if desired.
I have two deemphasis networks - one for mono and one for stereo - the mono output is taken to screw terminals for connection to an external audio amplifier. As shown, a jumper wire connects the deemphasis network to an optional stereo decoder.
The optional stereo decoder will be described in a subsequent post.
73,
Win W5JAG
Thanks for posting all that. Should make it easy for anyone to duplicate what's probably a quite good receiver. I know that Korean front-end; my company used it in an FM Modulation Monitor for broadcasters. That monitor product is being superseded by an SDR-based design this year, and we'll probably have a surplus of those tuners if anyone's interested... I believe we bought them in lots of 500 for a few bucks each. The mod-monitor was a dual-conversion design with a first IF at 10.7MHz and a second at 700kHz. FM demodulation was accomplished with a 'pulse-counting' scheme adapted from a circuit in the British RSGB's equivalent of the ARRL Handbook. ±75kHz deviation at 700kHz yields an impressive S/N figure. Being an old timer I have my suspicions of 'them newfangled digital chips,' not the least of which is that they are single-sourced and often hard to find in stock. That analog monitor will have had almost a 20-year run. In the beginning we used a Telefunken tuner, but they became unavailable and the Korean ones actually constituted an improvement at a much lower cost.
SIMPLE FM BROADCAST BAND RECEIVER - OVERVIEW / TUNER
About five years ago, the broadcast network for the local sports teams I follow, abruptly moved from a local AM station to a considerably more distant FM station.
I hadn’t built anything using RF in quite a while, so a simple broadcast receiver seemed like a good place to reenter the RF homebrew hobby. This thread will describe the receiver I actually constructed, which is not necessarily the receiver I would build if I were starting this project from scratch today. Those possible differences will be explored in a subsequent post.
The receiver shown here is functional, but not entirely complete. I have “designed” a breadboard chassis to match my homebrew SSB/CW transceiver and general coverage receiver, and also some other enclosures, but none have been printed out, and the receiver is still just a bare board right now. The large area of empty space on the perf board was (is?) to install some 10 turn pots that can be switched to channelize the receiver.
Probably the first critical decision a prospective builder of an FM broadcast tuner will confront is the receiver architecture, i.e., software defined radio ( “SDR” ) or superheterodyne, and, if the latter, how you actually get the FM broadcast band down to the chosen Intermediate frequency ( “IF” ). I did not consider any other architectures, such as regenerative.
Back in the day when SCA was a thing ( maybe it still is? ) Radio - Electronics ( now defunct ) ran a two part article by William Sheets and Rudolf Graf describing a simple FM broadcast receiver with a very simple tracking FET front end. The articles are appended here, extracted from the complete magazine issue that is available for download on world radio history. After carefully considering using the Sheets / Graf simple front end, I rejected it for the practical concern that it might not be quiet enough to receive the dx station I needed to receive.
Rather than design my own front end, I elected to use a commercial product. FM broadcast band tuners down converting to a 10.7 MHz IF, were once ubiquitous, in a wide variety of formats, as new production and surplus, and then virtually disappeared overnight. They are not unobtainium ( yet ), but they are awful darn close.
Searching every nook and cranny of my ham shack yielded two prospects - a really nice permeability tuned solid state AM/FM tuner module, likely for a car radio, probably out of a Radio Shack “Surprise Box” decades ago, and a solid state “three gang” modern tuner laying idle in a Velleman MK118 discontinued FM raio kit. After considering the pros and cons of each, I robbed the three gang tuner from the kit. Later, I found some identical tuner modules as surplus on ebay, so I was able to restore the kit.
I’ve attached the data sheet for the tuner module I used. I expect that all of the last production three gang tuner modules were a simple commodity item with virtually identical specifications regardless of the OEM, differing only in the vertical or horizontal mounting format.
As noted, these once common parts are now near unobtainium, with surplus or a friends stash being the only source as far as I know. They occasionally turn up on ebay. Some surplus houses may have a few if one wanted to call around. There are surplus vertical mount Mitsumi FE-352 three gang tuners being offered for sale on AliExpress at present. They look to be of good quality.
new MITSUMI FE 352 FM tuner , Mizmi, Beauty in Beauty|Amplifier| - AliExpress
I’ve attached my notes on how I implemented the tuner module in my receiver. Subsequent posts will describe the rest of the receiver.
73,
Win W5JAG
Good deal! Looks intriguing; sharpen that pencil and grab the yellow, lined pad! It does look to use a commercial front-end. Are those still available? I wonder what it would take to roll one's own.
It's a real radio. I guess - depends on how you define real radio ...
It's not SDR, although I've played with the SDR chips some and they work surprisingly well.
The radio is up and working - I'll try to start writing it up in installments over the weekend or next week, I built it about five years ago, and then got sidetracked on an SSB transceiver and the general coverage receiver.
73,
Win W5JAG
I've had experience with the 'one-chip-wonder' radios for FM; they are remarkable in performance and give you signal metrics that would be hard to derive otherwise. But these still require some peripheral chips and, as you remarked, Larry, are near-impossible to even see, let alone solder-down to a board or otherwise make connection to. Moreover, these are typical in-demand consumer parts, usually single-sourced, and many are either out of stock with suppliers or already have been superseded by an 'improved' successor that hasn't been released quite yet.
I, personally, like to work with time-tested, always-available discrete components that, if I drop them on the floor, I can find, pick up and keep on truckin'.
W5JAG, how about some particulars on the design? Is this a bare-bones-minimum radio or is it the performance equivalent of a home hi-fi? MPX-out for stereo decoding? Fairly sensitive? More info, please.
A real radio would be great
I've tried the surface mount chips. Bought everything I needed, solder equipment, paste, breakout boards, Skyworks SI4835-B30-GU, you name it. Anyone need a necklace made out of over soldered electronic discharge dead chips ?
Are we talking a 'performance' receiver, or one using these ubiquitous SDR chips? If it's a real radio, with coils and caps, I'm definitely interested.
Yes, me, that would be cool.