At my second house, I have no way of listening to CW or SSB, unless I hook up my homebrew 20 meter SSB transceiver, or my FT-817 transceiver, so I’m thinking I might try to build a simple communications receiver to solve this perceived problem.
My first receiver build was a direct conversion (DC) receiver, that did not work very well, but motivated me to learn how to build better superheterodynes. I’ve attached a copy of the magazine article for that first receiver. So, I’m thinking I might have a try at another DC receiver. These seem to be the new state of the art as SDR receivers.
As with the general coverage broadcast receiver, commercial off the shelf parts will be used if there is an advantage to do so, and the cost is reasonable. My goal will be realized with a simple kitchen table receiver that works well for its intended purpose and can be used on a daily basis. A secondary goal is to make it a true kitchen table receiver, by doing all construction at my second house on the kitchen table, with no resort to any of the tools / equipment in my ham shack. A multi band receiver would be nice, but probably is not essential, at least at first.
I have an extra working QRP Labs VFO left over from the development of my 20 meter SSB transceiver, so it will probably be used for frequency synthesis, at least in the early stages of development, if not permanently.
This will be an experiment / work in progress sort of thing, sharing time with a couple of other projects I want to do. My simple SSB transceiver took three years to satisfactorily complete, and the simple general coverage broadcast receiver took another year after that, so those are the sort of time frames I operate in when tinkering / experimenting with stuff.
73,
Win W5JAG
So, after the VFO debacle, I mostly put this project on the back burner while I decided whether or not to scrap it entirely, or continue on with it.
I located an extra DDS ( a spare for the DDS used in my 20 meter sideband rig ) in a drawer in a desk at my office, but I decided against using it here. It has switched outputs for front end filters, but they switch by amateur band, so not really suitable for the bandpass filter board that was constructed for this receiver without some additional work. Otherwise, it would be a good choice - it can directly drive a level 7 mixer, and is very feature rich. The most significant limitation is that it only goes up to 40 MHz or so, so useless for up converting superheterodynes.
The original goal, as best I recall, was just a simple communications receiver to receive the local 75 meter SSB nets, before project creep turned it into something somewhat more complicated.
At this point, I think I'm going to go back to the original goal of a simple one or two band receiver. I'll replace the VFO I fried, but I'm no longer inclined to use the replacement VFO in this project. I have all the parts for a raduino similar to the VFO Coildog has constructed, so I think I'm going to try my hand at constructing something similar. In the meantime, I have a FeelTech function generator that can do a sine wave up to 60 MHz that I can use as an LO.
In light of this, I've made some rework to the detector board. I removed the MMIC RF amplifier, and converted the audio output to single ended. I couldn't tell that taking the output double balanced made any difference, so why bother.
I added a simple filter to the front end. The goal was a narrow band 80/75 filter, but I wound up with a fairly decent bandpass filter for 80/75/60 meters. I've attached pdf's of the schematic and ideal plot generated by Elsie, as well as a plot of my realized filter using the same log graph as Elsie uses for a better comparison. Also attached is a frequency sweep with the Red Pitaya that I transferred to the log graph, and pictures of the reworked board. Left to right is the bandpass filter, the SA602 product detector, and a low level audio amp. At the top of the board are 8 and 5 volt regulators, Only the 5 volt device is in use. I used a mix of 1206, 0805, and 0603 parts for the filter.
73,
Win W5JAG
I’ve run into a serious problem, threatening the completion of the project. In a careless moment, I connected the frequency synthesizer directly to 12 volts, with the predictable outcome. It’s fried.
The back story is that I had (have?) successfully completed the bandpass filters, RF amp, product detector, two intermediate audio stages, and an audio power amp. All of this worked as expected when strung together. I thought I was ready to begin assembling a prototype temporary receiver on a temporary chassis, that could then be developed and refined into a final receiver.
All of this was being powered from a 78M05 regulator, on a board that would not be in the final radio, so I installed a couple of regulators unto the RF board - a LM317M at 8 volts to maybe give the SA602 a bit more dynamic range, and a second LM317M at 5 volts for everything else. This seemed like an inconsequential thing to me, but this is where it all went south.
When everything was strung together after this change, the receiver was unusable - it was so full of noise that even local BCB stations could barely be heard above it. Before, it was so quiet that FT8, CW, and SSB on 10 meters was easily copied.
Thinking I had an oscillation problem, I chased that (unsuccessfully) and in this process, I discovered that if I disconnected the RF board from the bandpass filter board, the noise went away. Further, hooking up the random wire antenna (high impedance) directly to the RF input on the RF board (low impedance) restored normal sensitive operation, albeit with all of the consequences of no front-end selectivity.
This caused me to suspect a ground loop problem between the frequency synthesizer board, the bandpass filter board, and the RF board, and in the process of (so far) unsuccessfully chasing that, the frequency synthesizer got carelessly zapped by over voltage. Never had that problem with tubes ....
Anyway, this catastrophe has put the outcome of the entire project in serious jeopardy. Possible recovery options so far include:
1) forget about it - move on - don't need it anyway - it'll never be better than my 51S-1;
2) hold my nose and buy a new frequency synthesizer;
3) try to build a simple frequency synthesizer, but that won’t switch my filter board unless I learn a lot more about coding. and I don’t want to learn to code;
4) my simple broadcast receiver has easily accessed IF outputs, the first at 45 MHz and the second at whatever I want (presently 455 KHz ) , so I could build a communications back end for that.
Anyway, here’s a picture of what survives.
73,
Win W5JAG
I have not had much opportunity to play with this.
Since the last post, I have added 4.7 nF (472 - .0047 uF) caps from each of the audio output pins of the SA602 (4,5) to ground to shunt some of the excess high frequency noise to ground. In the picture, these are visible as small disc ceramic caps on the board carrying the SA602. After installing the discs, I found that I had some 0603 caps in this value, so the discs may (or may not) be replaced at some point in the future.
Continuing left to right across the board, LO injection from the Si5351 is made through a series connection of a 10 nF 1206 capacitor and 1K resistor to pin 6 of the SA602. Nominal output of the Si5351 is 50 ohms at about 10-12 dBm. This arrangement reduces the output to approximately -10 dBm in the frequency range I can measure and is a better match to the oscillator transistor in the SA602.
I added a low-level audio amplifier, seen to the right of the SA602. This is the same common emitter amplifier used previously, except with an MMBT3904 transistor and modified to work at 5 volts. Input is taken through a 1 mH RF choke, shunted to ground by another 4.7 nF disc ceramic. The voltage divider at the base is a 33K / 10K resistor combination, the emitter resistor is 330 bypassed by 10 uF, and the collector load is 3K3. This gives about 1.1 volts bias, and about 0.4 volts at the emitter. Input and output caps are 1 uF non polarized ceramic caps. The RC network at the DC input is 100R and 100 uF ceramic cap. I have not tried to measure gain or harmonic or IM distortion, yet. It sounds okay.
I stepped on a lost 1206 LED laying on the floor, so I put it on the board to indicate the board is receiving power.
I haven’t had a lot of opportunity to play with it. There are no microphonics. Sensitivity seems satisfactory; it had no trouble copying SSB .on 10 meters this weekend. Running the audio output from this board into the audio amp on the other board through a twisted pair and then into my powered speaker, seems to provide adequate audio gain. There was no instability observed.
73,
Win W5JAG
I added a miniature output transformer to balance the audio output - it's one of the transformers that Radio Shack used to sell, I believe the primary is 1K CT, and the secondary is 8 ohms. Not really a good choice, but all I had in stock at the second house. The two ends of the primary are connected directly to pins 4 and 5 of the SA602, and the CT is grounded through a 10 uF electrolytic.
I can't tell any difference between taking the audio output balanced or single ended. Single ended might be slightly stronger. Going from a single ended to a balanced RF input did provide a significant performance improvement - basically changing the detector from unusable to well behaved, so I would always use the SA602 in that configuration if it used as a direct conversion receiver.
I also added a broadband Mini Circuits MAR-6+ MMIC ahead of the SA602, so I would not have to be making so much gain at audio. I used the data sheet circuit for the MAR-6 with a 5 volt supply. Impedance matching at the input to the MMIC is good, but poor at the output. It is probably not realizing its specified gain as a result. I will measure it at some point but have not yet.
Most of the components are 0603 and may be hard to see in the pic. It remains to be seen whether either of these changes are desirable or will persist to the final version of the receiver. Broadband and direct conversion may turn out to be an undesirable combination.
The product detector is working sufficiently well to temporarily move on to other parts of the receiver, but this may or may not be the final version of it.
73,
Win W5JAG
I played with it some more last night and just could not get it to work right. I believe the actual receiver module with the Tayloe detector is working properly, and the non working part is the polyphase network that is supposed to cancel the unwanted sideband. It just doesn't.
Reluctantly I pulled it all off the breadboard and consigned it (for now) to the box of broken dreams. I'll try to come back to it later when I'm farther into the project.
This morning I briefly considered alternative detectors, all of which would be DSB only - I want a doubly balanced detector, so in my travel parts kit the best candidates were a mini circuits diode DBM (no conversion gain), an HP IAM81008, and the ubiquitous Signetics (now NXP) SA602. I have experience with all three - the HP is the receiver mixer in my homebrew SSB transceiver, and the SA602 is the TX mixer in the same rig,
I pretty quickly dismissed the diode mixer as I want some conversion gain, for now, to get things up and running faster. The advantage of the HP is that it is a 50 ohm part but is very tolerant of impedance mismatching. It has good conversion gain, good IP3, but a horrific noise figure which would likely require an RF amp above 40 meters or so. The SA602 is a higher impedance part, has a decent noise figure (5 or 6 dB at 45 MHz), good conversion gain, but mediocre signal handling capability for an HF receiver. Both the HP and NXP part are fine with 5 volts, which I already had available for the VFO.
I had some SA602 already mounted on an adapter board, so that made the decision for me.
I brought it online a little bit at a time. I used an RC network of 91 ohm, and 100 nF to the chip, and verified if was drawing current - 2.1 ma at 5 volts.
Initially I grounded pin 2 for AC with another 100 nF cap and just clipped the random wire antenna to pin 1. I took the audio out from pin 5 through another 100 nF cap. I put the LO at pin 6 through a 22 pF capacitor, which may or may not be the correct LO level - not worried about that right now.
It worked right off the bat and was surprisingly sensitive, but the single ended input configuration suffered from the typical direct conversion gremlins of hum and broadcast station breakthrough, in my case a 5 KW station on 790 KHz about fifteen miles away.
I wound a broadband transformer on an FT23-43 core, fifteen or sixteen turns on the primary, and three on the secondary, and connected the primary across pins 1 and 2 to balance the input. This got rid of the hum and BCB breakthrough, but it was still responding to a lot of unwanted signals, probably because of all the harmonics and other junk thrown out by the Si5351 oscillator.
Next, I hooked up the bandpass filter board to the secondary of the input broadband transformer, and that got rid of all the unwanted signals - except of course the stuff coming in on the unwanted sideband. I was able to hear FT8 on 28.075 MHz, so it is reasonably quiet.
With the RF part working satisfactorily, I played with using a transformer to balance the audio output, but this was unsatisfactory - it actually reduced the audio output, probably because I did not have anything on hand with the right transformation ratio. So, for now it is just taking the audio single ended through a 100 nF cap.
Audio from the SA602 goes through a twisted pair to the input of the low-level audio amp, and then jumper leads to the aux input of the radio I am using as a powered speaker,
It works, shows some promise, and was easy to get working which I needed after the drama with the other detector.
73,
Win W5JAG
The sweet taste of success turned rancid, and now back to sweet again. Maybe.
I'm snowed in here at the lake, so Monday I hooked up the radio to play with it, and quickly discovered it wasn't working right - at all.
When I set it up to receive USB, I noticed the audio output was at least 20 dB lower than LSB. Worse, tuning around on twenty meters, USB was easily readable although the radio was set for LSB. Finally, audio in LSB mode had a raspy edge to it that I didn't care for. On the good side, it was easily hearing FT8 on 10 meters in the LSB setup., with only my random wire for an antenna.
Thus began a several day wander in the wilderness of fruitless troubleshooting, starting with checking basic voltages, making sure the components were in the right places, measuring actual component values, REMOVING components to measure actual component values free of in circuit influences, and even fruitlessly replacing components, all to no avail whatsoever. This was complicated by having two modules involved - both of which could be malfunctioning.
So, this afternoon I set up a signal generator and broke out my handheld oscilloscope (didn't feel like fooling with the Red Pitaya) and did what I should have done in the first place - old fashioned signal tracing. This was complicated by having four signal paths to deal with, each 90 degrees out of phase. After an hour or so, I concluded the receiver module was working - I had four 1 Khz audio outputs, equal amplitude, with 0, 90, 180, and 270 phases at all the correct places.
The network board was a lot harder to trace - the phasing networks are complex, the board is not at all intuitive or clear as to the circuit layout, and there are four signal paths that ultimately converge to one mono audio output. I went through it component by component and path by path looking at the signals with a scope, and after an hour or so, a pattern emerged - the more 22 nF capacitors the signals went through, the worse the signal looked in terms of phase and shape.
I removed all of the 22nF smd capacitors I installed and replaced them with the monolithic 22nF caps supplied with the kit, and that actually appears to have to have restored proper function. Both USB and LSB are now equal amplitude and sound undistorted, but until I have a suitable high gain low distortion audio strip or have a chance to look at it again with instruments, I am reluctant to say for sure. But I'm optimistic - it's done unless I'm forced to come back to it.
The receiver board does not look too much worse for all the wear and tear, but that polyphase network board looks really bad now - the 22 nF caps are just tacked on where the smd caps used to be. I saved the smd caps I pulled and will try to measure them and maybe find out why they appear to have been totally unsatisfactory in this application.
73,
Win W5JAG
A small taste of success - the Tayloe receiver module and polyphase audio network appear to be working properly.
A 5 vdc power supply is now on the far edge of the prototype audio board. The regulator is an smd 78M05 soldered to the backside of the board for a heat sink. Instead of electrolytic capacitors, I used non-polarized smd 1206 100uF and 100 nf capacitor at the input, and a non-polarized 10uF cap at the output. This powers the VFO module, Si5351 frequency synthesizer module, and detector / polyphase network.
While the power supply was burning in under the load of the VFO, I finished the receiver module and the polyphase module and wired everything together on the breadboard. Demodulated audio is taken from the 3.5mm jack on the receiver module, and then to the line input on a small portable receiver. The bandpass filter is not inline at present - I simply hooked my long wire antenna directly to the RF input on the receiver module.
Audio gain is clearly insufficient with this arrangement, but I can copy CW on 14 MHz (highest frequency I tried) and SSB on 7 MHz. The unwanted sideband is cancelled as it is supposed to be, and single signal reception results as with a conventional superhet receiver. This suggests both modules are working properly.
The receiver module requires LO injection at 4x the receiver frequency, and prefers a square wave input. The firmware in the VFO allows for 4X multiplication to be selected, while the displayed frequency remains at the receive frequency. Playing with this revealed an idiosyncrasy with the band limit selections that control the bandpass filter board - the band limits are controlled by actual VFO output frequency, which may or may not be the displayed frequency. In the case of 4X LO multiplication, clearly it is not. This is simply solved by multiplying each band limit by 4, which restores proper bandpass filer tracking with the displayed frequency. I cannot find where this is mentioned in the documentation for the VFO.
It’s only been working for an hour or so, so not much more to add at present. It's starting to hear some chatter on the local Arkansas SSB frequency on 75 meters.
73,
Win W5JAG
I have not had a lot of time to work on this the last month, but I have started to work on an audio board.
This is a simple NPN amplifier built around an MMBT2222 transistor intended to take low level audio from the product detector. It works okay - the pictures show it being driven with 1 Khz at 0.004 volts at the input. I'm not sure what kind of voltages will be coming out of the product detector, so component values are still tentative at the moment, but resistor values can be easily seen in the attached picture. The emitter bypass cap is 10 uF, and the coupling caps are 1 uF. The bypass cap at the B+ input is 100 nF. Audio comes into the amp through an RF block consisting of a 1 mH choke. and .0047 uF (4.7 nF) cap.
Gain looks to be high, maybe too high. Audio is flat out to 100 KHz or so (highest frequency I have looked at). Looking at the spectrum shots, it looks like the 2nd harmonic is around -40 dB (after subtracting the generator component) so that is 1% ish THD. The noise floor seems high, but my cabling is poor (practically nonexistent) and could be introducing noise. SIne wave output looks okay.
With 20/20 hindsight, a pnp transistor might have laid out better. This is built on a recycled board that has a lot of ground vias left in place, so I was using those for my convenience.
The first pic is just a shot of the smp - input at the left, output on the right.
The second picture is the generator output looped into the red pitaya spectrum analyzer input to establish a baseline. The generator is a FeelTech 60 MHz function generator (FY6600?). Sine wave output at 0.004 volts.
Third pic is amp output.
Fourth is output on the red pitaya scope function.
73,
Win W5JAG
Happy New Year!
After a few glitches, the bandpass filter board now appears to integrate properly with the frequency synthesizer VFO. The LED’s showing filter status proved to be useful in the debugging process.
When I first installed it, I was surprised to find that a positive 5 vdc appeared on each band output line, easily visualized by ALL of the LED’s being lit. Thinking this was a flaw in the VFO, I probed the same PCB points on the VFO in the general coverage receiver, with the same results. After studying the board a bit, I decided I had the band output lines in the wrong set of PCB holes. The board marking in this regard is ambiguous, in my opinion.
It turned out that I did have the band output lines in the wrong set of holes, and after transferring the leads, and programming some band limits into the VFO, all of the LED’s continued to remain lit - except when a particular band was engaged, then its LED extinguished. In that regard, it worked perfectly, as I tuned through the HF spectrum, I could watch each LED sequentially extinguish.
So I made a basic assumption - that when a band was selected by the VFO, it would output a high + 5 vdc on the band select line - that was completely wrong, In fact, it was the opposite: when a band select line is engaged, it drops to 0 volts. As far as I can tell, this is nowhere mentioned in the documentation for the VFO.
Anyway, at the expense of a few more hours of time, this was solved by using a PNP MMBT3906 switch to translate the zero-volt output from the VFO band select line to a + 5 vdc output to engage the filter.
The attached video shows the filters switching sequentially through the HF spectrum. The video starts at 0 MHz, and steps in 1 MHz intervals to 32 MHZ, then returns to zero.
The bands are as follows:
Blue - 160 meters
Green - 80/75/60 meters
Orange - 40/30 meters
Yellow - 20/17 meters
Red - 15/12/10 meters
This will provide seamless tuning across the HF spectrum without operator intervention. The simple communications receiver will be fully general coverage like its kitchen table broadcast receiver stablemate.
73,
Win W5JAG
I had an extra breadboard (that was made for the general coverage receiver), so I installed the VFO and bandpass board unto it to make further testing / construction easier.
I had an unexpected, and time consuming, difficulty getting the VFO to power up. I traced this to an apparently bad ground trace somewhere in the multi layer main board. Apparently in its prior installation, the module was being grounded through the coax shield, so this flaw went unobserved. Then, I managed to miswire the encoder, something I would not have thought possible, and it took a while to figure that out.
The ground trace being bad in the multilayer board is troubling. If other traces are boogered up, may be that the board needs to be replaced.
73,
Win W5JAG
It won't win any beauty contests, but the front end bandpass filter board is done, except for relocation of the SMA RF connectors.
I broke down and added color coded LEDs corresponding to the band select line for some eye candy and confirmation switch voltage is reaching the correct filter.
The filters will be automatically switched in and out by a 5 VDC line according to received frequency without need for operator intervention, just like a real store bought radio. At least that's the plan for now.
The LEDs are matched for brightness by using a common current limiting resistor at the ground end of the display, with the green and red LEDs having an additional current limiting resistor in the line from the switch to the LED.
I tested everything AGAIN and detected no difference with my crude test equipment.
In hindsight, multi band operation was apparently more important than I thought. This was an interesting learning experience and can be improved on, I think, with higher quality components and better test equipment.
73,
Win W5JAG
The filter board is probably finished except for, maybe, the location of the RF connectors, or if there will even be RF connectors,
I used an XH connector for the DC switch lines. I'm not crazy about this type of connector on this board, but the advantages of not having to unsolder a bunch of leads outweighs the cons.
I added 10 nF bypassing caps to the DC lines on the filter board. I thought about adding color coded LED's that would light to show which filter was active but decided that was too much work for a dubious benefit, as there is little room left in the areas where the LEDs would have to go.
I ran a final test on the board by energizing each filter and checking the response. I made an overlay graph showing the response and overlap of the bandpass filters out to 50 MHz. The low pass filter is not shown. The notch around 19 MHz shows pretty clearly.
This has made me want to get some kind of VNA, or the VNA add on for my Red Pitaya.
73,
Win W5JAG
I tweaked the highest band filter a bit - it's marginally better at 10 meters, and closed some of the gap around 19 MHz.. The - 3 dB points are 19 and 30 Mhz.
19.5 - 30 MHz bandpass filter
9 - 18.5 MHZ bandpass filter
5 - 12 MHz bandpass filter
I decided the top three bandpass filters were unsatisfactory, so, using the free version of Elsie from Tonne Software, I redesigned them for parts values that I have in stock. Elsie, while old, is a highly useful program, even for the non-technical, like me.
http://tonnesoftware.com/elsie.html
These new filters are substantially better for general HF coverage and especially good for all of the amateur bands, except the top end of ten meters which is down about 4 dB. These new filters use 0805 inductors and 0603 capacitors, so there should be little stray capacitance floating around to distort the results. When you compare my realized results with the Elsie generated theoretical plots, they correspond quite well, with my filters having as good as, or often better skirts, albeit with more ripple across the passband.
The revised real-world coverage is:
0-2.85 MHz (low pass only)
2.75 - 6.75 MHz
4.9 - 12.5 MHz
9-18.5 MHz
19.5 - 30 MHz
There is a small gap around 19 MHz that I don’t think is of much significance in the real world. Also, the top band should be better, it’s peakier than I would like. I expect that the results here are an artifact of poor component tolerance. I may order some 1 or 2% parts for this filter when I next place an order with Mouser to test this theory.
Given my (lack of) technical expertise, very modest test equipment, and unknown tolerance of the parts I am using, I think these are acceptable results.
For now.
I'll post the schematics / plots in separate posts in the next day or so, so as not to over crowd this one.
73,
Win W5JAG
Not a lot of progress lately.
Here is a Tayloe doubly balanced detector that gives I and Q output:
Receiver module (qrp-labs.com)
And the matching phasing network that takes the I and Q output and produces a single channel, sideband cancelled, output.
Polyphase network (qrp-labs.com)
These are about 98 % complete. Assembly was a bit dicey as they are supplied with through hole parts, but I elected to use 1206 SMD instead. 0805 might have been easier. but I didn't have all the necessary values in 0805 in stock, and I wanted to avoid a hodge podge of different size parts.
I thought hard about omitting the 3.5mm stereo jack, but decided it might be useful in some circumstances, and can always be removed later.
I'm probably a ways off from testing these - a rudimentary audio channel needs to be made and also power supplies to run everything. The RF bandpass filter board also needs to be finished out, breadboard chassis printed, and probably other stuff.
There are also other detector circuits I'd like to play with.
73,
Win W5JAG
You can see that in the ideal filter I was striving for, the second harmonic of any given amateur band falls outside of the bandpass for that particular filter.
Unfortunately, in the actual results realized, in the instance of the 6 -16 MHz filter (which should be 5-12 MHz) the second harmonic of 7 Mhz will pass right through that bandpass filter with no attenuation.
That is problematic for a direct conversion receiver, as it will allow it to generate interference on two bands instead of just one. I will probably need to use the next lower bandpass filter for 7 Mhz even though it has a few dB of attenuation on 40 meters.
73,
Win W5JAG
I used SOD-323 style 1N4148 diodes for the remainder of the switches. These diodes are about the size of a 0805 component. Because the cathode marking is so difficult to read on my parts, when constructing the switches, I first installed only the diodes, and then checked them with an ohm meter to make sure they were correctly installed. After verifying they were correctly oriented, I constructed the rest of the switch. After the switches were installed, I again "swept" the filters and verified the response had not changed.
I haven't decided whether or not to install a connector for the band switching control leads or hardwire them. Likewise for the RF input / output. So, a decision on those two items is deferred for right now. Also, eye candy LED's to indicate which filter is active remain a possibility.
The last bandpass filter is more of a not very good high pass filter than a bandpass. A high pass filter would actually be okay here, but this has unacceptable ripple.
I'll try to get the schematic drawn and posted later today, or as soon as possible after that.
73,
Win W5JAG