This radio was previously described on the old RadioBoard forum under the title “A Simple Superhet”, and is re-presented here in more detail than previously. It is a general coverage broadcast receiver.
OVERVIEW:
The radio features digital tuning and frequency readout with memory channels, a low VHF first intermediate frequency for good image rejection, and broadband circuits eliminating alignment. No unobtainium parts are used. Commercial products are used where there is an advantage to do so.
The radio is constructed modularly. Three 50 x 80mm BusBoard systems SMD prototype boards ( 200 x 100 format ) hold the RF and audio circuits. The power supplies are on simple perfboard. The completed modules and VFO are installed on a breadboard “chassis” designed on TinkerCad and printed on a simple and inexpensive 3D printer with PLA plastic, thus no complicated metal work is required. A rotary encoder is used for tuning, and directly mounted to the front panel, eliminating the tricky mechanical alignment problems associated with obsolete LC or Varactor VFO’s. The main tuning knob is also 3D printed. Volume is set by an electronic attenuator controlled by a simple DC voltage, eliminating cumbersome shielded wiring to the front panel.
The heart of the radio is a commercially produced QRP Labs VFO kit, available for about $33 USD, using the now ubiquitous Si5351 frequency synthesizer chip. The VFO can generate a tunable LO well past 200 MHz, quadrature output if required, tuning steps as fine as 1 Hz, at a square wave level sufficient to drive commercial level 7 mixers without additional amplification. A digital frequency readout and memory channels are provided. Further, an additional fixed frequency local oscillator is provided. The firmware allows the frequency displayed to account for the IF offset from the tunable oscillator. The kit is supplied with a white on blue background LCD, but 5 volt type 1602 LCD readouts are available in a wide variety of color combinations. My radio at this time uses a black LCD with red digits.
The Si5351 has many good features and one, perhaps two, significant drawbacks - there is a degree of cross talk between the oscillator channels that is inherent to the device. The output is a square wave, by definition harmonic rich. This is ideal for diode mixers. The Si5351 has a DC component on its output, so a DC blocking capacitor of 10 or 100 nF should be used in circuits where this DC component cannot be tolerated.
For up converting superheterodyne receivers, the general rule of thumb is that the first intermediate frequency should be at least 1.5 X the highest frequency to be received.
The intermediate frequencies in this receiver are 45 MHz and 450 KHz. Crystal filtering is used at the first IF, and a ceramic filter is used at the second IF. The use of a low VHF first IF, greatly simplifies the construction of the receiver, while enhancing its performance compared to older design techniques. Tunable LO injection is 45 MHz higher than the frequency to be received. The use of the low VHF IF puts the image frequency 90 MHz above the received signal, thus a simple low pass filter is satisfactory to protect a strong first mixer, while eliminating front end alignment and tracking issues.
The low pass filters and diode double balanced mixers used are commercial products by Mini Circuits. The IF amplifiers are 50 ohm broadband amplifiers. A TDA1072 Integrated Circuit handles final IF amplification, AM detection, AGC, and provides a meter output voltage that is used for signal strength indication and an additional AGC loop. Audio output is from a DA7052A BTL Integrated Circuit.
Each of the modules in the radio will be described in further detail in subsequent posts.
73,
Win W5JAG
This receiver was built expressly for hi fi reception of the big one, 5085 Mhz, and now that it is permanently QRT, with all of the talent and programming having moved to WRMI, I find myself in the unfortunate position (literally) of being in between the two 31 meter beams, and at a less than optimal distance from the transmitter, to add insult to the injury.
So, after a few weeks of observing received signal strength at my lake house QTH, which is typically 50 to 60 dB less than what I was getting, I pulled the four pole 9 KHz filter and replaced it with a four pole 6 KHz filter. The new alternate 4980 KHz frequency puts in a pretty good signal here (although it is beamed at south America, so I am off the back side), but is often compromised by a strong RTTY signal on 4987.5 KHz that would spill over into the edge of my wide passband during QSB of the main signal. The 6 KHz filter has stopped that spill over.
Not to wax philosophical, but whenever one door closes, another one opens. Sensitivity is marginally better with less noise in the passband, enough so that I can now Q5 copy some weak MW stations that were just too weak previously. And since I had no 450 KHz 6 KHz filters, I installed a 455 KHz filter that I pulled from something a long time ago, and I have some SMD crystals for 44.545 MHz, which means with sufficient motivation, I can make a crystal controlled second local oscillator to go from 45 MHz to the second IF at 455 KHz. That could improve performance even more since there is unavoidable cross talk in the Si5351 oscillator channels.
But another anomaly has reared its head - like Coildog experienced with his ceramic filters - my filter is off frequency by about 1 kc. It has the same i/o impedance as my 9 KHz filter, which was spot on, so it's not an impedance matching problem. I looked at the specs, and sure enough, the specification for nominal center frequency on these inexpensive four pole ceramics is +- 1 KHz, so this is within spec. Other than annoying, it's not that big a deal - I just adjusted my second LO by 1 KHz to compensate. Easy to do with the Si5351. But it annoys me, so I ordered another group of five of the 455 KHz 6 KHz filters, and hopefully one will be on center frequency. If I get two or more on a common center frequency, I might try putting two in series.
Or I might ditch work on the simple communications receiver, take what I have learned from building this receiver, and make a higher performance broadcast receiver that is better equipped to deal with these annoyances.
73,
Win W5JAG
After about fourteen months of fairly heavy use, I had my first component failure - the TDA7052A BTL audio amp. The symptom was intermittent audio output, restored by a slight adjustment of the volume potentiometer.
I incorrectly diagnosed this as a failure of the volume control pot, as it has a DC voltage applied to it which I suspected as having caused the failure of the pot. Replacement of the pot did not cure the problem, so I replaced the chip, which restored normal operation. I substituted a 100K linear pot for the previous 10K audio taper, to at least reduce the current in the pot, and spread the control range out, which was pretty compressed in the prior iteration, but not annoying enough to replace it for that reason alone.
In the development phase of the receiver, I stressed the chip pretty heavily exploring its voltage range, at one point making it so hot I feared it would unsolder itself. I probably should have replaced it then, but it continued to work so I let it go.
Replacement of the chip has restored normal operation. Even though these chips are rated for relatively high voltage operation, I do not recommend this. I think 5 - 6 volts DC is optimum.
73,
Win W5JAG
In addition to the 1980's era books by Collins Radio Co, ("Single Sideband Systems and Circuits"). Dr. Rohde ("Communications Receivers, Principles and Design"), and Wes Hayward ("Introduction to Radio Frequency Design") there are a couple of good books by a less well known author:
Radio Communications Receivers, Cornell Drentea, 1982
https://archive.org/details/cornell-drentea-radio-communications-receivers-1982-tab-books
https://worldradiohistory.com/BOOKSHELF-ARH/TAB-Books/TAB-1393-Radio-Communications-Receivers-Drentea.pdf
Modern communications receiver design and technology, Cornell Drentea, 2010
The first three titles are probably only available in print, but the copyright on Mr. Drentea's books does not look to be vigorously enforced and they are available as pdf downloads. I have a pdf of the 2010 book, and it is 487 page tome that is terrific, so it is out there somewhere and well worth looking for. Mr. Drentea's credentials are top notch, and it seems pretty clear to me that he is a ham, like most of us. I am reluctant to post the pdf here because it is clearly marked as copyrighted.
The answer to the original question "How much gain is too much at 455 KHz?" is "It depends".
The overall receiver gain for good sensitivity probably needs to be at least 100 dB, but there is no law that says you have to get all that at RF. You really need only as much RF/IF gain as is necessary for the amount of AGC control range desired, if AGC is even desired. That could be as little as one IF stage, or lots of them.
73,
Win W5JAG
For additional context, the issue I was investigating with broadband IF amplifiers was basically "what is the minimum number of IF tuned circuits required in a single-conversion superhet with a regenerative detector?" I thought I might get it down to one -- an LC filter on the mixer output, followed by broadband IF amplification, followed by a crystal-controlled regenerative detector at IF -- but I never succeeded with the "broadband IF amplification" stages in my trials, which admittedly were not carefully designed. I got tired of trying and failing, and finally just went with the old-fashioned way of making every IF amplifier tuned.
Well, I'm not entirely sure I agree with his premise, that untuned amplifiers necessarily lead to an increased background noise level.
In my particular receiver, it may be important to note that I used low noise bipolar transistors - mmbt5179, the smd equivalent of 2N5179. The strip is fixed gain, with attenuation, as needed, handled by the PIN diode attenuator at the front end.
My IF amplifiers are split over two frequencies, 45 Mhz and 450 KHz, and are filtered at each end - I have a 20 KHz two pole crystal filter at the entrance to the IF strip, and a 9 KHz four pole ceramic filter at the tail end of the untuned IF strip.
I am a believer in the benefits of a filter at the tail end of an IF strip, even an LC filter. This is pretty well known, I think, and probably mentioned in every textbook on superheterodyne receiver design.
I have also found tuned IF strips to have a tendency to oscillate. I intend to try common base amps, as you suggest, the next time I make a tuned IF strip from bipolar transistors,
At the moment, I have a decided preference for broadband designs.
73,
Win W5JAG
Following up on previous discussion in this thread about untuned vs. tuned IF amplifiers: I commented earlier that I didn't have much luck with untuned IF amplifiers, whereas the receiver of this topic is using 2 untuned IF amplifiers with success.
I found a discussion on another forum that briefly explains some of the risks of using untuned IF amplifiers, at https://www.vintage-radio.net/forum/showpost.php?p=1465261&postcount=4 . Quote:
I think that is partly what I was experiencing when I had very noisy results with untuned IF amplifiers. Furthermore, my use of an unbalanced mixer meant that there was a very large LO signal still present at the mixer output, which may have driven the following untuned IF amplifiers into saturation, further increasing distortion and noise and obliterating the comparatively tiny IF signal that should have been amplified.
Tuned IF amplifiers vastly improved the performance (using 2N3904 transistors running at 1.2 volts) in my experience.
I would guess that untuned IF amplifiers require more careful design than tuned IF amplifiers to keep the noise within acceptable levels. On the other hand, tuned IF amplifiers may be able to provide acceptably low-noise gain in a wider variety of sub-optimal conditions -- such as impedance mismatches, low signal levels, noisy transistors, and transistor bias -- thanks to the narrow-banded resonant gain of a tuned amplifier. Creating a chain of several tuned IF amplifiers brings with it a risk of oscillation, but I think the easiest way to avoid that is to use tuned common-base IF amplifiers, which all but eliminate unwanted oscillation thanks to the shunting of the instability-causing collector-to-base Miller capacitance to ground.
I'm afraid I am not yet familiar enough with the NanoVNA to start a thread on it. I haven't really used it much yet. The main reason I bought it is for adjustment of the small coupling loop on a small transmitting loop antenna, for a perfect 50-ohm resistive match (and 1:1 SWR) at the desired frequency. This is a pretty fiddly operation requiring small and precise geometric adjustments to the coupling loop's size, orientation, and position. I had given some thought as to how to do this with completely homebrew equipment (noise bridge and dip meter), but I finally realized it would be vastly easier just to use a NanoVNA, so I bought one. As a test, I grabbed a USB cable and rolled it up into a 3-turn coil. By hooking a small coupling loop onto the NanoVNA and holding that loop inside the 3-turn coil, I could see that the loop was resonant around 50 MHz, and by looking at the real-time SWR graph, and adjusting the size and position of the coupling loop, I could very quickly achieve a perfect 50-ohm resistive match (1:1 SWR). That quick experiment gives me confidence that I will be able to do the same thing with an actual small transmitting loop antenna.
I'm also hoping I can use it to measure crystal parameters for building a crystal filter.
I should have been more clear - I still use the 2M FM transceiver ...
At the time of construction, I used the dip meter as an RF wavemeter. The TX strip was from a kit ( GLB Electronics ) a classic frequency multiplier type from 12MHz ish to 2 meters, so it was important to make sure the multiplier stages were being tuned up on the right harmonic which the dip meter was well suited for, as well as for tuning the crystal oscillator tanks, the RF output tank, etc.
I still use it as a wavemeter for a quick check for high level spurious emissions, etc.
I was never successful in using it to reliably check the resonant frequency of tuned circuits, likely for some of the reasons you mention above. Since I tend to use toroids for tuned circuits, which are self shielding, I don't think dip meters get a good coupling into the field, unless the toroid is so large that the dip meter coil can be inserted through the middle of the core.
Dip meters would probably warrant a thread of their own, because, as you note, they can be tricky to use and give misleading indications if you are not careful.
I would be interested in a thread on your NanoVNA and it's capabilities for the hobbyist. There is a VNA adapter for the Red Pitaya but it is a bit pricy ( I'm cheap when it comes to hobbies ) and the price of the NanoVNA looks real appealing.
73,
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