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.
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.
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.
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.
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.
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.
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.