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
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