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
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