I built the direct conversion receiver described on the Soldersmoke blog a while back but ran in to several problems. The audio chain was very unstable and, perhaps ironically, the audio output level was very low.
Hackaday has also been covering this receiver project.
After some very enlightening discussion on the latest podcast I went back and re-built the audio amplifier chain.
One insight was that the gain of the three stages depends on the gain of the transistors and testing mine I get hfe of 350 which is up at the top of the range and might have helped with instability. The other insight was that there are bad 1K:8R audio transformers around and I suspect mine is causing low output audio.
Finally, I was unsure about how much drive a diode ring mixer needs. On the podcast they mentioned +7dBm which should be measured into a 50 ohm resistor and not into the mixer. I hadn't done that before. My VFO put 0.8V into 50 ohms which I calculate to be about +2dBm so too low.
Adding a 0.1uF capacitor in parallel with the resistor in the FET's source raised output a bit and I'm now getting the desired squared off waveform at the input to the mixer. I have struggled to get enough drive out of the oscillator and have tried a variety of tricks including adding extra gain stages.
I've just had a tune around 40m and can hear stations from all up the east coast quite well. Distances over 1,000km. (Although the lack of AGC means my ears hurt sometimes).
VFO tuning is very sensitive but I know things can be done about that. Actually, I love this VFO with the PTO tuning.
Note that while I have built the band pass filter I don't find it is needed at my home so I've left it out for now.
Thanks again to Bill, Dean, Pete and Hackaday for the prompt to have another go at this amazing home brew project.
Update: NP0 Capacitors and now it is very stable
My PTO drifted badly with ceramic capacitors. I went looking for NP0 caps and found a little bag given to me by Ralph VK3ZZC at some point. These have made all the difference as you can see from this video.
Tuning is very sensitive but when I stop touching it all is very stable.
For reasons I can't understand my VFO puts out less power to the mixer than Bill's so I've added an amplifier stage which has done the trick.
2 comments:
Mark
When I did my initial training I was taught that the magnetizing inductance of a good transformer should be 5 time the nominal impedance of the circuit. Over the years since this seemed to have been revised down to at least 2 times.
Recently I purchased a surplus batch of what were advertised as 1000:8 audio transformers from an Australian vendor.
When I received them I measured the primary magnetizing inductance as being 0.34 H. If we assumed that the audio band exists from say 200 Hz up then this is a reactance of only 427 ohms at 200 Hz. Measuring the secondary inductance I observed a turn ratio (unloaded) of 11 so the transformers had the correct turns ratio.
My initial thoughts were that the transformers were designed for higher frequencies, maybe above 1000Hz. Then I looked at some other transformers in my junk box and found magnetizing inductances of some older 1000:8 transformers that I had previously used successfully, that were only a bit greater than that of my recent acquisitions.
I decided to test them out and built a single transistor amplifier that used the new 1000:8 transformer. It worked okay, not the best fidelity in the world but okay for communications type work.
My next exercise was to use two in a back to back circuit intended as an impedance matching and isolation device. The performance was abysmal.
After some thought I realized that performance was totally dependent on application. For small audio amps driven off a collector or drain I think such transformers are okay. For higher fidelity or other impedance matching tasks maybe you should use a transformer that is closer to the old 5 times rule of thumb.
From my reading of what has been said about bad batches of transformers I suspect maybe someone fell into the trap I did and applied the 5 times rule without thinking it through.
Thanks Anonymous. The transformer I have is tiny and I suspect gets saturated well before it can drive a speaker with any power. I have some on order but until then I'm using a little audio amp which, as you can hear, sounds very good.
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