Now and then I see some mention of using a Franklin oscillator as a regenerative receiver. Has anyone tried one?
It seems it may have some interesting properties, like higher loop gain, as described here:
According to that discussion:
Simulation of your Franklin oscillator shows an open loop gain of ~60 (~8x gain per stage), whereas one-transistor oscillators typically have single digit gain. [...] I suspect the difficulty in attenuating the Franklin's enormous gain is the reason why it is not traditionally used as a regenerative receiver.
One reason I'm interested in this is that I'm thinking that higher loop gain, by using 2 transistors in the feedback loop, may lead to better detection efficiency. I've recently (again) been simulating AF output from my low-voltage transistor regenerative detectors, and it's always low -- microvolts in yield microvolts out. By adjusting the regen right to the edge of oscillation (for a -3 dB bandwidth of about 80 Hz) I can get a little more gain (e.g. 1 uV modulated RF in becomes 4 uV AF out). Examining the regeneratively amplified waveforms at the LC tank indicates that as regeneration is cranked up right to critical threshold, the modulation depth becomes shallower. I think that this then leads to detection inefficiency, because the envelope detector then simply generates less output as it is traversing the smaller modulation peaks/troughs. This phenomenon of modulation compression is described here:
https://www.amplitudemodulation.com.au/regen.html
So maybe an oscillator with a fundamentally different loop gain behavior (like the Franklin) might be able to provide higher regenerative gain while still avoiding modulation compression, which then might lead to better detector efficiency. This is all still highly speculative and more work is required.
Any comments or discussion would be appreciated!
I speculate that the Franklin offers two benefits to the regen configuration:
allows Air-band and, perhaps UHF regens to be implemented using general purpose, nondescript transistors like BC108 / BC548 / 2N2222/etc. Common transistors like BC108, or BC548 don't have enough gain to oscillate at VHF and above. Hence, cascading the transistors as in the Franklin can overcome the gain deficit.
the Franklin's higher gain allows lighter coupling to the resonator - this may improve selectivity & stability.
Thanks for that. It seems to work OK. Nice straightforward circuit too.
https://www.youtube.com/watch?v=8I0xQOIYcas "Trying a Franklin regenerative receiver for HF". In this receiver a J-FET is used as a separated infinite impedance detector.
Hi. I've tried a couple of times. The signal to noise remained the same, when you reach the critical point. You can get more audio output from it. I've never been convinced there is much advantage. I'd be interested to here what results other people have found.