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Post by qrp-gaijin » Sat Jun 22, 2013
dpnewkirk wrote:As a result of this, I think you're going to want to operate your regen-as-PLL at IF, behind a very flat AGC system (one that takes hold as closely as possible to the band noise floor, and holds the IF-strip output to within a dB or three as signals ramp from noise to very strong). (But then: If you don't tune your detector, you don't need regen-level AGC to keep your detector at a particular regeneration level as you tune...)
My goal -- it remains to be seen if it is attainable -- is to make a very simple regenerative shortwave set tuning, say, from 5 MHz to 15 MHz, that allows me to listen to SWBC stations without needing to adjust the regeneration control. I did try a simple superhet approach with a one-transistor self-oscillating mixer ahead of a regenerative IF stage, but there was too much interaction between the self-oscillating mixer and the regenerative stage to allow the regenerative stage to sit just at threshold while I tuned the LO and RF stages. Taking the superhet idea further would have pushed the complexity past what I considered minimalist (though I may be forced to return to the superhet idea). Hence my exploration of the AGC-controlling-regeneration idea.
I think that a reasonable plan of attack is:
1. Make a regen where the base is DC-grounded and the emitter resistance controls regeneration. I'm currently using this circuit:
(see related thread: http://theradioboard.com/rb/viewtopic.php?f=4&t=5031) 2. Connect base to the wiper of a pot wired as a voltage divider with a diode voltage doubler in the bottom half. 3. Amplify the RF voltage from the regen and feed it into the diode doubler. 4. Adjust emitter resistance and base voltage for "automatic" regeneration control. Based on some preliminary simulations I expect that the AGC responsible for controlling the regeneration level -- without special measures like hybrid feedback -- will not be able to regulate the oscillation voltage over the entire 3 octave tuning range because the amplitude (with the chosen Hartley feedback topology) varies too greatly over the range; greater amplitudes seem like they will cause squegging. Nevertheless, I think it's very possible that the AGC can maintain the oscillation at a very low level (that enables phase-locked reception) over a narrow range of, say, a few MHz. This would at least reduce, if not eliminate, the need to adjust the regeneration control. dpnewkirk wrote:Don't be disappointed if your detector unlocks on fades and this cannot be easily corrected. Were both sidebands always present at the same level, a regen-as-PLL would always be equally pulled up and down by sideband energy, and locking to the carrier would always prevail But selective fading may notch one sideband or the other and/or the carrier over time, and the resulting imbalance of energy on one side or the other of signal center may cause momentary unlocks. Makes sense. Frankly, I'm not expecting much performance out of this scheme, but I've always wanted to see how such a contraption works in practice. After I get it working as a proof-of-concept, I'll probably go back and resume work on the superhet.