Post by qrp-gaijin » Wed Aug 14, 2019
Martin Luther King Jr.'s famous "I have a dream" speech concluded with repeated phrase, "free at last!" That's how I am feeling now -- I am free, at last, from the unpredictable and temperamental behavior of the regenerative detector. I have finally built a stable, sensitive -- and in my opinion, simple -- shortwave superheterodyne receiver that nowhere depends on regeneration. I am free at last!
The circuit grew out of my experiments with a regenerative superhet (viewtopic.php?p=81539#p81539) . I finally realized the regenerative stage, in my circuit, required such massive IF amplification at the input, and such massive AF amplification at the output, that in end effect the regenerative stage added very little, if any, benefit, and definitely was contributing to the overall instability of the set by feeding back stray regenerated IF signals back into the IF strip. Note that my interest is in receiving AM SWBC signals (not ham band CW or SSB signals), which -- if a regenerative detector were used -- requires below-threshold operation, which generally is less sensitive than above-threshold operation.
So I freed myself from regeneration, and made the following circuit changes:
Replaced varactor tuning with polyvaricons.
Removed the secondary 9V power supply that was powering the varactors. The entire circuit is now powered off of a single 1.2V cell.
Removed the regenerative detector. Selectivity and gain are now determined primarily by the 3-stage IF strip, which is resonant at 2.86 MHz.
Attempted to insert, then again removed, a fourth IF stage for more gain. This resulted in instability of the IF strip and a slow motorboating effect.
Connected the IF output from the final IF amplifier to the AF input of the 3-transistor direct-coupled AF amp, forcing the AF amp to perform detection as well.
A floating diode was also tried between the last IF amp and the AF amp (ahead of C3), but the floating diode was found to offer no benefit. The signal levels at the output of the last IF stage are strong enough such that the transistor(s) in the AF amp can effectively perform envelope detection. The current scheme may however be excessively loading down the last IF tank.
super.jpg (81.98 KiB) Viewed 2226 times
The sensitivity and selectivity are adequate, considering that the receiver has a severely compromised antenna -- a 5 cm ferrite rod antenna at shortwave. The IF of 2.86 MHz gives better image rejection (compared with a 455 kHz IF), at the expense of poorer adjacent-channel selectivity. More selectivity (and more IF gain) could be had by adding a second converter and second IF strip resonant at 455 kHz. I may try that later.
The current selectivity is still adequate for casual SWBC listening, which is the goal of this receiver. In fact, increased selectivity would make it more difficult to tune the receiver due to the small knobs on the polyvaricons, which have no reduction drive. I plan to use larger knobs in the final build, but will not implement any reduction drive mechanism because this should be a physically compact and very portable receiver.
I'm having problems with my video recording setup, so instead I am uploading here a photo and a two 1-minute audio files of tuning quickly through a crowded shortwave band.
Next I will add AF buffer(s) to drive low-Z headphones. I also may try capacitive coupling of the LO into the mixer, which could allow easier switching of coils for bandswitching (no link winding would needed, hence allowing an easily-swappable two-terminal coil). Then, to increase the sensitivity, I might try adding an RF amp.
Now, with a reasonably stable IF strip including AGC, I think my future receivers will focus on the converter part: the LO, mixer, and maybe an optional RF amp. I might even try a hybrid receiver where the converter is implemented with a tube.