I'm continuing the previous discussion (started in the thread about a tube-based regenerative superhet) here into a new post.
There are no final schematics yet, since the circuit is still a work in progress. This thread will hopefully describe that progress.
In my linked post above, I was discussing with @dayleedwards about the general problem in a regenerative superhet (with a fixed IF) that the LO tuning can change the regeneration level slightly.
I mostly fixed this by using an amplitude-stable LO, the cross-coupled BJT oscillator. I connect this LO to an unbalanced, single-BJT mixer (through a link winding from the oscillator tank onto the BJT emitter), and take the mixer output from the collector, which leads to the regenerated LC tank. In this configuration, the LO tuning almost does not affect the regeneration level. But, I noticed something funny.
I previously wrote:
With this new, mostly-amplitude-stable LO, I can indeed set the regeneration so that it is just below threshold over the entire tuning range, and leave it there when I tune the LO over its entire range. There is a slight decrease in regeneration as the set is tuned higher in frequency, but this is not enough to significantly affect the sensitivity of the set, as tested with signals from a small ferrite rod antenna.
I thought that the decrease in regeneration, when tuning the LO to higher frequencies, was due to the oscillator waveform, when tuned higher, placing an increased load on the regenerated tank and reducing the regeneration.
But just now I noticed that the decrease in regeneration was slow and gradual. To combat a separate problem of unwanted AF oscillation due to supply voltage ripple, I recently have been modifying a high-C RC decoupling network in the power line of the receiver. The high-C RC network serves to temporarily stabilise the supply voltage if the AF power amplifier suddenly draws a lot of current on AF peaks, by preventing sudden changes in supply voltage and only allowing the supply voltage to change very slowly.
Therefore, a slow -- and not instantaneous -- change in the regeneration level is indicative of a slowly changing supply voltage. And I observed that the regeneration level changed slowly, not instantaneously, as I tuned the LO. Even when snapping the LO from maximum to minimum very quickly, the regeneration level changed not instantly, but slowly over a few seconds. So I now think that what is actually happening is that at higher frequencies, the oscillator is pulling more current, which causes the supply voltage to sink (after a second or two as the RC network stabilises), which finally causes the regeneration level to drop lower. Note that my power supply is a single 1.2 volt cell, so I don't have the luxury of regulating a higher voltage down to a lower stable voltage.
With previous oscillators (Colpitts, Hartley, or Vackar), I also had the problem of LO tuning affecting the regeneration level of the fixed-frequency regenerative IF stage. These oscillators will change their output amplitude with frequency. I had assumed this varying output amplitude was causing the change in regeneration level. But now I'm not sure anymore.
To summarise, there seem to be two factors at play here:
Sagging supply voltage at higher LO frequencies causing a decrease in regeneration.
Differing LO output levels depending on tuning, which leak through the unbalanced mixer into the intermediate-frequency LC tank, thus placing a differing load on this regenerated LC tank and changing the regeneration level.
For testing, factor 1 can be eliminated by powering the LO off of a separate battery. Then, tests can be conducted again with a Colpitts, Hartley, or Vackar oscillator (which will change the output amplitude with frequency). If these oscillators no longer change the regeneration level with LO tuning, then we can conclude that factor 2 is not significant.
Continued in part 2.
Single-diode voltage regulation actually works in this 1.2V receiver!
I am now powering all stages directly from the 1.2 volt battery, except for the following three stages: the cross-coupled LO, the cross-coupled regenerative IF stage, and the last IF amp attached to the regenerative stage's tank. Those three stages are running off of 725 mV -- Vcc goes through 50 ohms through a forward-biased 1N4148 to ground, and the "regulated" 725 mV supply voltage is taken off of the top of the diode (with a large-uF filter capacitor also at the top of the diode). We're using about 24 mA of current with this simple "voltage regulator" arrangement (1.2 volts through 50 ohms through the diode to ground).
It was enjoyable to confirm that the good old cross-coupled oscillator -- for both the LO and the regenerative stage -- can run off of this small 725 mV. The one IF amp connected to the regenerative stage's tank is also, necessarily, powered off of 725 mV, which very likely leads to poor gain in this IF stage.
However, the poor gain of the last, 725 mV-powered IF stage is more than offset by the massive gain increase in the rest of the receiver. Thanks to the single-diode voltage regulation applied to the LO and the regenerative stage, these stages are now sufficiently stabilised (with regard to supply voltage) that no motorboating occurs and all of the other stages can now be powered directly off of the 1.2V battery, with no RC decoupling network needed and hence no voltage drop across the RC network. This full 1.2V power for the 3 IF amps, for the detector stage, and for all the AF stages leads to a quite noticeable boost in volume -- the no-signal mixer noise is now uncomfortably loud in the headphones (I will add a manual volume control potentiometer later).
As for frequency stability: Reception of strong CW reception still causes some chirp, but it seems less than before. SSB signals are more intelligible than before, but there still is some warbling present.
So single-diode regulation has improved the receiver gain and has improved the frequency stability, though the oscillation frequency of the LO and regenerative stage still can be influenced slightly by the dynamic AGC action. Further improving the frequency stability would require buffer stages, but I don't want to increase the receiver complexity any further, so I'm going to accept the current frequency stability as is.