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.
This is Part 2, continued from the original post. I wrote above:
It seems that both factors are at work here.
Confirming factor 1, I observed that when the LO is powered by the same battery as the rest of the receiver (with its supply voltage coming off of a low-pass RC network in the power supply line), the regeneration level of the following regenerative IF stage decreases slightly at the LO is tuned higher, at a slow rate consistent with voltage sagging due to increased current draw from the LO, which then pulls down the entire available voltage for the receiver, reducing the regeneration level slightly.
Confirming factor 2, I observed, strangely, the opposite effect when the LO was powered by a separate battery from the main receiver's battery (but sharing a common ground). In this case, the regeneration level of the following regenerative IF stage increases slightly when the LO is tuned higher. Because the LO is powered by a separate battery, I think that in this condition, the LO tuning (and varying current draw due to tuning) cannot affect the main receiver's supply voltage. The regeneration level changes almost instantly (very quickly) with the change in LO tuning, consistent with the hypothesis that it is the oscillator's variable, frequency-dependent signal energy (and not the variable current draw) that is causing the changing loading on regenerative stage.
Moving on, let's assume the above two factors are negligible. In practice, they cause a very small, but acceptable, variation in the regeneration level as the LO is tuned. This is actually a pretty good result, since other oscillator circuits (e.g. Hartley) tend to have a widely variable output amplitude (depending on frequency) that then causes large variations in loading of a subsequent regenerative IF stage.
So this LO circuit functions well for a regenerative superhet. This LO circuit is the low-voltage, two-transistor cross-coupled oscillator, shown in the post above, which has a mostly stable amplitude due to signal limiting by the transistor's B-E junctions. I think it is this stable LO amplitude that limits the undesirable variation in the load placed on the following regenerative IF stage.
To simplify things further, I was able to design in LTspice (the circuit simulation software) a similar, low-voltage, one-transistor oscillator that also limits its amplitude in a similar way. In the below circuit, the diodes D1 and D2 serve to clip any excessively large voltage swings. The emitter resistor R4 is chosen to have a low value, to drive the transistor hard into oscillation so that the unclipped tank voltage over the entire tuning range will exceed 0.7 volts, thus causing the diodes to conduct and the tank voltage to be clipped at an almost-constant 0.7 volts over the entire tuning range. The C6/L6 tank is intended for later use as the IF tank, when Q1 is converted into a mixer. The C6/L6 tank does not interfere with the operation of the oscillator, whose frequency is controlled by the L1/L2/C3 tank.
One consequence of running the transistor so hard into oscillation is that the tuning range becomes slightly smaller than it would be if we ran the oscillator at a lower amplitude (which would then be frequency-dependent and cause the undesired variable loading on the regenerative stage). Measuring the frequency spectrum of the simulated oscillation signals, we see that the oscillator oscillates from about 6.5 to about 16 MHz, when using a 1.9 uH coil (0.5 uH + 0.5 uH + 2*mutal_inductance, where mutal_inductance = 0.45 uH given the K=0.9 coupling factor) and varying the capacitance from 300 pF to 30 pF. Analytically, we would expect tuned circuit to cover from about 6.7 MHz to 21.0 MHz in the absence of stray capacitance, but the oscillator as simulated covers less than this range. (The mechanisms causing the reduction in oscillator range may be complex and may involve more than just stray capacitance; see https://www.radiomuseum.org/forum/relaxation_oscillations_in_lc_oscillators.html.)
As shown in the above schematic, it should be possible to convert the one-transistor oscillator into a self-oscillating mixer, by injecting the RF signal into the Q1 base. Furthermore, the constant oscillator amplitude, and the constant loading on the IF tank C6/L6, should make it possible to regenerate the IF tank without having the regeneration level be affected by the LO tuning.
Next steps:
Build the above oscillator and confirm it works.
Convert the oscillator to a self-oscillating mixer, feed that into my current 2-transistor regenerative detector, and confirm that the LO tuning does not affect the regeneration level.
Simplify the 2-transistor regenerative detector into a 1-transistor regenerative detector.
If all of that works, the result will be a two-transistor regenerative superhet (minus AF amplifier): one BJT to convert from RF to IF, and one BJT to regenerate the IF and convert from IF to AF -- powered off of 1.2 volts. It may be barely conceivable to drive a crystal earphone directly from the two-transistor circuit, though I think that would be very difficult to achieve, since the signal levels from my antenna (a small ferrite rod) are so small to begin with.
Continued in Part 3.