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.
Toroidal IF strip is stable!
I was poking around the IF strip -- currently composed of a mixer followed by three IF amps, for a total of 4 self-wound toroidal IF transformers -- with some copper tape in an attempt to figure out if minimal shielding would suffice. In the process of poking around, the oscillation stopped, and the IF strip became stable!
I think that what may have happened was that the low-impedance secondary winding wire of one previous IF transformer, which was supposed to go into the low-impedance emitter input of the following IF amplifier's transistor, accidentally was making contact with not only the emitter of the next IF amp, but also with the base of the next IF amp. The base is supposed to be grounded at RF by a 100 nF capacitor, but it's possible that if the signal is fed into both the emitter and the base that some sort of instability can develop. Now that the set is stable, I can't even reproduce the oscillation if I want to! Ah well, such is the nature of RF debugging -- I'm not going to spend much more time worrying about it unless it causes a problem. (As an amusing aside, while reading up about shielding, I found out that at microwave frequencies, the shield cavity itself can become resonant, which then causes all kinds of weird instabilities: https://www.edn.com/shields-are-your-friend-except-when/ .)
As a stress test I even tried squeezing the IF transformers as closely as possible next to one another, so that the spacing between adjacent toroids was less than 1 cm. Even in this condition, the IF strip did not oscillate.
And the reception is quite good. No AGC is implemented yet, but I could easily receive several shortwave stations with good volume on the ferrite rod antenna. But I think more IF gain -- a 4th IF stage, as originally implemented before when using commercial IF transformers -- is still needed. It seems that I am not quite or am only barely hearing the mixer noise. With the receiver located in a electromagnetically-quiet location indoors and with no off-air signals received, shorting the emitter of the first IF stage to ground with a short wire only barely changed the noise level of the receiver. What is being shorted to ground here is essentially the mixer noise from the mixer output (since no signals are present), and if the noise level changes only slightly when shorting the mixer output to ground, then this means the IF strip is not providing enough gain to loudly hear the mixer noise at the mixer output. Ideally, the mixer noise should be loudly audible, which will then lead to over-sensitivity for strong signals, which will then be dialed back via AGC.
So tentatively I can conclude that:
The toroids seem to have sufficient self-shielding such that even fairly close spacing does not cause oscillation, with 3 IF amps. We'll see what happens when I add in a 4th IF amp and regeneration onto the 4th IF amp.
Hand-alignment of each toroidal LC tank in isolation is sufficient to achieve high IF strip gain. To review, each isolated LC tank was connected to its unpowered IF amp circuitry, and the resonant frequency of each LC tank in isolation was peaked to the same IF frequency. After each individual tank was aligned in this way, all IF amplifiers were hooked together (secondary winding of previous IF transformer goes into the emitter of the following IF amp's transistor), and power was applied. No further alignment was done. A perfect alignment procedure would require re-peaking the IF transformers after they are connected into the circuit to confirm final alignment -- but very good reception is possible without doing this. So, the divide-and-conquer approach of winding and aligning each IF transformer individually is good enough, thanks to the intentionally low-L/high-C IF tanks which help reduce the effect of parasitic capacitances on each tank's resonant frequency.
So far, so good -- the dependence on commercially-wound IF transformers has now been removed.
I'm a little hesitant to post pictures of the quite ugly construction, but anyway, here are a couple of pictures to show the IF strip layout and the overall receiver layout.
In the below picture you see the ferrite rod antenna and its resonating variable capacitor at the bottom-left. Above the RF variable capacitor is the LO variable capacitor and its inductor, the yellow T50-6 toroid (swinging freely in the air), connected to a cross-coupled pair oscillator. At the top of the board, the top horizontal wire is the unregulated 1.2-volt Vcc line, with the regulated 0.7-volt Vcc line below it (currently used only for the LO, but later to be used for the regenerative IF stage as well). The detector transistor and AF amp stages are on the right of the IF strip. It should be possible to shrink everything down to fit in a pocket-sized, 3D printed case, which is the ultimate goal here.
Below, we see a close-up of the IF strip. The 4 toroids are pressed fairly close to one another, but currently there seems to be no instability.