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.
Part 10: AGC
AGC is working. Latest schematic is below.
AGC circuit description
AGC voltage is taken off the collector of the last AF power amp via a 100 nF capacitor. This feeds backwards into Q141, which is biased non-linearly as a detector. Increasing average AF voltage causes an increase in average current flow through Q141, lowering the base bias on the first IF amp Q132. VR18 sets the bias for the AGC detector Q141.
The AGC bias setting is somewhat tricky:
Set AGC detector bias to zero.
Set the regenerative stage somewhat below the oscillation threshold.
Disconnect and reconnect the mixer from the IF amp and confirm the mixer noise becomes inaudible and then audible again.
Slowly increase AGC detector bias until noise goes down, indicating a reduction in IF gain. Then decrease AGC detector bias slightly until the noise comes up again.
Tricky part: as the AGC begins to take effect, it will affect the supply voltage seen downstream of the R192/C195 decoupling network on the battery. This in turn will affect the regeneration level. Therefore, the regeneration level must again be adjusted to be somewhat below the oscillation threshold, and the entire procedure should again be repeated from step 3.
To test the AGC, hold a noise-generating device (like a modern smart phone 😀) near the RF tank and mixer. The noise level audible in the headphones should barely change. If the noise level jumps up, it indicates that the AGC is not working properly.
Where to derive the AGC voltage, and its interaction with manual AF volume control
AGC voltage is currently derived from the output (collector) of the last AF power amp.
An attempt was made to take the voltage from the input (base) of the last AF power amp, or equivalently from the output of the 3-transistor direct-coupled AF voltage amplifier Q128/Q129/Q130. This did not work and AGC action was not noticeable.
So the AGC voltage needs to be derived from the output of the last AF power amp. This then requires that the manual AF volume control for the receiver be placed at this same point in the audio chain -- right at the headphones. It is not possible to place the manual AF volume control anywhere before the last AF power amp, because in that case the manual AF volume control would then undesirably affect the AGC voltage.
Undesirable voltage fluctuation on the regenerative stage
The Q-multiplier Q139/Q140 is powered downstream of the Vcc decoupling network R192/C195.
Unfortunately, as the AGC is operating to vary the gain of the IF stage, the voltage seen downstream of the Vcc decoupling network will fluctuate due to the varying current draw.
This causes fluctuation of the voltage available to the regenerative stage, affecting the regeneration level and possibly also slightly shifting the frequency of the regenerated tank.
It is not possible to power the Q-multiplier Q139/Q149 (and the attached IF amp Q135) directly from Vcc. Attempting to do so causes AF motorboating of about 5 Hz.
Garbled SSB reception due to AGC affecting regeneration level and frequency of the regenerative stage
SSB signals around 7 MHz were tuned in.
Case 1. Pushing the regenerative stage over the oscillation threshold yielded warbling, barely-intelligible speech, consistent with a rapidly varying signal strength and frequency, likely caused by the AGC action. When tuning in to strong CW signals, "chirp" could be heard as the AF signal strength caused the AGC to reduce the IF gain, affecting the regenerative stage's frequency and causing the CW signal's received pitch to change.
Case 2. Running the regenerative stage below threshold, and tuning a separately-powered BFO (a GDO) to 7 MHz -- the signal frequency -- yielded good and clear SSB reception with almost unnoticeable levels of warbling or distortion. This indicates that the LO frequency is sufficiently stable even in the face of varying current draw caused by AGC.
Case 3. Tuning the BFO to 3 MHz (the IF) again yielded garbled, warbling, barely-intelligible speech. Speech quality was very bad, but slightly better than Case 1. Presumably, the AGC action is causing rapid fluctuation of the regeneration level (due to voltage fluctuation of Vcc_2) and possibly rapid frequency shifts of the regenerated tank's frequency.
Though SSB reception is possible in Case 2, it is not desirable to run the BFO at the signal frequency. It would be more desirable to run the BFO at the IF frequency. But the regenerated IF amplitude (and possibly frequency) are unstable due to Vcc_2 being unstable. And we cannot connect the regenerative stage directly to Vcc because this causes motorboating.
I need to think about this some more. A separate battery to power the regenerative stage would probably work. But having been successful thus far in using only one 1.2V battery, I would prefer to continue using only one 1.2V battery to power the set.
A compromise solution might be to connect the AF power amp to a separate RC decoupling network -- which I previously determined introduced AF distortion at high AF levels. But now with AGC working, the AGC might be able to keep the signal levels below the point where bothersome AF distortion begins. Then, having isolated the power-hungry AF power amp with its own RC network, I might then be able to connect the regenerative stage directly to Vcc without the emergence of unwanted AF oscillation.
Other problems: 1 Hz AF oscillation
When a very strong signal is tuned in -- like the local BFO signal -- an AF oscillation of about 1 Hz can occur. Probably, the RC decoupling network on the battery needs to be increased to combat this.