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.
Here are some thoughts and experimental results about voltage stability.
The two most voltage-sensitive stages -- the cross-coupled oscillator for the LO, and the cross-coupled oscillator for the regenerative IF amplifier -- can probably run on quite low supply voltages, such as 0.7 volts.
By using a forward-biased silicon diode in series with a resistor, and connecting the free end of the resistor to Vcc, the voltage at the top of the diode will be, more or less, 0.7 volts, due to the voltage drop across the diode. The available voltage and current will likely be enough to power the cross-coupled oscillator. The voltage available on top of the diode will not be a perfectly stable 0.7 volts, of course -- there will be some ripple if the Vcc supply voltage has ripple. Some LTspice simulations (which I can't seem to locate now) indicated that with a 1.2 volt supply voltage, a millivolt-level voltage ripple on the Vcc line would be reduced by about a factor of 10 at the top of the diode, thus providing a crude form of voltage stabilisation, even at the low supply voltage of 1.2 volts.
Separately from the above considerations about diode-based voltage regulation, I did some AGC tests in hardware, increasing the standing AGC bias to a very high quiescent level -- thus greatly attenuating all signals, and making the AGC level fairly stable since no signal was now strong enough to "activate" the AGC any further. This then yielded clear SSB and CW reception. So it's definitely the dynamic AGC action that is causing the frequency instability when listening to SSB and CW.
Even if the supply voltage could be stabilized with a diode, it's still possible that AGC action might undesirably change the LO frequency and the oscillating regenerative detector's frequency. It's conceivable that when AGC changes the gain of the IF strip, then the input impedance and output impedance of the IF strip might also change, and these impedance changes might then affect the frequency of the LO and the oscillating regenerative stage. Avoiding this might then require buffer stages with stable impedances, thus increasing the receiver complexity.
For SSB or CW reception, probably the simplest practical method is simply to disable AGC when listening to SSB or CW. But some manual gain control would still be needed to avoid overloading the regenerative stage. Therefore, disabling AGC while still allowing manual gain might be most easily accomplished by adding a switch in the AGC signal line, which when closed allows normal AGC operation, and when opened disconnects the AF signal from feeding back into the AGC transistor. In this open-switch condition, then the AGC transistor's bias (and the IF gain) is determined solely by the variable resistor on its base, which can then be used as a crude form of manual IF gain control (crude because its action will be exponential; a small increase in base current will cause a large increase in transistor current and hence a large decrease in IF gain).
Since it's fairly easy to do, I think I will try using the one-diode voltage regulator to power three stages: the LO, the regenerative stage, and the last IF amp attached to the regenerative stage's tank. I can foresee four possibilities: (1) the 0.7 volts supplied by the one-diode regulator is insufficient to power the LO or the regenerative stage; (2) the set may start to motorboat again due to the different supply voltages in different parts of the receiver; (3) it may work and SSB/CW reception may be possible without distortion; or (4) SSB/CW reception may still be distorted due to insufficient voltage regulation, due to impedance changes caused by AGC action, or due to a combination of both.
If that fails, then I'll just go with the switchable AGC option.
Part 11: Voltage stability optimisation
This is a small update from part 10 concerning voltage stability.
Since this receiver is powered off of only 1.2 volts, there is no good way to regulate the voltage, since voltage regulation chips or diodes would require a much higher voltage in order to be able to regulate it down to a lower voltage.
Powering everything directly off of Vcc leads to motorboating due to internal battery resistance and small Vcc voltage ripples causing unwanted AF feedback and oscillation.
Therefore, part of the receiver must be powered downstream of an RC decoupling network (to isolate and filter out Vcc voltage ripples and prevent them from reaching another part of the circuit where they can cause a feedback loop), and part of the receiver must be powered directly from Vcc.
The final power supply scheme I settled on is shown below. The most voltage-sensitive parts of the receiver are the LO and the regenerative Q multiplier. Even small voltage ripples on these stages will cause noticeable shifts in the LO frequency, the oscillation frequency of the regenerated IF, and the amplitude of the regenerated IF signal. Because these stages are so sensitive to supply voltage variations, these stages should be powered from the most stable voltage available, which is the full Vcc voltage across the battery. In addition, the last IF amp shares the LC tank with the Q multiplier, so it too should be powered directly from Vcc. This scheme is the best we can do to ensure voltage stability, and hence frequency and amplitude stability, of the LO and Q multiplier stages. This scheme isn't perfect, because there is still some internal battery resistance that may cause the actual voltage across the battery to vary with the current draw, but this cannot be avoided.
All of the other parts of the circuit are then powered through a 100 ohm/1000 uF decoupling network.
Results:
No motorboating or unwanted AF oscillation was observed.
Distortion: Now, because the AF power amp is powered through an RC network, very strong signals can cause the final power amp to generate distorted output. This was previously determined to be caused by the presence of the RC network. The distortion sets in at a lower signal level than when the power amp was powered directly from Vcc. But, the good news is that the AGC can compensate for this. The AGC bias just needs to be set a little higher (setting the AGC transistor a little more into conduction), resulting in less IF gain. This then keeps the IF signal strength below the level where it would cause AF distortion in the final power amp.
Reduced AGC dynamic range: Ideally, we want the IF gain to be set so high that mixer noise (which corresponds to the weakest signal that can be received) to be amplified to a loud and comfortable listening level. However, due to the distortion described in point 2 above, we can no longer run the IF gain at this high of a level, and need to reduce the quiescent IF gain to avoid AF distortion on strong signals. Unfortunately, by reducing the quiescent IF gain, this also reduces the gain on very weak signals like the mixer noise. This means that weak signals (and mixer noise) are only weakly audible. Previously, when the AF power amp was powered directly from Vcc, very weak signals and very strong signals could be regulated by the AGC to be about the same strength. But now, since very weak signals are reduced in strength to allow sufficient AGC action on very strong signals, the end result is a reduction in AGC dynamic range.
Frequency stability: Setting the Q multiplier into a moderately-strong oscillating condition, strong CW signals still exhibit a noticeable of "chirp" or LO frequency change when the CW signal changes from off to on. I couldn't get many SSB signals in my reception test this evening, but the few SSB signals I heard seemed to be somewhat intelligible, but not very clear. There still seems to be some warbling in the speech, which again likely indicates some fluttering of the LO frequency caused by varying current draw as the AGC is dynamically responding to the signal strength and modulating the IF gain. Anyway, I think there is nothing more that can be done to combat this phenomenon, as long as the receiver is limited to being powered off of a single 1.2-volt cell. It's as good as it's going to get.
Excessive AF gain: As a test, powering the 3-transistor direct-coupled AF amplifier directly from Vcc (and not from the filtered Vcc_2) led to higher AF gain, but the AF gain was actually too much for comfortable listening and led to noisy audio.
Excessive IF gain: As a test, powering the IF stages directly from Vcc led to noticeably higher IF gain and a noticeably higher noise level, because the no-signal mixer noise was now being amplified to a very high AF level, which reduces the available dynamic range for signals above the mixer noise. The excessive IF gain could be more-or-less compensated by adjusting the AGC bias even higher.
Optimum gain: The excessive gain described in point 5 and point 6 above indicates that the current receiver (as shown in the above schematic) seems to have reached an almost optimum amount of gain -- enough for loud headphone listening, but not so much that there is excessive noise in the no-signal condition, which might overload the Q multiplier or overload the AF amplifiers. However, this optimum amount of gain may partially depend on the Q of the inductors in the IF LC tanks. If one uses inductors with a different Q than my current inductors (which have unknown Q), then this might change the total IF gain and lead to insufficient of excessive IF gain. For the IF tank inductors I am currently using shielded, red OSC coils, intended for the LO of AM BCB receivers, that I rewound. At some point, I hope to try hand-wound toroidal inductors for my IF tanks, although this will make aligning the IF tanks more difficult as toroidal inductors aren't really adjustable (except for adjusting the turn spacing).
Now, I think the core parts of the receiver are working about as well as can be expected. Next steps will be minor improvements:
Adding an LED to show signal strength (the AGC current).
Adding a manual volume control, which may need another AF buffer stage.
Implementing LO-RF tracking.
Adding a bandspread capacitor. The bandspread capacitor, like the main tuning capacitor, will tune both the LO and the RF sections simultaneously, and must be a dual-gang capacitor. I'll probably aim for around 500 kHz of bandspread at the low end of the tuning range (around 5 MHz), which will then probably cover 2-3 MHz at the high end of the tuning range (around 20-ish MHz).
Maybe adding a BFO. Because the Q multiplier can already be set into the oscillating mode for CW/SSB reception, the only reason to add a BFO would be for single-signal selectivity, running the Q-multiplier below threshold as a narrow filter and tuning the BFO to one or the other side of the filter peak to emphasize one or the other sideband. I'm not sure if it will be worth the trouble to add a BFO in this receiver, since it's not really optimised for CW/SSB listening due to the lack of voltage regulation and some small LO drift.
Some other major improvement ideas, which I may or may not tackle, would be:
Increase the AGC dynamic range by controlling two, not just one, IF stages via AGC.
Investigate whether different LO topologies (instead of the cross-coupled oscillator) might have improved frequency stability when faced with ripples on the Vcc voltage. The cross-coupled oscillator's frequency is somewhat sensitive to voltage, whereas traditional LC oscillators may be less sensitive. An LTspice simulation might be a quick way to check this.
Superhets with common-base IF amplifiers (like mine) seem to be unusual. But I found a circuit with an IF strip similar to mine, using common-base amplifiers. On p. 17-25 of the following document, a "pocket superhet" is described: https://worldradiohistory.com/BOOKSHELF-ARH/Author-Groups/Edwin-Bradley/Transistor-Circuits-for-the-Constructor-No-1-Edwin-Bradley.pdf . The text states that:
I have to say that this mirrors my experience also. I have 4 tuned IF stages. When I tried common-emitter stages, the whole IF strip oscillated. It seems that neutralization, with some empirically-determined negative feedback capacitor(s), is needed to inhibit oscillation. Also the collector output generally needs to be connected to an inductive tap on the LC tank (not the top of the LC tank) to avoid oscillation; this requirement for a tapped coil makes self-wound IF tanks more troublesome.
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.
To stabilise the LO voltage when adjusting IF gain, I'm now powering the LO (Q54/Q55 in the below schematic) directly from Vcc instead of the filtered Vcc_2. This has mostly eliminated the problem of the LO changing frequency when the IF gain is adjusted. I still think there is a several-hundred Hz shift (tested with the LO at about 7 MHz) that may occur when IF gain is altered from zero to maximum. This is a lower frequency shift than before (when the LO was powered from the Vcc_2), but still is potentially problematic for SSB signals. So, SSB reception may not be possible, or may be distorted, when AGC is used.
The latest schematic is below.
However, I am now noticing some unwanted AF oscillation with very strong signals, when IF gain is at maximum and when regeneration is near, but below, threshold. I can't say for sure, but I don't remember this happening previously when the LO was powered from the filtered Vcc_2. When the problem occurs (only on strong signals), with IF gain at maximum, a sputtering AF oscillation of about 10-20 Hz appears. Lowering the IF gain slightly increases the frequency of the AF oscillation. Lowering IF gain further stops the AF oscillation. This provides some hope that an AGC system will be able to avoid this AF oscillation problem, by keeping the IF gain on strong signals low enough that the AF oscillation does not occur.
It may also be worth trying to power the Q multiplier Q62/Q63, and the last IF amp Q67, directly from the most stable voltage Vcc instead of from the potentially sagging voltage Vcc_2.
Bingo! The suggestion from @coildog is right on the money -- the supply voltage, seen by everything downstream of the 100 ohm / 1000 uF Vcc decoupling network, is sagging as IF gain is increased. With IF gain at 0, supply voltage to the LO as measured with my multimeter is 0.933 volts. With IF gain at maximum, it drops to 0.921 volts. Then, increasing regeneration to maximum further reduces the voltage to 0.896 volts.
These voltage swings will certainly cause variation in the LO frequency.
On the other hand, the measured voltage at the battery is a constant 1.190 volts regardless of the setting of the IF gain or regeneration. So, the battery voltage itself seems stable enough, indicating negligible internal resistance of the battery.
The next step is to try powering the LO directly off of the battery (Vcc), instead of from the RC decoupling network. This is going to require long wires to the battery, due to the way I have the circuit physically laid out. Hopefully, decoupling capacitors at the ends of the long Vcc wire will prevent unwanted RF currents from flowing on it.
@coildog wrote:
I haven't had time to try rewiring the circuit yet, but I did perform a quick test monitoring the radiated LO frequency on my RTL-SDR. (I tend to use my RTL-SDR most frequently for this purpose, as a handy piece of test equipment!)
The results were that increasing the IF gain caused the LO frequency to increase. Unexpectedly, increasing the regeneration above threshold (into hard oscillation) also caused the LO frequency to increase.
This is just a theory for now, but these results are consistent with the hypothesis that the supply voltage for the LO is sagging due to increased overall current draw caused when IF gain is increased or when regeneration is increased. Lower supply voltage, with the cross-coupled oscillator, will indeed cause the oscillation frequency to rise, by reducing the tendency for relaxation oscillations and allowing the oscillation frequency to move upwards closer to the natural resonant frequency of the LC tank.
A quick test revealed that adjusting the manual IF gain (bias on the first IF stage) unfortunately can still affect the regenerative IF stage and push it above or below the oscillation threshold, even with 3 IF amps between the gain-controlled stage and the regenerative IF stage.
This means that when the regenerative stage is moderately or heavily oscillating, adjusting the gain of the 1st IF stage can still affect the loading on the regenerative stage, which may cause its oscillation frequency to vary.
Furthermore, it is still possible that the gain adjustment of the 1st IF stage may be affecting the LO tuning as well.
I plan to do careful monitoring (on my RTL-SDR receiver) of both the LO oscillation frequency and the IF oscillation frequency (of the regenerative IF stage) while adjusting the IF gain, to see which oscillator is being pulled, or if both oscillators are being pulled.
To implement AGC, perhaps the easiest possible workaround to solve this problem of oscillator pulling is simply to control regeneration level as the IF gain (leaving the gain fixed for all non-regenerative IF amps). In this case, four IF amps and 1 mixer stage separate the LO from the gain-controlled regenerative stage, so adjusting the regenerative gain should have no effect on the LO. (Of course I plan to confirm this by monitoring the LO frequency.) This approach would require that the regenerative stage always be run below threshold, because above-threshold operation would be pulled down below threshold again by the AGC. A separate beat oscillator would be needed to listen to SSB or CW. But that is the superior approach anyway, because it will allow single-signal reception by tuning the regenerative amplifier away from the BFO frequency (away from zero beat), so that the regenerative amplifier boosts signal energy on only one side of zero beat.
The challenge with this approach will be how to ensure that the interaction between the manually-set regeneration level and the AGC-controlled bias is smooth and predictable. I'm thinking of using a common-base regenerative IF stage with manual regeneration control via emitter resistance, and AGC control via base bias. I could imagine that while adjusting the manual regeneration control upwards, as we approach threshold the noise and signals will become stronger, so the AGC tries to pull the base bias down, causing everything to become quieter, possibly causing an unwanted feedback loop and AF oscillation. Slowing the AGC reaction time should hopefully allow the AGC loop to stabilize even in the face of volume changes caused by manual regeneration adjustment.
Listening tests tonight with extremely strong SWBC signals revealed no problems with the newly-configured regenerative IF stage, with the separate detector taking the RF output from a capacitive tap (not the top) on the regenerated LC tank. It was always possible to cure overload by either reducing IF gain or by detuning the RF section. In all cases, after adjusting the signal level accordingly, it was always possible to adjust the regenerative stage to near critical threshold and to achieve a narrow bandwidth, clear audio, and minimal noise. In short, it's working!
I could also successfully listen to CW and SSB signals by reducing IF gain slightly and increasing regeneration above the oscillation threshold.
Listening to SSB signals, I could unfortunately confirm that tuning the RF section slightly affects the LO tuning. Similarly, changing the IF gain also slightly affected the LO tuning. These small changes in tuning are almost unnoticeable when listening to strong AM SWBC signals, but when listening to SSB, the changes in tuning are noticeable and shift the tuning away from the correct frequency needed for SSB reception.
Therefore, the next challenge is to design a variable-gain IF amplifier, or some variable attenuation scheme, that does not pull the LO at all, such that it does not affect SSB reception. The varying gain (or varying attenuation) also must not affect the regeneration level at all. It will require some experimentation to determine where in the signal chain to place the variable-gain stage (or variable attenuator), so as to affect neither LO nor regeneration level. It may be necessary to add isolation between stages (e.g. a buffer after the LO), to reduce the coupling between stages (e.g. using small-value coupling capacitors), or to investigate balanced mixer schemes.
I previously had a simple AGC system working in an earlier, non-regenerative version of this receiver, but in hindsight, I think that the old AGC system must have been pulling the LO frequency as it varied the gain, but I failed to notice this due to the wide IF bandwidth and due to listening to AM signals instead of SSB.
Part 9: Properly-behaving, low-noise, narrow-bandwidth regenerative IF stage
I finally got the regenerative IF stage working properly, with low noise and low distortion. The key point is connecting the separate BJT detector stage (nonlinearly biased) not to the top of the regenerated IF tank, and not to the emitter of the cross-coupled amplifier, but instead to a capacitive tap on the regenerated IF tank. Only in this condition does the regenerative amplifier yield low-noise, minimally-distorted audio.
Also, I previously reported occasional motorboating problems when the regenerative IF stage was tuned to critical threshold, which I attempted to combat with heavy and multi-stage power supply decoupling measures. In the current receiver, with only nominal levels of power supply decoupling, no motorboating is observed and the regenerative amplifier is far more stable than before.
Also, I previously reported that the regenerative stage could not be operated above threshold, because it generated too much noise. This is also now solved. The regenerative stage can be set into the lightly-oscillating condition to listen to CW or SSB signals (LO stability permitting). The heterodyne whistle is clean and noise-free. If the regenerative stage overloads, it is sufficient to simply reduce the IF gain (bias on the first IF stage).
Also, I previously reported that the regenerative stage didn't seem to be narrowing the bandwidth. Now, it definitely is narrowing the bandwidth correctly, and if adjusted too narrowly, a seashell sound properly occurs, without excessive noise as previously experienced. Adjacent channel selectivity is now quite good, and sensitivity is also very good -- sufficient to hear the mixer noise, and very weak signals, with IF gain at maximum.
From this I conclude that:
Connecting a BJT detector (non-linearly biased amplifier) to the top of a regenerated IF tank has a high risk of causing instabilities, distortion, and noise at critical threshold. Any tiny variation in circuit parameters -- such as variable loading caused by a detector stage -- can affect the regeneration level, and the dynamic effect of such variations might be AF oscillation or excessive noise around critical threshold.
Connecting a BJT detector to the emitter of a cross-coupled regenerative amplifier yields also distorted audio, compared to connecting that same detector to a capacitive tap on the tank. I think that the signal at the emitter of a cross-coupled pair is inherently distorted, and is further distorted when passed through the separate detector. On the other hand, by using a capacitive tap on the tank, the RF signal from the tank will be cleaner (closer to sinusoidal) than at the emitter.
Excessive noise at or above critical threshold may be a sign of improper and rapidly-varying loading on the regenerated tank.
When not using regeneration, connecting the BJT detector to the top of the tank works very well and yields clean and loud audio with no noticeable distortion. But taking RF from the top of the tank doesn't work as well when using regeneration.
The way I discovered this problem was first to replace the cross-coupled oscillator (used to regenerate the last IF tank) with a common-base oscillator. This was much quieter. I almost jumped to the incorrect conclusion that the cross-coupled oscillator was inherently noisy, but that didn't seem right. So I connected an alligator clip to the BJT detector's base input and tried connecting the detector to each of the following three points: RF_0 (top of tank), RF_1 (capacitive tap on tank), and RF_2 (emitter of the cross-coupled pair), and came to the above conclusions.
It's tricky business working with regenerative stages. Regens always make noise at critical threshold. But only through repeated investigation did I finally discover that the threshold noise I was hearing, in this case, was indeed excessive.
It's worth noting that B. Kainka's shortwave regenerative receiver (https://www.b-kainka.de/bastel3.htm), using the same cross-coupled pair for the regenerative amplifier, also connects the separate BJT detector to a tap (inductive) on the tank, instead of to the top of the tank. That does have the disadvantage that the signal levels will be lower at the tap than at the top of the tank, making weak signal reception difficult. In a straight regenerative receiver, that disadvantage is difficult to overcome. But in a regenerative superhet, that problem can be overcome simply by adding more IF gain ahead of the regenerative detector, as I have done with my 4 IF amplifiers.
Here is the latest schematic.
Four IF amplifier stages are needed to enable hearing the mixer noise, and any very weak signals only barely above the mixer noise, when the regenerative stage is adjusted below critical threshold. Fewer IF stages might be enough if weak signal reception is not needed and if reception of only strong signals is desired. As described on the schematic, it is useful to test the receiver's sensitivity by temporarily disconnecting/reconnecting the mixer and confirming that the mixer noise becomes inaudible and then audible again.
Second attempt at double-conversion: still no-go
I also tried methodically introducing double-conversion again. The result was still the same as before: lots of closely-spaced spurs when tuning the LO, and general RF instability as indicated by random heterodyne whistles whooshing up and down occasionally.
I followed this procedure:
Remove the Q-multiplier and attach detector directly to top of last 3 MHz IF tank. The set is now a single-conversion superhet without regeneration. Reception is stable.
Add the 3.470 MHz oscillator, but don't connect it to anything yet. The 3.470 MHz oscillator was confirmed to be oscillating. Still, reception was stable with no noticeable new spurs or instabilities.
Add the second mixer and connect it to the LO. But, the detector is still connected to the 3 MHz IF tank. Reception is still stable with no new spurs or instabilities.
Disconnect the detector from the 3 MHz IF tank, and instead connect it to the 470 kHz IF tank. At this point, all the previously-experienced spurs and instabilities returned.
Rewire the second LO and second mixer with shorter wires, physically farther away from the first LO circuitry. No improvement was observed.
So double conversion just doesn't seem to be a simple technique for a simple receiver like this. A well-behaving regenerative IF stage seems to be the simpler, better way to improve selectivity. And now, with this latest circuit, I have finally got the regenerative stage working as I hoped it would. My testing just now was during daylight hours, but I'll try again later during nighttime hours when signal levels are much stronger. I expect no problems.
In an attempt to reduce FM broadcast station breakthrough, I removed the link winding on the ferrite rod antenna and instead connect the mixer transistor's base through a 100 nF capacitor to the top of the ferrite rod's LC tank. This seems to have slightly improved the problem, but not solved it completely. In particular, at the high end of the LO (which tunes I think around 20-25 MHz), the FM broadcast stations are quite loudly audible. I guess the 4th or 5th harmonics of the LO are mixing with the FM signals.
Next, I will try to replace the LO (currently a differential pair oscillator, which gives a distorted waveform at the tank) with a tapped-coil Hartley oscillator, which maybe will give a cleaner oscillator output with less harmonic energy. As suggested by the Penfold book I referenced before, a cleaner LO is supposed to help with this problem of VHF signal breakthrough.
While listening to these powerful FM stations, and while fiddling with the manual IF gain control, I also noticed that manually adjusting the gain (bias) on the first IF stage pulled the LO tuning. This means that AGC, if implemented on the first IF stage, will also pull the LO tuning, which is not good. But if we implement AGC on the second or third IF stages, then there is the danger that the changing IF amp bias will affect the regeneration level of the following regenerative IF stage.
Graphically, we have:
LO -> mixer -> IF1 -> IF2 -> IF3 -> IF4 -> regenerative detector -> AF amp
If we apply AGC to earlier IF stages, they might pull the LO. If we apply AGC to later IF stages, they might affect the regenerative detector's regeneration level.
I did more tests on the regenerative IF stage last night and it's really hard to tell if it's actually narrowing the bandwidth or not. If I reduce the IF gain low enough so the regenerative detector does not overload, then increase the regeneration near threshold for high gain and narrow bandwidth, the noise increase due to regeneration becomes quite noticeable and objectionable -- and I really can't tell if there is any bandwidth improvement, or not.
Once I get the FM breakthrough problem solved or at least minimized, I'll probably try double-conversion again, which was much quieter than applying regeneration.
I previously noted that strong FM broadcast signals are audible when I am tuning across shortwave frequencies.
I now think these strong FM signals can actually pull the local oscillator signal. I was listening to a strong shortwave broadcaster, with a heterodyne whistle present. I assume that heterodyne whistle was coming from a nearby interfering shortwave station. There was also some strong interference from a local FM broadcast station. I noticed that the heterodyne whistle seemed to vary randomly up and down in tandem with the strength of the audio from the interfering FM signal.
This would be consistent with the theory that the strong FM signal is somehow pulling the local oscillator off-frequency, causing the heterodyne whistle to warble.
This may also have been a problem with the double-conversion design. It is conceivable that both LOs were being pulled by strong FM broadcast signals. That might explain the apparently randomly varying heterodyne whistles that plagued me when I tried double conversion.
I also remember, in the double-conversion set, that simply touching my finger to the toroidal coil of the second IF tank (tuned to 470 kHz) led to very strong FM signals being received regardless of LO tuning. So there is such strong ambient FM broadcast signal energy that my body as an antenna can couple so much signal into the second IF tank that the receiver is swamped with those FM signals.
Reducing the strength of these interfering FM broadcast signals is the next step. I'll first try a capacitive divider on the tank to replace the link winding, as suggested below by DrM. I suppose it's also possible that some of my wiring is too long.
Regenerating the ferrite rod antenna
I think the current limiting factor in the receiver performance is the ferrite rod antenna. My IF strip has enough gain to hear everything (including mixer noise) that comes out of the mixer. And what goes into the mixer is the signal from the ferrite rod antenna.
The obvious solution is to amplify the signal from the ferrite rod antenna, but this has its limits. I think that the best possible solution is to apply regenerative amplification to the ferrite rod antenna, as I previously described here: https://www.theradioboard.org/forum/solid-state-radios/q-multiplied-ferrite-antenna-for-portable-shortwave-set. The reason that regenerating the antenna works so well is that it increases the ability of the incoming electromagnetic wave to excite large currents in the antenna. The regenerated antenna currents, in phase with the incoming signal, then re-radiate and strengthen the local EM field, distorting it, and causing the regenerated antenna to interact with a larger portion of the EM field than it would in the un-regenerated state. This article by Polyakov briefly mentions this effect, and contains references to related analyses: https://www.theradioboard.org/forum/solid-state-radios/2010-article-on-regens-by-the-famous-polyakov-ra3aae .
The disadvantage of introducing regeneration into the ferrite rod antenna is that the receiver will be much more touchy to use. Adjusting tuning of the antenna by more than say 10 kHz will require re-adjusting the antenna's regeneration level. Also, due to the narrow regenerated front-end bandwidth, it will probably be nearly impossible to implement accurate tracking between the RF and LO sections. Therefore, tuning will require slow, painstaking adjustment of three controls: Tuning the LO slightly to find a new signal. Then tuning the antenna to peak the new signal. Finally, adjusting regeneration of the antenna to be near critical threshold.
The advantage of this scheme will be to extract maximum performance from the receiver, because, as stated above, the small ferrite rod antenna is currently the bottleneck in receiver performance. Regeneration will increase the volume of the ferrite rod antenna's near field, enabling it to capture more signals.
Spurious FM broadcast signals and LO purity
In the first post of this thread, I originally chose the cross-coupled oscillator for the LO because it provided a stable output level, which in turn provided relatively constant loading on the regenerative IF stage connected directly to the mixer, allowing a fixed regeneration level.
However, this fixed LO output level comes at the price of distortion of the LO signal at the tank, due to clipping by the transistors.
I'm starting to think that this LO distortion is causing an excess of energy at LO harmonics, which is then mixing with strong local FM signals, frequently causing these strong FM broadcast signals to be audible when I am tuning across shortwave frequencies. This problem is somewhat present in the single-conversion design, and seemed much worse in the double-conversion design (which used two cross-coupled oscillators, so that both LO signals probably had significant harmonic content).
P. 14 and 15 of this reference on shortwave receiver construction (https://worldradiohistory.com/UK/Bernards-And-Babani/Babani/276-Penfold-Short-wave-superhet-receiver-construction.pdf) mentions that the recommended solution to this problem is a cleaner LO, instead of front-end filtering.
Therefore, I may change my LO to a more traditional LO like a Hartley (tapped coil) or Colpitts oscillator. But these will have a non-constant output level as they are tuned, with higher amplitude at higher frequencies. This would normally place variable loading on the regenerative IF stage, but now that I have 4 IF amplifiers ahead of the regenerative IF stage, they should be able to isolate the regenerative stage from the variable loading caused by LO amplitude variations. Also, with the high gain provided by 4 IF amplifiers, the regenerative stage is run far below critical threshold, so that even if there is some small amount of variable loading on the regenerative stage, its effect will not be enough to significantly alter the gain of the regenerative stage.
(Note: The amplitude of the LO can, with effort, be made both constant-amplitude and non-distorted, using hybrid feedback methods as described in https://www.kearman.com/vladn/hybrid_feedback.pdf. I've done that before, but I don't want to go through that trouble for this simple receiver. It relies on carefully balancing two feedback paths, one inductive and one capacitive, and requires a lot of fiddling with tickler turns and exact values of feedback capacitance.)
Double conversion: No go
Did a couple of experiments with double conversion over the weekend. Results are mixed. The conclusions are that it delivers much cleaner audio than the regenerative IF, but is too unstable and has lots of spurs.
Circuit details were as follows. The first IF was at 3 MHz and second IF was at 470 kHz. Second IF tank was made with 13 turns on an FT50-61 toroid, resonated with 10 nF. The second LO was set to 3470 kHz so that the 3 MHz first IF signals get translated down to the second IF of 470 kHz. A 2-transistor differential pair oscillator was used for the second LO (LC tank was about 20 turns on a T50-6 toroid resonated with 1000pF). Emitter resistance for the LO was 470 ohms. The mixer was a common-emitter amplifier with its collector load being the second IF tank, going to Vcc. Base bias came a from 100k resistor onto the collector. Emitter connected through 1k ohms to ground. LO was injected into the emitter through a grounded 1-turn link winding through the LO coil, connected to 10 nF, connected to the emitter. (Grounding the emitter at DC through the link winding loaded the LO too heavily and stopped oscillation; hence, the requirement to use the current-limiting 1k resistor on the mixer's emitter.) The mixer's RF input signal was taken from the top of the last IF tank in the first 3 MHz IF strip, and was connected to the second mixer's base through a 100 nF capacitor.
Results: When it worked and was stable, the audio was much clearer than when using a regenerative IF stage. Surprisingly, even when strong signals, the set almost never seemed to overload, and the audio remained clean. Also, weak signals were clearly audible. There almost seemed to be some kind of inherent AGC action.
Unfortunately, very frequently, the audio would cut out completely. I think this is due either to the 3 MHz IF stages oscillating, or due to the 3470 kHz second LO stage dropping out of oscillation, or due to the 3470 kHz second LO stage exhibiting spurious oscillation.
Even when the receiver was apparently stable and was tuned in to a clear, strong signal, yielding clear and strong audio, there were still audible, random heterodyne warbles moving up and down in frequency, indicating some kind of RF instability. Moving my hand near the second LO seemed to shift the frequency of these warbles.
Even when the receiver was apparently stable, tuning across the band yielded several heterodyne whistles and/or whooshing/hissing/popping sounds indicative of spurious signals, spaced very closely at maybe 50 kHz intervals, or even more closely. These are likely caused by unwanted mixing among the harmonics of the two LO stages, incoming RF signals, and/or strong local FM signals that blast their way into the receiver wiring. I never observed these kinds of extremely closely-spaced spurious signals when using the regenerative IF stage in a single-conversion configuration.
Probably, to get this double-conversion configuration working stably, mechanical shielding (like large metal plates and metal casing) would be needed to isolate the second LO from the rest of the circuit. For me, that represents too much work for this kind of a simple receiver.
So, with great regret, I gave up on the double conversion approach.
Back to regeneration
I then tore out the second LO and second mixer, and then added back (once again) the two-transistor differential pair regenerative amplifier/detector onto the LC tank of the last 3 MHz IF amplifier. In its current configuration, the receiver has 4 IF stages (and 3 IF tanks) preceding the final IF tank, with the final 4th IF tank connected simultaneously to both the last non-regenerative IF amp and the just-added regenerative amplifier.
As expected, the received audio -- compared with the double conversion receiver -- had noticeably more static and distortion. Maybe a different kind of regenerative detector could be lower noise -- this particular differential pair amplifier has a reputation for being noisy in this kind of a circuit.
With 4 IF amps (and 3 IF tanks) before the regenerative stage, the regenerative stage must be run extremely far below threshold to avoid complete overload. This has the advantage that threshold noise is avoided, and threshold motorboating problems (fringe howl) also seem to be avoided -- I observed no unwanted AF oscillation. Although running far below threshold, still the regenerative selectivity seems to be slightly better than without regeneration.
With the regenerative stage run extremely far below threshold, and with the 4 IF amps (and 3 IF tanks) preceding the regenerative stage, it was now possible to manually adjust the gain (via a potentiometer controlling base bias) on the first IF stage, which yielded the expected smooth decrease in input signal levels into the regenerative stage. This gain adjustment can allow implementing AGC. Previously, with only 2 IF amps (and 1 IF tank) before the regenerative stage, I reported that manual gain control was not possible because IF gain control undesirably affected the regeneration level too drastically. Now that we have more gain, more isolation, and are running with greatly reduced regeneration, manual gain control and automatic gain control should be possible, in order to prevent overload of the regenerative IF stage.
To test for sufficient IF gain, as a test the mixer was connected and disconnected from the first IF amplifier. The regeneration control was set far below threshold (more than 1 turn on the ten-turn pot), to the level used for receiving strong signals. With no signal tuned in, disconnecting the mixer resulted in a drop in the static level; connecting the mixer resulted in an increase in the static level. This is the mixer noise. This test proves that the regenerative stage, preceded by 4 IF amps and run far below threshold, is definitely able to hear the mixer noise. (A similar test was done with the double-conversion receiver configuration -- with 4 non-regenerative IF amps followed by a second mixer+second LO -- and the double-conversion receiver could also clearly hear the mixer noise.)
Because the regenerative detector can hear the mixer noise, even when the regenerative detector is in a relatively stable condition that is run far below critical threshold, the regenerative detector can detect weak signals, right down to the mixer noise. This is thanks to the gain of the 4 IF amplifiers ahead of the regenerative detector. It is not necessary to run the detector in weakly oscillating mode to hunt for weak signals. It is also not possible to run the detector in weakly oscillating mode, as noted previously, because, at the moment, this overloads the AF stages with noise when the regen is near or above critical threshold.
Regeneration: one last chance
I have almost given up on regeneration in this receiver, but there is one last hope. Now that I have again restored the set to use 4 non-regenerative IF stages, there's enough IF gain to drive a detector transistor with no need for regeneration.
But, maybe we can still use regeneration to narrow the bandwidth.
One way would be to use regeneration at the end of the IF chain, maybe by applying regeneration to the last IF stage (regenerating the existing tank on the last IF stage), or by adding a new regenerative IF stage (with its own separate tank). With so much IF gain before the regenerative detector, we should be able to run the regenerative stage far below threshold and still get good detection efficiency and high levels of AF output. This far-below-threshold regeneration setting should correspondingly generate almost no threshold noise because we are not near threshold. So weak signal reception should be possible, thanks to massive IF gain before the detector and the avoidance of threshold noise. But, the regenerative stage will be in constant danger of overloading from too-strong signals, so we would need to apply AGC. I previously said that AGC didn't work, because the gain-controlled stage adversely affected the regeneration level, when I tried using 2 IF amplifiers plus the regenerative stage. But now I would have 3 IF amplifiers preceding the regenerative stage, or even 4 if I use a separate tank for the regen. The hope is that with 3 or even 4 IF amplifiers before the regenerative stage, an adjustment of the gain on the first IF amplifier should be sufficiently isolated from the regenerative stage so as to avoid affecting the regeneration level. If not, I can still add yet another IF stage in between, as additional isolation.
The other way of using regeneration would be to place it at the beginning of the IF chain. Here, the regenerative stage would act only as a Q-multiplier -- again, operated far below threshold so as to avoid threshold noise, but with enough regeneration so as to improve the selectivity somewhat. The Q-multiplied RF output -- still weak, but with a sharper selectivity peak -- would then get massively amplified by 3 or 4 non-regenerative IF stages, then get fed into a separate detector transistor to generate the AF. In this arrangement, to prevent overload of the regenerative stage, AGC would have to control the regeneration level, which would also have the side-effect of widening the bandwidth on strong signals as the regenerative gain is lowered.
Both of these attempts to re-introduce regeneration into the receiver face one fundamental obstacle: instability. Currently, in its non-regenerative form, the Vcc decoupling resistor must be about 100 ohms -- less, and motorboating occurs; more, and IF gain drastically drops. However, previous experiments with the regenerative stage indicated that the Vcc decoupling resistor needed to be 200 ohms -- less, and motorboating occurred when the regenerative stage was near threshold.
Limitations of the regenerative IF stage
The last iteration of the regenerative superhet circuit (Part 8, posted previously) works okay. Not great, but okay.
The good thing is that thanks to the regeneration, adjacent channel selectivity is good. The bad things are that, compared to a non-regenerative version of the circuit with more IF stages:
The dynamic range seems limited.
Sensitivity seems lower. Strong signals are audible, but weak signals are not (see also next point).
The regenerative stage cannot be used in weakly-oscillating mode for hunting for weak signals because the AF output in oscillating mode (including the threshold noise) is so high that it overloads the high-gain AF stages. The high-gain AF stages are optimised for the low AF output when the regenerative stage is in non-oscillating, below-threshold mode.
I attempted to add AGC to solve #3, with the hope of being able to run the set in weakly-oscillating mode, without overloading the AF stages, in order to search for and receive weak signals. Unfortunately, this attempt failed.
Starting from the circuit in Part 8, I first tried varying IF gain manually by adding a 500k variable resistance from Q4 base to ground and adjusting it to see what happened. This could adjust the IF gain, but had an insurmountable, show-stopping obstacle: as the Q4 base was brought closer to ground potential, thus reducing the IF gain, the regeneration level undesirably also increased. This is because the reduced current flow through the IF amp Q4 places less loading on the IF tank, thus reducing the tank losses and causing the regenerative stage to slip above the oscillation threshold the more that the IF gain is reduced. This is completely unacceptable. Normally the set is to be operated with the regenerative stage just below critical threshold. If a real AGC loop, based either on IF signal or on AF signal, were implemented, then it would fail, because an attempt to reduce the signal by reducing the IF gain would result in the set suddenly starting to oscillate, generating more AF output (and IF output), which would cause the feedback loop to further try to reduce the IF gain (Q4 bias), further pushing the regenerative stage harder into oscillation.
I tried to fix this by adding a second tuned IF stage before Q4, and adjusting the bias on that additional stage. The hope was that the variable current in the preceding IF stage (which has its own LC tank) would not affect the loading on the following regenerated IF tank. Unfortunately, this hope was dashed -- the same problem occurred. Reducing bias on the new, preceding IF stage still resulted in the following regenerative IF stage slipping over the oscillation threshold.
I also tried varying the bias on the mixer stage, again by adding a 500k variable resistance from the mixer's base to ground. Adjusting the variable resistance didn't seem to affect the regeneration level, but I also was unable to receive any signals, presumably due to the change in mixer bias caused by presence of the 500k variable resistance (which, even at max resistance, will lower the base current compared to when the variable resistance is omitted from the circuit). I didn't pursue this further.
Another problem was that adjusting the bias on the IF stage caused the occasional return of the dreaded motorboating problem, when the regenerative stage was right at critical threshold and a strong signal was tuned exactly in the center of the passband.
With much effort, an AGC could probably be implemented (e.g. adding an RF stage, and applying AGC there instead of to the IF stages), but there's still no guarantee that the AGC would work well enough to achieve the goals of increasing the dynamic range and allowing stable and sensitive operation both above and below the oscillation thresholds.
I'm pretty much finished messing with the regenerative IF stage.
For a simple regenerative superhet, the circuit of Part 8 works okay. But it's still a regenerative stage, right on the critical threshold, and overly-sensitive to any small dynamic changes in the circuit.
I'm in the process now of taking out the regenerative IF stage and adding back in two more tuned IF stages, so that the circuit is in its original configuration -- four non-regenerative IF stages. That will be less selective than the current circuit, but overall, if memory serves, it will be more stable and have better dynamic range.
To improve the selectivity, I'll probably investigate a double-conversion approach with a second IF at 455 kHz. Or, I might just convert all my IF cans to be 455 kHz cans instead, and live with the reduced image rejection.
Receiver working without regeneration
It's done. The regenerative stage has been ripped out. During the failed attempts to investigate AGC, I had already added 2 non-regenerative IF stages, so all that remained to do was to add back in 2 more IF stages for a total of 4. I also changed the collector resistor on the AF preamp Q13 from 4.7k to 10k for increased non-linearity, so the preamp serves as a detector, connected to the top of the last IF tank. RC decoupling network's resistor R18 in the Vcc line was reduced from 200 ohms to 100 ohms, for a very significant increase in overall IF gain. (However, reducing R18 to 0 ohms led to motorboating.)
Works great, just as I remembered. Everything acts much more predictably and the audio is slightly clearer than with the regenerative stage present. Selectivity is noticeably reduced compared to the regenerative configuration, but is still acceptable for separating close shortwave broadcasters (although the main signal of interest, centered in the passband, is audible, nearby strong signals "bleed through" and are also audible, due to the wider IF bandwidth). Manually adjusting the bias on the first IF stage very controllably reduces the gain. No signs of motorboating or instability at all.
I'll post updated schematics soon.
I found an interesting tidbit of information about European legal limits to headphone levels. According to a comment at https://www.edn.com/turn-a-smart-phone-into-a-signal-generator/ :
I found a bit more information by an audio enthusiast: https://addictedtoaudio.co.nz/blogs/how-things-work/why-some-digital-audio-players-just-aren-t-loud-enough , which says:
A technical document is here: https://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.174.1466&rep=rep1&type=pdf , which says:
I still can't tell if this 150 mV is a peak value or an RMS value, but I guess they're probably talking about RMS values when using the specified input test signal with the specified power spectral density.
An LTspice simulation shows that my current amp configuration, when heavily overloaded with a 1 mV input signal, generates a square wave of about 515 mV peak-peak amplitude, which equals 257.5 mV peak amplitude, which equals about 182 mV RMS. So blasting one's ears continuously with overloaded noise from the amplifier is probably not a good idea.
But for pleasant listening at undistorted levels, the amplitude must be reduced anyway to below about 100 mV peak, which should be a safe volume at which to listen. This assumes that there isn't any ultrasonic oscillation happening in the amplifier which might generate inaudible but damaging levels of sound.
As for normal levels of listening, the previously-linked audio enthusiast at https://addictedtoaudio.co.nz/blogs/how-things-work/why-some-digital-audio-players-just-aren-t-loud-enough notes that:
Interesting. My 1.2-volt amp can generate 27 mV RMS (38 mV peak, 76 mV peak-peak) into a 32-ohm loa