1. It is a little strange. They are intended to be coarse and fine control but R2 (R3 in first schematic) shifts the bias point of the transistor which isn't such a good idea although the audio quality is probably irrelevant anyway.
2. It is basically a phase shift oscillator with a control to lower the gain below oscillation threshold. The frequency is decided by the three feedback capacitors and the resistors following them so they should be ganged so their values track each other for best 'Q'.
3. Yes, the ground is necessary but missing from the schematic. It should be at the bottom end of R2.
4. No. But the exact point of oscillation may be difficult to predict and may not be repeatable. You will have to try both types and possibly with the log ends reversed to see which gives you best control.
Brian.
Be aware that as a 'Q multiplier' at low gain it will amplifiy the audio applied to it but if the gain is set high enough for self oscillation, the audio level my jump to many times its previous volume!
It probably doesn't matter where the audio is injected in to the circuit but my preference would be directly at the base so the preceeding circuits have less effect on the tuning of one of the RC networks.
Brian.
1. Why the peaking pot and R2 pot is used instead of a single pot. Is it just for "fine tuning"?
This kind of circuit will always be influenced by the level of input signal. It isn't a true filter in the normal sense, it is an amplifier with positive feedback at a particular frequency and given enough signal it will "go over the edge" and burst into oscillation.Let me put it like this: Say I receive a weak signal. and set the regeneration just before oscillation, for max filtering effect. Then I retune the RX to another frequency, where a higher level signal is. Will the filter go into oscillation then (need retuning)? In other words, does it depend on the gain of the amplifier or on the signal strength?
It might work but personally I would leave it in. The final stage of phase shift is the final capacitor in the feedback loop and the resistor from the base to ground. It is already compromized by the transistor impedance and the other bias resistor, keeping the impedance feeding it as high as is practical will help to keep the 'Q' high and minimize other tuning effect from outside influences.Regarding to the second point, would it be enough to remove the 10k input resistor and connect a 100nF in series with a 10k, directly to the transistor base, to feed the signal in?
The circuit has several potentiometers, implying it may be finicky to operate. Of course that is all right since it can be instructive too. The purpose of the circuit is novel, as a transistor-based filter that selects one of two close pitches. When I was making my morse code translator, I found I needed a filter for a similar purpose. I wish I'd known about this circuit, because I ended up building a more complex one with 3 op amps (either biquad or state variable type).
The danger is of course that you need the filter to isolate a weak signal but a nearby strong one trips it into oscillation. Don't think of the gain potentiometer as a bandwidth control, it only optimizes the peaking at the set frequency. It will have an effect on the bandwidth but it will be quite small.
If you do a search on "Phase shift oscillator" you will find the identical circuit many times but without the audio input coupling. It will give you a better insight into how it works.
Brian.
Here is another one
[found at www .qsl .net /va3iul /Homebrew_RF_Circuit_Design_Ideas /Regenerative_Audio_Filter_WB6IOM .gif]
It uses 2 transistors though but it's ok, I would like to try it to see if it has better performance.
Any clues about the C1 and C2 values to use?
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