Twicer: adding double and fourfold frequencies
 
Diagram
A full wave rectifier (IC1A&B) is followed by a DC remover (IC1C) to double the frequency. It works well with triangular waves because we end up with a triangle with twice the frequency. It don’t work with square wave. With sine wave, we do not obtain exactly a sine wave but it could be interesting (some kind of distortion).
IC1D is an inverter (shift of 180°) or an all pass filter with a shift around 90° depending on the frequency. To obtain one or the other, we have to select R11, R14 and C10. The switch S1 bypasses this filter.
IC2 does exactly the same and therefore produce frequency X4.
Finally, IC3 mixes the input with the wave having twice frequency and fourth frequency. That why it is called Twicer.
Oscillograms at the test points (Yellow is the Input)
Test point 1 (TP1)
TP2
TP3
TP4
Output for different settings
X2 @ 0% and X4 @ 0%
X2 @ 50% and X4 @ 0%
X2 @ 0% and X4 @ 50%
X2 @ 50% and X4 @ 50%
X2 @ 50% inverted and X4 @ 50%
X2 @ 50% and X4 @ 50% inverted
X2 @ 50% inverted and X4 @ 50% inverted
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