# About the combination of two signals with capacitors

Here is a link of an article I've been reading. http://www.analoguehaven.com/moog/thereminblack/hotrod.pdf

In page 4 there is a circuit schematic. Notice there is a part called variable pitch oscillator and a part called fixed pitch oscillator. At the output of each of the two oscillators mentioned there is a capacitor. At page 6, the article states that:

C2 and C6 combine the pitch oscillator signals, while D4, R23, R24, and C23 extract the difference frequency. C2 and C6 also provide weak coupling between the two pitch oscillators, which has the effect of synchronizing the pitch oscillators when their frequencies get very close together. This has the desirable effect of providing a stable 'zero beat', so that the instrument, once properly tuned, is silent when the player steps away from it.

Question:

What I would like to understand is how the two output signals of the two oscillators are combined with the use of the capacitors C2 and C6.

Is it correct to say that the two capacitors work as modulator? I'm saying this because the produced signal as you can see goes through a detector. Could you explain what exactly happens at the capacitors or how they combine the signal?

Thank you.

## 1 Answer

The capacitors just act as relatively high impedances to sum the two oscillator signals at D4.

D4 then provides the non-linearity to cause mixing (multiplication) of the two signals and create sum and difference signals in the frequency domain.

In this case the two oscillators are running at approximately 250kHz. So the sum frequency will be >500kHz and the difference will be in the audio range. The audio frequency is filtered by C23 and associated resistors and then amplifier to create the audio output.

• Thanks for the reply. I would like to ask this: it's wrong to say we have modulation in this circuit, isn't? It's more of a terminology question but I think I'm a bit confused. Thanks again. – Constantine Black Jul 25 '15 at 8:43
• I wouldn't describe that as a modulator; the term modulator is normally used when the intent is to vary a signal in some way to impose information on it. A more common term in this context is to describe the diode and the capacitors feeding it as a "mixer". This is not to be confused with the audio mixer which is a summation process. – Kevin White Jul 25 '15 at 18:06