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.
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?