# What is the purpose of this capacitor in the SA602A circuit and how do I choose its value?

I'm trying to use the SA602A mixer and I'm not sure what the capacitor between the emitter and the base does. I'm using this mixer because I've seen it being used in the Norcal40. However in the Norcal schematics it's a trim pot, which leads me to believe it's part of the LC tank?

I'm working at 7 MHz, the tank has two 30 pF caps and the inductor is 33 uH. The decoupling capacitor is 0.1 uF. How do I pick the last cap?

If I remember rightly, that cap is part of the circulating-current loop.

You may view that circuit as a PI of form CLC, and you want some stepup ratio from emitter to base.

So make the cap from emitter-base about 2x the other resonant cap.

Does this help: -

$$\C_0\$$ in the common collector Colpitts oscillator on the left has an extra varactor diode in series for auto tuning but, other than that they are identical. You might also look up the SA605 because it has more details about the oscillator that may prove useful to you.

Colpitts Oscillators are commonly referred to as having "a tank" but that is a misnomer. The Colpitts oscillator relies on phase change (like all sinewave oscillators) and that is brought about by a tuned circuit but, that tuned circuit is not at resonance so, calling it "a tank" can be misleading: -