Passive mixer design

[Jason Chang]

Hello
I have a question about how to decide the size of the switching transistor and the capacitor at the load when I design a sampling mixer.
thank you~

 

[BradtheRad]

Is this like a sample-and-hold circuit? If so...
1) You expose a capacitor to an incoming volt level briefly,
2) disconnect it for a length of time (idle period),
3) then send its volt level to a readout circuit.

Selecting its value is a tradeoff between small and large Farad value.
Small value quickly charges/discharges through the incoming resistance Whatever is the incoming resistance, you must adapt it to the length of window time available to expose the capacitor.

Farad value must be large enough to hold its voltage during idle period. Readout should be taken through a high ohm value.
After taking a reading you may find it necessary to discharge it rapidly and completely, through a low ohm resistance.