Problem in Double balanced mixer

[nikhilsigma]

Hi everyone...

I am simulating a double balanced mixer....as shown below...



I am giving 2Vrms at LO input and 1Vrms(from Capacitor voltage divider) on RF input...

And I am getting 516mV DC at output(on Oscilloscope)
but I am not able to think...why am I getting such output voltage...?? since output of the mixer should be multiplication of input signals...

Please help....

 

[mtwieg]

Your RF and LO signals are at the same frequency, so when they mix the result will have a DC offset (depending on their relative phase). So your mixer may be working properly.

 

[nikhilsigma]

yup that is ok....but How much should be the DC level(Amplitude) of the DC signal...

 

[mtwieg]

It's hard to estimate precisely from just a schematic. But 516mV doesn't sound too unreasonable. Your RF port signal (1Vrms) is probably too strong though, in general you want to keep it at least 10dB below your LO power.

A better test of the simulation would be to create a separate voltage source to drive the RF port with a small amplitude (like 0.1Vrms) and a small frequency offset (1KHz). Then you should get a sine wave on the IF port, and that should allow you to calculate your conversion gain.