Five questions about RF mixers

dear all

i have some questions about RF mixer.

1. We can find the conversion gain for mixer by using
a. voltage gain
b. power gain
c. S21 parameter ( forward gain versus frequency)

Actually what is the differrence from these three gain?

2. To find linearity of the mixer, normally we calculate the IIP3. We will set two-tone frequeny, f1 and f2 for RF input and see its IIP3. My questions is what is the criteria to set the frequency spacing for f1 and f2? I see some set in khz, or Mhz.. Is freqeuncy spacing will effect the accuracy of the measurement for IIP3?

3. For down conversion mixer, is output impedance matching very critical? As i know, the output frequecny of mixer is is KHz or MHz range. For low frequency, impedance matching is not critical.

4. When we said DCR ( Direction conversion receiver), the output will convert to zero frequency. How the zero frequency work? Or is it very low freqeuncy, maybe in Hz range instead of zero frequency?

5. Is common mode feedback very important in Mixer? How about PSRR?

Thank you and looking for the discussion and your idea. Thanks a lot.

Voltage gain generally is used when the impedances are ignored. Power gain and S21 needs to know and to specify the ports impedances.
Frequency spacing of the two-tone signals is generally given by the channel spacing of the communication system.
Impedance matching is important at any mixer ports. The output port impedance match of the mixer is important not only at fundamental, but also at harmonic frequencies.