any one can give me some advice on active mixer design?

I don't know what do you mean by "as a topic", but I can just give you some hints from the literature. Mixers with active elements can be devided in two, transconductance (bias as active device, has gain) and resistive ones (basically working as a diode mixers but have better linearity). I understand that you want to build a transconductance one. For that solution you usally connect LO and RF on the gate of transistor and extract IF from the drain

The story in short will be:

Bias gate near pinch off, have enough LO power to swing the gate voltage near 0V (but not to drive gate in direct regime)

Short cicruit the drain on all LO harmonics (F output filter)

Short circuit gate on IF frequencies (usually a shunt stub)

There is an ultimate guide for designing mixers in two books by Steve Maas available in electronic form on the web from MCU - The RF and Microwave Circuit Design Cookbook and Microwave Mixers.

flyhigh

thanks for advice
but where can I get these two book? where can I downlaod?

They still are at mcu.cz/atm/userfiles/eirp/RFmicrowave or very similar. You will have to registr first. There is very fine stuff laying there for free!

Flyhigh

to flyhigh:

could you tell me why "have enough LO power to swing the gate voltage near 0V"?

Thanks!

There is a system level thing to keep in mind with mixers. They are more distortion producing than amplifiers and you should block out your system so that they have the minimum signal level that allows your system to function. Usually, you put a bandpass filter in front of it and have only enough RF amplifier gain to set the system noise figure.

Active mixers are much worse than passive mixers (diode ring) and should be avoided whenever possible.

Active mixers like Gilbert cell have better gain and could suppress the LO harmonic on ouput. why should be "avoided whenever possible"?

Active mixers have much worse IMD performance than diode ring mixers. They also frequently have much worse suppression of feed through of LO and RF signals. For a data point, diode ring mixers have 40 dB of LO and RF suppression.

High performance systems, such as used in the military, always use diode ring mixers for these reasons. It is the low performance low cost consumer products that use the active mixers.