passive mixer feedthrough & input impedance.

[victoriya]

I have designed a front-end for 2.4g zigbee applications.i have some problems with my mixer. my mixer is passive(double balanced with single rf input) i have ac grounded the other port of rf.it has too much feedthrough between all of it's ports specially lo to rf and when i connect it to the lna it desensitized the lna wholly the rf input corrupts completely and the signal level drops drastically. of course i know the latter problem stem from the low input impedance of my mixer.
i use some techniques to decrease the feedthrough at rf input. i put an inductor between lo inputs(lo+ and lo-) and remove the lo buffers but it resolves the problem only for low input sinal levels partly and the inductor value must be very big.and i use lc tank at the output ports but it doesn't work properly. and i have large lo in both of rf and if ports.
also i don't know how to increase the input impedance of passive mixer.
thank you.

 

[davenn]

what is the input impedance of your mixer ? what is the output impedance of the previous stage ?
Should normally be around 50 Ohms for most systems

A couple of suggestions. If the input level is too high, either
a) back off the front end gain or
b) place a "T" or "Pi" attenuator pad between the front end amplifier and the mixer input

cheers
Dave

 

[victoriya]

thak you for your reply.
lna output impedance is 490 ohm and it's gain is 25.5db mixer input impedance shows 340 ohm by tran analysis but i think it's wrong i think it must be greater than this. my swiches on resistance is 634 ohm. i don't want to attenuate my lna gain because our system require such as large gain for lna.our system's front end gain should be 21db.if my mixer loss be -4(but now it is -2.5) the lna gain will be suit for this case.i think it isn't neccassary for lna ouput and mixer input impedance to be matched to 50 ohm where we don't have any off chip filter such as image reject filter between them.but the mixer input impedance must be greater than 980 ohm because we have two mixers(quadrature).i have wonder my mixer doesn't work properly but it's characteristics such as cg nf and ip3 are good!! do you know how to simulate the mixer impedance by pss+pac?