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Design >> RF Design >> current Passive mixer
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Message started by Muhammed on Mar 6^th, 2012, 5:23pm

Title: current Passive mixer
Post by Muhammed on Mar 6^th, 2012, 5:23pm

Hello .. i have a question about current passive mixing

there are many topologies which make LO drives the gate of the MOS and other make LO drives the Drain of MOS

Is there any difference ?????

Title: Re: current Passive mixer
Post by RFICDUDE on Mar 6^th, 2012, 7:29pm

I don't recall a practical CMOS passive mixer with LO drive on the drain or source.
Do you have an example or two you can reference?

Title: Re: current Passive mixer
Post by aaron_do on Mar 6^th, 2012, 8:06pm

Hi,

usually the LO drives the gate as far as I know. One obvious difference is the loading conditions at the gate and the drain/source.

The operating regions also sound like they would be different. You should check the effect on linearity, gain, harmonics etc through simulation.

regards,
Aaron

Title: Re: current Passive mixer
Post by Muhammed on Mar 7^th, 2012, 5:01am

so the difference between them is loading condition ..

i read that ... usually in RX architectures they make LO drive the gate but i don't know why ???

by the way ... do you knew any tutorial about simulation direct conversion mixer in cadence ??

thanks for your reply :)

Title: Re: current Passive mixer
Post by Muhammed on Mar 7^th, 2012, 5:04am

RFICDUDE wrote on Mar 6^th, 2012, 7:29pm:

I don't recall a practical CMOS passive mixer with LO drive on the drain or source.
Do you have an example or two you can reference?

Look at this two photos

Title: Re: current Passive mixer
Post by Muhammed on Mar 7^th, 2012, 5:06am

RFICDUDE wrote on Mar 6^th, 2012, 7:29pm:

I don't recall a practical CMOS passive mixer with LO drive on the drain or source.
Do you have an example or two you can reference?

and this photo

here the LO drives the drain not the gate

Title: Re: current Passive mixer
Post by aaron_do on Mar 7^th, 2012, 6:50am

Hi,

I think the second one is not typical. With the first, you can design a current-mode passive mixer which is very commonly used for its high linearity and robustness to variation.

For the second design, I'm not sure about the operating region. It seems like when the LO is high, M1 and M2 are in the saturation region with the LO connected to the drain. When the LO is low, the devices are still in the saturation region, but now the LO is connected to the source. You could try and simulate this to be sure, but anyway in this case, the linearity may not be good.

Anyway I'm not so sure, so you should run some simulations to find out.

cheers,
Aaron

Title: Re: current Passive mixer
Post by RFICDUDE on Mar 7^th, 2012, 5:44pm

It is difficult to see how the second circuit would work well as a passive mixer.

It might work as an active mixer if the LO signal provides enough positive voltage to pull the MOSFET pair into saturation. Then the ON devices would have some transconductance gain.

As a passive mixer the modulated ON resistance of the MOSFETs would be small because of the small RF signal on the gates, but maybe this works out somehow?