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Design >> RF Design >> question on how to bias the lpf after mixer
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Message started by James Bond on Jul 16^th, 2007, 4:18pm

Title: question on how to bias the lpf after mixer
Post by James Bond on Jul 16^th, 2007, 4:18pm

Hi, all,

Can someone give me a suggestion on how to bias this low pass filter which does not affect mixer?

The left block is passive mixer which is very sensitive to the load I think , the right block is active low pass filter.

Title: Re: question on how to bias the lpf after mixer
Post by aaron_do on Jul 16^th, 2007, 6:51pm

I take it you don't want to use resistors...

Title: Re: question on how to bias the lpf after mixer
Post by James Bond on Jul 17^th, 2007, 12:09pm

Hi, aaron_do,

You are quite right, and you know exactly what I am talking about. That's true, I can use two resistor to connect between + and - input, with the ground connecting to the middle of the resistors, but this will load the mixer, usually , we want capacitive load instead of resistive load. Could you suggest me any ideas ? Any comments would be appreciated. Thank you very much.

James

Title: Re: question on how to bias the lpf after mixer
Post by aaron_do on Jul 17^th, 2007, 6:30pm

Hi James,

If you don't want to use resistors and you don't want to load down the mixer, then your only other option is to make the passive mixer provide the DC level to the filter and remove those coupling caps. If you put this biasing on the LNA side of the passive mixer, then the biasing will load down the LNA, but you can get away with using a much smaller resistor.

Aaron