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Quadrature mixer: want to keep frf+flo, How?

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dd2001

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As I know Quadrature mixer only keep positive frequecy, e.g., if frf>flo, the result is fif=frf-flo, and the other portion image, flo+fif, will be depressed, that is called image injection, however, if I want to keep frequecy, frf+flo, and depress flo+if, is it can be implimentated?


looking for guru's help!
 

Is the flo-frf image frequecy?Is it not the 2flo-frf?
 

Sorry, I just make some correct on my first post.

I just want to say on Quadrature mixer, how to keep frf-flo=fif? and how to keep frf+flo=fif?

Both cases, how to apply I, Q to mixer inputs?
 

I think it's dependent on how you add the I-Q path after mixing RF and LO signals.

1. Vrfi*Vloi + Vrfq*Vloq will keep the frf+flo part
2. Vrfi*Vloi - Vrfq*Vloq will keep the frf-flo part
 

If the RF input is a real signal then using a complex mixer with I and Q you get a complex IF output with both frequency ranges. Then you could apply a image rejection filter. It is called polyphase filter. It could reject the unwanted image to some degree.

If you have a complex RF and a complex LO you mix down to IF a true single sideband. Both mixer outputs have to be added together.
 

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