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How IQ imbalance causes Image frequency?

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yuvalkesi

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Hi,
Well, the title speaks for itself... (I guess it's pretty basic question, but help me, please)
I read in a few places that the IQ imbalance causes the Image frequency. How come?
I know that the IQ imbalance is created due to non-similar gain and phase in I and Q signals. What causes the image in this context?
Thx,
Tom
 

The outputs of the two mixers part of the IQ modulator are combined at a summing node.
– The two lower sideband terms of the mixers add in-phase and produce the desired low-side output.
– The two upper sideband terms of the mixers are out-of-phase and cancel.

If I and Q inputs are swapped, or the polarity of either I or Q is reversed, the operation is the same, except that the lower sideband terms cancel, and the upper sideband terms add, producing the desired high-side output

In an ideal IQ modulator the image output completely cancels if IQ inputs are perfectly balanced and the LO signals fed to the mixers are in perfect quadrature.

IQ_Imbalance.swf
 

Hi,
Thanks. Based on your explanation I found also this link:
http://www.ti.com/lit/an/slwa059/slwa059.pdf

But what I still don't understand is why when the two upper sideband terms are not precise out-of-phase and thus - do not cancel, where is the image frequency (the value of LO-IF) coming from?


Ok, I found this link. Explains everything!
**broken link removed**
 
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