zero IF, image and iq imbalance

[yuvalkesi]

Hi,
This question is regarding an article I recently read about zero IF receivers.
In zero IF receivers there's no image frequency, right? because it has a zero IF frequency. On the other hand, it does have an image frequency, due to IQ imbalance.
Well, I'm confused....

Tom

 

[mtwieg]

Could you link to the article? IQ imbalance is an issue with zero IF receivers (like it is with any receiver with IQ demodulation, I think), but I don't see what that has to do with it having an image frequency. Are you sure it wasn't talking about DC artifacts? Because that could be interpreted as an image frequency (at f=0).

 

[yuvalkesi]

Ok, actually it was few articles, apparently I missunderstood something...
But, still, the cause of an image frequency is IQ imbalance (that's what I know), but in zero IF there's no image frequency but there's IQ imbalance.
So, I'm confused.

 

[BigBoss]

Explanation is hidden in methematical expressions...
Do some math with complex variables and see the result..

 

[yuvalkesi]

Ok, thanks.
Still, if someone could explain the logic, rather than mathematics it would be great...

 

[vfone]

Do not confuse Sideband Suppression with Image Rejection.
Direct Conversion Receivers doesn't have an image frequency.
In an IQ receiver, ideally the I and Q channels of the radio signal carry orthogonal, non-interfering, channels of information.
Mismatch (imbalance) in the Gain or Phase of the I and Q channels results in interference between the channels, which makes it more difficult to recover the information they contain.

 

[yuvalkesi]

Do not confuse Sideband Suppression with Image Rejection.
Direct Conversion Receivers doesn't have an image frequency.
In an IQ receiver, ideally the I and Q channels of the radio signal carry orthogonal, non-interfering, channels of information.
Mismatch (imbalance) in the Gain or Phase of the I and Q channels results in interference between the channels, which makes it more difficult to recover the information they contain.

Mismatch (imbalance) in the Gain or Phase of the I and Q channels results in interference between the channels - isn't that the cause of image frequency? In few articles I read this is the cause...

 

[Mazz]

I won't go for a math explanation, but just to get you some feeeling about it.

Try to imagine the ZIF as it is called or defined, as the IF frequency is zero. The signal is double sideband, so half of the spectrum is in the negative frequency axis both for I & Q. If there is (as it happens in real systems) some I/Q impairment, it acts as part of the positive sideband is flipped into the negative sideband and viceversa after I/Q recombination; there is a SNR degradation due to this mismatch.

After all there is something similar in non-ZIF systems (for example LOW IF or Weaver architecure), except for the fact that the image frequency is not part of signal bandwidth but comes from different part of the spectrum and noise or interferer coming from the image frequency are rejected ideally by I/Q quadrature system.

Nice to read at: **broken link removed**

I hope it can help.

Mazz

 

[yuvalkesi]

Thank you all for your explanations!
I would definately read the pdf, Mazz.
Much appreciate the help...!
Tom.

 

[yuvalkesi]

Btw, does IQ imbalance issues occur at transmitter and receiver, or just at one of them?

 

[Mazz]

We are referring to the receiver issues, but, as in real implementation there is always some imbalance, also in TX there could be some issues due to I/Q imbalance, provided that the transmitter has I/Q path, of course.

The main drawback in transmitter is degradation of EVM (error vector magnitude) due to I/Q in direct conversion transmitters (this architecture is widely used today).

Mazz