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Design >> RF Design >> minimum NF of Low-IF receiver
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Message started by aaron_do on Jul 1^st, 2008, 12:44am

Title: minimum NF of Low-IF receiver
Post by aaron_do on Jul 1^st, 2008, 12:44am

Hi all,

if i use an image-reject architecture, in theory i can completely cancel the image noise that appears in-band due to noise folding in a mixer.

However, in some receivers i've seen, image rejection is done in the digital domain. Just intuitively, since the signal is already digitized, the bit errors due to the noise in the signal are already there, so in theory the image noise cannot be canceled using digital domain image rejection.

Is that right?

thanks,
Aaron

Title: Re: minimum NF of Low-IF receiver
Post by pancho_hideboo on Jul 1^st, 2008, 1:39am

aaron_do wrote on Jul 1^st, 2008, 12:44am:

if i use an image-reject architecture, in theory i can completely cancel the image noise that appears in-band due to noise folding in a mixer.

No. you are misunderstanding. Even though you apply image rejection mixer such as combination of I/Q down mixer and complex BPF, holding noise from mixer itself can not be canceled.

aaron_do wrote on Jul 1^st, 2008, 12:44am:

However, in some receivers i've seen, image rejection is done in the digital domain. Just intuitively, since the signal is already digitized, the bit errors due to the noise in the signal are already there,

I can't understand why bit errors already exists there.
If quantized noise is enough small, there is no degradation in baseband BER.

aaron_do wrote on Jul 1^st, 2008, 12:44am:

so in theory the image noise cannot be canceled using digital domain image rejection. Is that right?

No, it is not correct.
Assume an architecture where I/Q-ADC after I/Q down mixer and digital complex BPF or complex Bandpass-Delta-Sigma-ADC. you can realize image rejection architecture.

Title: Re: minimum NF of Low-IF receiver
Post by aaron_do on Jul 1^st, 2008, 1:52am

Hi pancho_hideboo,

thanks for the reply. Actually I have no experience at all with ADC design so maybe that's where my misunderstanding is coming from.

Anyway what do you mean by holding noise? Is that the intrinsic noise of the mixer? So the noise at the image frequency which is present before the mixer can be cancelled by the image reject mixer right?

thanks,
Aaron

Title: Re: minimum NF of Low-IF receiver
Post by pancho_hideboo on Jul 1^st, 2008, 1:55am

aaron_do wrote on Jul 1^st, 2008, 1:52am:

Anyway what do you mean by holding noise? Is that the intrinsic noise of the mixer?

Right.

aaron_do wrote on Jul 1^st, 2008, 1:52am:

So the noise at the image frequency which is present before the mixer can be cancelled by the image reject mixer right?

Right.