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Image-reject IF downmixing

When digitally mixing an IF down to baseband, one is left with a spectrum that consists of the baseband (Fif - Fmix = 0Hz) and an image (Fif + Fmix). If the IF is greater than the Nyquist freq, the image will wrap back into the first Nyquist zone (0 to Fn).

Normally the next step in demodulation is decimation, which consists of lowpass filtering out the image (often with CICs) and then dropping some of the resultant samples to get a lower sample rate.

Is there a method by which an image-reject mixer could mix the IF down to baseband while simultaneously cancelling the image? If so, it seems like there would be no filtering required for decimation, which would consist entirely of throwing samples out. Then it also seems that the mixer itself could run at the slower decimated rate.

Or is that all just another way of saying "undersampling"?

-Kevin

Reply to
Kevin Neilson
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Look at sinrle-sidebamd receiver design. It's all there for you.

Jerry

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Engineering is the art of making what you want from things you can get.
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Reply to
Jerry Avins

I/Q demodulation - a complex mixer using the sine and cosine to deliver the baseband without image - provides the single-sideband demodulation Jerry mentions.

Decimation is a simple way of getting "processing gain" by taking a high digital frequency IF with a lower number of bits per sample and - through decimation - increasing the effective number of bits by filtering out the system and quantization noise from the original A/D that is mixed to outside the baseband. If you don't need the processing gain, subsampling can be used but realize that this effectively aliases the IF into several "folds" such that any spurious signals or thermal or quantization noise gets added to the desired IF.

Your description of the image comes off a little peculiar in my perception suggesting you might not be getting your desired point across. If the IF is analog-filtered then subsampled, this aliased pseudo-IF can be I/Q mixed to provide the baseband; it can even *be* the baseband depending on the sampling rate. If the IF is analog-filtered then sampled by at least twice the IF, I/Q demodulation is still required to isolate the baseband from the

*mixed* image of Fif+Fmix. The topology you're thinking about isn't clear.

Are your questions answered by the interpretation I took?

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Reply to
John_H

I'd forgotten about the decimation processing gain--that's a good reason to stay with a high sample rate and conventional decimation process.

-Kevin

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Reply to
Kevin Neilson

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--Ray Andraka, P.E. President, the Andraka Consulting Group, Inc.

401/884-7930 Fax 401/884-7950 email snipped-for-privacy@andraka.com
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"They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759

Reply to
Ray Andraka

Ray, I've seen that idea presented before but I don't think I can combine mixing/filtering in my case because I'm using the IF mixer to do carrier synchronization so I need to be able to precisely control the LO with an NCO.

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Reply to
Kevin Neilson

--

--Ray Andraka, P.E. President, the Andraka Consulting Group, Inc.

401/884-7930 Fax 401/884-7950 email snipped-for-privacy@andraka.com
formatting link

"They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759

Reply to
Ray Andraka

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