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
Image-reject IF downmixing
Started by ●April 19, 2004
Reply by ●April 19, 20042004-04-19
Kevin Neilson wrote:> 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"? > -KevinLook at sinrle-sidebamd receiver design. It's all there for you. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●April 19, 20042004-04-19
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? "Kevin Neilson" <kevin_neilson@removethiscomcast.net> wrote in message news:HfYgc.173403$JO3.100798@attbi_s04...> 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 someof> 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 mixeritself> could run at the slower decimated rate. > > Or is that all just another way of saying "undersampling"? > -Kevin > >
Reply by ●April 19, 20042004-04-19
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 "John_H" <johnhandwork@mail.com> wrote in message news:iaZgc.4$913.203@news-west.eli.net...> I/Q demodulation - a complex mixer using the sine and cosine to deliverthe> baseband without image - provides the single-sideband demodulation Jerry > mentions. > > Decimation is a simple way of getting "processing gain" by taking a high > digital frequency IF with a lower number of bits per sample and - through > decimation - increasing the effective number of bits by filtering out the > system and quantization noise from the original A/D that is mixed tooutside> the baseband. If you don't need the processing gain, subsampling can be > used but realize that this effectively aliases the IF into several "folds" > such that any spurious signals or thermal or quantization noise gets added > to the desired IF. > > Your description of the image comes off a little peculiar in my perception > suggesting you might not be getting your desired point across. If the IFis> analog-filtered then subsampled, this aliased pseudo-IF can be I/Q mixedto> provide the baseband; it can even *be* the baseband depending on the > sampling rate. If the IF is analog-filtered then sampled by at leasttwice> the IF, I/Q demodulation is still required to isolate the baseband fromthe> *mixed* image of Fif+Fmix. The topology you're thinking about isn'tclear.> > Are your questions answered by the interpretation I took? > > > "Kevin Neilson" <kevin_neilson@removethiscomcast.net> wrote in message > news:HfYgc.173403$JO3.100798@attbi_s04... > > When digitally mixing an IF down to baseband, one is left with aspectrum> > 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 backinto> > 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 downto> > baseband while simultaneously cancelling the image? If so, it seemslike> > > 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 by ●April 19, 20042004-04-19
You need a filter in there (unless you are not downsampling and there is no out-of-band signal), however you can play some optimization tricks with the math to roll the mixer into the filter if the LO is a multiple of your input sample rate. What you end up with is a polyphase filter that has the mixer convolved with it so that you get both mixing and filtering. If you want to run the mixer slower, that can be done by bandlimiting the signal before the mix and subsampling. Kevin Neilson wrote:> 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-- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 email ray@andraka.com http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759
Reply by ●April 20, 20042004-04-20
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. -Kevin "Ray Andraka" <ray@andraka.com> wrote in message news:408466B4.85D6AF45@andraka.com...> You need a filter in there (unless you are not downsampling and there isno> out-of-band signal), however you can play some optimization tricks withthe> math to roll the mixer into the filter if the LO is a multiple of yourinput> sample rate. What you end up with is a polyphase filter that has themixer> convolved with it so that you get both mixing and filtering. If you wantto> run the mixer slower, that can be done by bandlimiting the signal beforethe> mix and subsampling. > > Kevin Neilson wrote: > > > When digitally mixing an IF down to baseband, one is left with aspectrum> > 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 backinto> > 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 someof> > the resultant samples to get a lower sample rate. > > > > Is there a method by which an image-reject mixer could mix the IF downto> > baseband while simultaneously cancelling the image? If so, it seemslike> > there would be no filtering required for decimation, which would consist > > entirely of throwing samples out. Then it also seems that the mixeritself> > could run at the slower decimated rate. > > > > Or is that all just another way of saying "undersampling"? > > -Kevin > > -- > --Ray Andraka, P.E. > President, the Andraka Consulting Group, Inc. > 401/884-7930 Fax 401/884-7950 > email ray@andraka.com > http://www.andraka.com > > "They that give up essential liberty to obtain a little > temporary safety deserve neither liberty nor safety." > -Benjamin Franklin, 1759 > >
Reply by ●April 28, 20042004-04-28
Which is why I stated that if the LO is a multiple of your input sample rate.... It should have read if the input sample rate is an INTEGER multiple of the LO. Kevin Neilson wrote:> 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. > -Kevin-- --Ray Andraka, P.E. President, the Andraka Consulting Group, Inc. 401/884-7930 Fax 401/884-7950 email ray@andraka.com http://www.andraka.com "They that give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety." -Benjamin Franklin, 1759