Image-reject IF downmixing

K

Kevin Neilson

Guest
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
 
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
Click to expand...
Look 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.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ
 
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 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
Click to expand...
 
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 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 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
Click to expand...
 
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
Click to expand...
--
--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
 
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 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
Click to expand...
 
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
Click to expand...
--
--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
 
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