Discussion:
SAR matched filters?
(too old to reply)
Bo
2006-05-19 20:30:02 UTC
Permalink
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.

In particular, I am interested in finding out how matched filters can be
implemented for X band SAR radars. I know that one method is through DSP of
sampled data---but in our particular case, needing 500MHz- 1GHz bandwidth, I
don't see that sampling I/Q data for those bandwidths is practical. (is
it?). We may be used coded CDMA waveforms as well--which would as I
understand it, even further widen our bandwidth requirements.

Could matched filters be done with analog or RF circuits? and CDMA coded
matched filters? Can anyone point me to some good tutorials/ references,
websites, or mfrs of these type filters?

Thanks,

Bo
Armin Doerry
2006-05-19 21:03:13 UTC
Permalink
See embedded answers below...
--
========
Armin Doerry
Post by Bo
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.
In particular, I am interested in finding out how matched filters can be
implemented for X band SAR radars.
*** Note that all (at least the useful and used ones) SAR processing
algorithms attempt to implement a matched filter to the scene pixel
locations. Where they fall short is in the approximations employed for
efficient processing techniques - usually some kind of transforms. The
specific assumptions and approximations made leading to specific techniques
employed then distinguish the various image formation algorithms.
Post by Bo
I know that one method is through DSP of sampled data---but in our
particular case, needing 500MHz- 1GHz bandwidth, I don't see that sampling
I/Q data for those bandwidths is practical. (is it?).
*** As a matter of fact, this is posible with the latest A/D converters...
This notwithstanding, a technique known as "stretch" processing for Linear
FM chirps allows 'de-chirping' the echoes for substantial bandwidth
reduction. This is how state-of-the-art radars can achieve 4-inch range
resolution (>1.5 GHz of resolution bandwidth). Note that 'de-chirping' the
echoes is in fact a partial compression scheme, that is, a partial
implementation of a matched filter in analog RF.
Post by Bo
We may be used coded CDMA waveforms as well--which would as I understand
it, even further widen our bandwidth requirements.
*** Note that the latest FPGA technology can operate at clock frequencies
greater than 1 GHz.
Post by Bo
Could matched filters be done with analog or RF circuits?
*** Yes... LFM chirp range compression with SAW filters is well known...
Post by Bo
and CDMA coded matched filters?
*** I suspect this requires at least some minimal digitization of the
signals, but I don't know...
Post by Bo
Can anyone point me to some good tutorials/ references, websites, or mfrs
of these type filters?
Thanks,
Bo
Bo
2006-05-22 14:05:22 UTC
Permalink
Please see follow-up questions below.
Post by Armin Doerry
See embedded answers below...
Post by Bo
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.
In particular, I am interested in finding out how matched filters can be
implemented for X band SAR radars.
*** Note that all (at least the useful and used ones) SAR processing
algorithms attempt to implement a matched filter to the scene pixel
locations. Where they fall short is in the approximations employed for
efficient processing techniques - usually some kind of transforms. The
specific assumptions and approximations made leading to specific
techniques employed then distinguish the various image formation
algorithms.
Post by Bo
I know that one method is through DSP of sampled data---but in our
particular case, needing 500MHz- 1GHz bandwidth, I don't see that
sampling I/Q data for those bandwidths is practical. (is it?).
*** As a matter of fact, this is posible with the latest A/D converters...
I know there are 3GHz 8bit ADCs available--but that leads to further
questions---like

1) will 8 bits provide enough SNR?
2) re-iterating the earlier thread questions about I/Q sampling---how could
one use these 8b 3GHz ADCs to perform I/Q sampling?
3) if 8 bit is too low for system SNR, how could this be improved?
4) I assume that at these data rates all, or almost all, processing
algorithms to implement a matched filter would *have* to be implemented in
an FPGA--that not even the fastest DSPs from TI/Analog Devices could process
data this quickly? Is this a valid viewpoint? I don't know the length/types
of coding that will be employed on this SAR yet--but discussion is leaning
toward digital encoding of perhaps length 32 or 64 PN codes. How much
(ballpark) would such PN codes spread the bandwidth of say a nominal 1GHz BW
LFM chirp?
Post by Armin Doerry
This notwithstanding, a technique known as "stretch" processing for Linear
FM chirps allows 'de-chirping' the echoes for substantial bandwidth
reduction. This is how state-of-the-art radars can achieve 4-inch range
resolution (>1.5 GHz of resolution bandwidth). Note that 'de-chirping'
the echoes is in fact a partial compression scheme, that is, a partial
implementation of a matched filter in analog RF.
Can you explain what 'partial compression' means in this context? Or provide
any links on the method or available HW for analog RF matched filter?
Post by Armin Doerry
Post by Bo
We may be used coded CDMA waveforms as well--which would as I understand
it, even further widen our bandwidth requirements.
*** Note that the latest FPGA technology can operate at clock frequencies
greater than 1 GHz.
Point taken.
Post by Armin Doerry
Post by Bo
Could matched filters be done with analog or RF circuits?
*** Yes... LFM chirp range compression with SAW filters is well known...
And by using these SAW filters for matching, I could then beat the SAW
output signal down to baseband for sampling/processing?
Post by Armin Doerry
Post by Bo
and CDMA coded matched filters?
*** I suspect this requires at least some minimal digitization of the
signals, but I don't know...
Thanks again,

Bo
Armin Doerry
2006-05-27 01:07:34 UTC
Permalink
See answers embedded below...

Armin

========
Armin Doerry
Post by Bo
Please see follow-up questions below.
Post by Armin Doerry
See embedded answers below...
Post by Bo
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.
In particular, I am interested in finding out how matched filters can be
implemented for X band SAR radars.
*** Note that all (at least the useful and used ones) SAR processing
algorithms attempt to implement a matched filter to the scene pixel
locations. Where they fall short is in the approximations employed for
efficient processing techniques - usually some kind of transforms. The
specific assumptions and approximations made leading to specific
techniques employed then distinguish the various image formation
algorithms.
Post by Bo
I know that one method is through DSP of sampled data---but in our
particular case, needing 500MHz- 1GHz bandwidth, I don't see that
sampling I/Q data for those bandwidths is practical. (is it?).
*** As a matter of fact, this is posible with the latest A/D
converters...
I know there are 3GHz 8bit ADCs available--but that leads to further
questions---like
1) will 8 bits provide enough SNR?
*** generally, yes... The image dynamic range is the sum (in dB) of the
processing SNR gain and the ADC dynamic range...
Post by Bo
2) re-iterating the earlier thread questions about I/Q sampling---how
could one use these 8b 3GHz ADCs to perform I/Q sampling?
*** same as any other ADC... Look up quadrature demodulation... For
example
http://members.tripod.com/michaelgellis/mixerscom.html
There are two basic techniques for achieving quadrature (I/Q) data
1) form analog I/Q channels and then sample each channel with separate ADCs
2) Sample the IF with a single ADC and do digital baseband conversion and
formation of I/Q channels
Post by Bo
3) if 8 bit is too low for system SNR, how could this be improved?
*** If you de-chirp (stretch processing) for LFM waveforms you will need
more bits than if you do not de-chirp. The difference is due to the SNR
gain of de-chirping.
Post by Bo
4) I assume that at these data rates all, or almost all, processing
algorithms to implement a matched filter would *have* to be implemented in
an FPGA--that not even the fastest DSPs from TI/Analog Devices could
process data this quickly? Is this a valid viewpoint?
No... real-time SAR systems generating digital data and using DSP to form
images were around before FPGAs... Remember that systems are often
pulse-Doppler radars, and that a rate buffer can follow the ADC to slow the
data rate from the burst rate of the ADCs.
Post by Bo
I don't know the length/types of coding that will be employed on this SAR
yet--but discussion is leaning toward digital encoding of perhaps length 32
or 64 PN codes. How much (ballpark) would such PN codes spread the
bandwidth of say a nominal 1GHz BW LFM chirp?
*** Why would you use a PN code on top of a LFM chirp? It is not necessary
merely to achieve fine resolution.
Post by Bo
Post by Armin Doerry
This notwithstanding, a technique known as "stretch" processing for
Linear FM chirps allows 'de-chirping' the echoes for substantial
bandwidth reduction. This is how state-of-the-art radars can achieve
4-inch range resolution (>1.5 GHz of resolution bandwidth). Note that
'de-chirping' the echoes is in fact a partial compression scheme, that
is, a partial implementation of a matched filter in analog RF.
Can you explain what 'partial compression' means in this context? Or
provide any links on the method or available HW for analog RF matched
filter?
*** mixing the received echoes with a local oscillator chirp removes the
chirp characteristic from the received signals, thereby compressing its
bandwidth with no loss of signal. This generates SNR gain in addition to
bandwidth compression. The result is a partial compression along the way to
a matched filter. A matched filter is the ultimate (in a minimum mean
square error sense) compression of the signal, i.e. maximizing the SNR.
Post by Bo
Post by Armin Doerry
Post by Bo
We may be used coded CDMA waveforms as well--which would as I understand
it, even further widen our bandwidth requirements.
*** resolution is the same function of bandwidth regardless of the waveform
used. The system impulse response is the autocorrelation of the waveform,
which is the Fourier transform of the power spectral density of the
waveform, regardless of the exact signal itself.
Check out the appendix in
http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2006/060821.pdf
Post by Bo
Post by Armin Doerry
*** Note that the latest FPGA technology can operate at clock frequencies
greater than 1 GHz.
Point taken.
Post by Armin Doerry
Post by Bo
Could matched filters be done with analog or RF circuits?
*** Yes... LFM chirp range compression with SAW filters is well known...
And by using these SAW filters for matching, I could then beat the SAW
output signal down to baseband for sampling/processing?
Post by Armin Doerry
Post by Bo
and CDMA coded matched filters?
*** I suspect this requires at least some minimal digitization of the
signals, but I don't know...
Thanks again,
Bo
rge11x
2006-05-27 11:34:15 UTC
Permalink
If I may add one sentence to Armin Doerry's explanation

"...mixing the received echoes with a local oscillator chirp removes
the chirp characteristic from the received signals, thereby compressing
its bandwidth with no loss of signal. This generates SNR gain in
addition to bandwidth compression. The result is a partial compression
along the way to a matched filter. "

stretch becomes "matched filtering" when the resulting heterodyne tone
at th eoutput of the mixer is filtered by a bandpass filter whose
bandwidth is the reciprocal of the chirp length; the filtering is
usually done in an FFT that will immediately gives a bank of parallel
filters, so you can also estimate the frequency (range) not only detect
the target's presence. It is only approximately matched because the
impulse response of the resulting filter is not exactly a finite square
pulse.
Armin Doerry
2006-05-28 00:25:41 UTC
Permalink
Subtleties of signals having both finite time duration and finite bandwidth
notwithstanding, the stretch-processed impulse response (IPR) is pretty darn
close to a true matched filter... or at least can be...

A matched filter's output for the input signal to which it is matched is the
signal's autocorrelation function, which is also the Fourier transform of
the signal's Power Spectral Density (PSD). A constant amplitude, finite
duration, LFM chirp with large time-bandwidth product has a PSD that is very
nearly a rectangle function. Consequently, its IPR is very nearly a sinc()
function, that is, sin(x)/x in character, especially near its mainlobe peak.
Typical SAR systems operate with time-bandwidth products in the hundreds to
the many ten-thousands for high-performance systems, e.g. in fact a 100 usec
chirp with 1800 MHz bandwidth = 180000.

Deramping the received chirp echoes doesn't by itself lose any information.
The received signal can always be reconstituted by adding back the chirp.
If the sampling interval is long enough such that all echo energy from all
ranges of interest is contained in the samples, then a deskewing (removing a
residual video phase error) operation can align the deramped echoes in time,
and superfluous time samples can be trimmed. At this point then no energy
has been lost. A FFT applied will result in an IPR that is again very much
like a sinc() function, especially in its mainlobe, to within what the
digital sampling will allow (i.e. more samples make the mainlobe more
sinc()-like)...

The bottom line is that for large time-bandwidth LFM chirps, the IPR in the
region of its mainlobe will have inconsequential differences. Both will
exhibit essentially sinc() behavior in their IPR.

I apologize for being too wordy... radar design and analysis is actually
fun for me... so I get carried away sometimes... ;-)

Armin
--
========
Armin Doerry
Post by rge11x
If I may add one sentence to Armin Doerry's explanation
"...mixing the received echoes with a local oscillator chirp removes
the chirp characteristic from the received signals, thereby compressing
its bandwidth with no loss of signal. This generates SNR gain in
addition to bandwidth compression. The result is a partial compression
along the way to a matched filter. "
stretch becomes "matched filtering" when the resulting heterodyne tone
at th eoutput of the mixer is filtered by a bandpass filter whose
bandwidth is the reciprocal of the chirp length; the filtering is
usually done in an FFT that will immediately gives a bank of parallel
filters, so you can also estimate the frequency (range) not only detect
the target's presence. It is only approximately matched because the
impulse response of the resulting filter is not exactly a finite square
pulse.
rge11x
2006-05-29 00:41:41 UTC
Permalink
You are not too wordy, on the contrary. I think we should all be
thankful to you for the rare effort in usenet to write coherent
explanation at all levels of readership. Most of sci.phys and sci.math
are by now have become nearly unreadable drivel. While this newsgroup
is not as widely read as those, we should be, and I certainly am
always, glad to read your opinions.
Bo
2006-05-31 13:09:07 UTC
Permalink
Post by rge11x
You are not too wordy, on the contrary. I think we should all be
thankful to you for the rare effort in usenet to write coherent
explanation at all levels of readership. Most of sci.phys and sci.math
are by now have become nearly unreadable drivel. While this newsgroup
is not as widely read as those, we should be, and I certainly am
always, glad to read your opinions.
Well put.

Thanks again for the responses. Being new to radar I am still struggling to
digest all of what has been said and am sure to have some more follow-up
questions.

Best regards,

Bo
Bo
2006-05-31 13:03:35 UTC
Permalink
Armin,

Thanks for the reply. I've been out a few days and am just getting back to
this--see my replies below.
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.
In particular, I am interested in finding out how matched filters can
be implemented for X band SAR radars.
*** Note that all (at least the useful and used ones) SAR processing
algorithms attempt to implement a matched filter to the scene pixel
locations. Where they fall short is in the approximations employed for
efficient processing techniques - usually some kind of transforms. The
specific assumptions and approximations made leading to specific
techniques employed then distinguish the various image formation
algorithms.
Post by Bo
I know that one method is through DSP of sampled data---but in our
particular case, needing 500MHz- 1GHz bandwidth, I don't see that
sampling I/Q data for those bandwidths is practical. (is it?).
*** As a matter of fact, this is posible with the latest A/D
converters...
I know there are 3GHz 8bit ADCs available--but that leads to further
questions---like
1) will 8 bits provide enough SNR?
*** generally, yes... The image dynamic range is the sum (in dB) of the
processing SNR gain and the ADC dynamic range...
Post by Bo
2) re-iterating the earlier thread questions about I/Q sampling---how
could one use these 8b 3GHz ADCs to perform I/Q sampling?
*** same as any other ADC... Look up quadrature demodulation... For
example
http://members.tripod.com/michaelgellis/mixerscom.html
There are two basic techniques for achieving quadrature (I/Q) data
1) form analog I/Q channels and then sample each channel with separate ADCs
2) Sample the IF with a single ADC and do digital baseband conversion and
formation of I/Q channels
Post by Bo
3) if 8 bit is too low for system SNR, how could this be improved?
*** If you de-chirp (stretch processing) for LFM waveforms you will need
more bits than if you do not de-chirp. The difference is due to the SNR
gain of de-chirping.
Post by Bo
4) I assume that at these data rates all, or almost all, processing
algorithms to implement a matched filter would *have* to be implemented
in an FPGA--that not even the fastest DSPs from TI/Analog Devices could
process data this quickly? Is this a valid viewpoint?
No... real-time SAR systems generating digital data and using DSP to form
images were around before FPGAs... Remember that systems are often
pulse-Doppler radars, and that a rate buffer can follow the ADC to slow
the data rate from the burst rate of the ADCs.
Post by Bo
I don't know the length/types of coding that will be employed on this SAR
yet--but discussion is leaning toward digital encoding of perhaps length
32 or 64 PN codes. How much (ballpark) would such PN codes spread the
bandwidth of say a nominal 1GHz BW LFM chirp?
*** Why would you use a PN code on top of a LFM chirp? It is not
necessary merely to achieve fine resolution.
Because this is not a single radar-- but rather N radars and we are
contemplating use of PN codes to allow each radar to distinguish the other
radar's returns. Either that or find a way to sync the radars very precisely
so that only one transmits at a given time. The final signal
coding/LFM/combo scheme is very much up for grabs right now. I'm looking
into the +/- of each type and how one can implement the system once the
decision is made.
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
This notwithstanding, a technique known as "stretch" processing for
Linear FM chirps allows 'de-chirping' the echoes for substantial
bandwidth reduction. This is how state-of-the-art radars can achieve
4-inch range resolution (>1.5 GHz of resolution bandwidth). Note that
'de-chirping' the echoes is in fact a partial compression scheme, that
is, a partial implementation of a matched filter in analog RF.
Can you explain what 'partial compression' means in this context? Or
provide any links on the method or available HW for analog RF matched
filter?
*** mixing the received echoes with a local oscillator chirp removes the
chirp characteristic from the received signals, thereby compressing its
bandwidth with no loss of signal. This generates SNR gain in addition to
bandwidth compression. The result is a partial compression along the way
to a matched filter. A matched filter is the ultimate (in a minimum mean
square error sense) compression of the signal, i.e. maximizing the SNR.
Post by Bo
Post by Armin Doerry
Post by Bo
We may be used coded CDMA waveforms as well--which would as I
understand it, even further widen our bandwidth requirements.
*** resolution is the same function of bandwidth regardless of the
waveform used. The system impulse response is the autocorrelation of the
waveform, which is the Fourier transform of the power spectral density of
the waveform, regardless of the exact signal itself.
Check out the appendix in
http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2006/060821.pdf
Is it _possible_ to implement a matched filter for CDMA in an analog
fashion? I'm thinking not. Which leads me to the question-- _must_ a CDMA
signal be directly sampled---or can it be sampled after mix down to IF?
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
Could matched filters be done with analog or RF circuits?
*** Yes... LFM chirp range compression with SAW filters is well known...
And by using these SAW filters for matching, I could then beat the SAW
output signal down to baseband for sampling/processing?
I looked into SAW filters after your reply--but could find nothing available
beyond the 2-4GHz range. The ones I found also had fairly limited bandwidth
as well. Perhaps one existed for X band I have not yet found...(?)
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
and CDMA coded matched filters?
*** I suspect this requires at least some minimal digitization of the
signals, but I don't know...
Thanks again,

Bo
rge11x
2006-06-03 00:47:05 UTC
Permalink
Yes, it is possible to build matched filters in a purely analogue
fashion. This is how it was done in the good old days, 50 years ago
with delay lines made out of LC circuits (group delay equalized
filters), there is a huge literature exists on how to design that kind
of stuff but the invention of SAW technology quickly made them
obsolete. Bulk Acoustic technology can get you to a few GHz range but
conventional LC is not practical above a few hundred MHz IF.

If I understand your problem then I think you will be better off using
a correlator instead of a match filter. You should be able to do that
at any carrier frequency directly with a homodyne mixer followed by an
integrator.
Post by Bo
Armin,
Thanks for the reply. I've been out a few days and am just getting back to
this--see my replies below.
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.
In particular, I am interested in finding out how matched filters can
be implemented for X band SAR radars.
*** Note that all (at least the useful and used ones) SAR processing
algorithms attempt to implement a matched filter to the scene pixel
locations. Where they fall short is in the approximations employed for
efficient processing techniques - usually some kind of transforms. The
specific assumptions and approximations made leading to specific
techniques employed then distinguish the various image formation
algorithms.
Post by Bo
I know that one method is through DSP of sampled data---but in our
particular case, needing 500MHz- 1GHz bandwidth, I don't see that
sampling I/Q data for those bandwidths is practical. (is it?).
*** As a matter of fact, this is posible with the latest A/D converters...
I know there are 3GHz 8bit ADCs available--but that leads to further
questions---like
1) will 8 bits provide enough SNR?
*** generally, yes... The image dynamic range is the sum (in dB) of the
processing SNR gain and the ADC dynamic range...
Post by Bo
2) re-iterating the earlier thread questions about I/Q sampling---how
could one use these 8b 3GHz ADCs to perform I/Q sampling?
*** same as any other ADC... Look up quadrature demodulation... For
example
http://members.tripod.com/michaelgellis/mixerscom.html
There are two basic techniques for achieving quadrature (I/Q) data
1) form analog I/Q channels and then sample each channel with separate ADCs
2) Sample the IF with a single ADC and do digital baseband conversion and
formation of I/Q channels
Post by Bo
3) if 8 bit is too low for system SNR, how could this be improved?
*** If you de-chirp (stretch processing) for LFM waveforms you will need
more bits than if you do not de-chirp. The difference is due to the SNR
gain of de-chirping.
Post by Bo
4) I assume that at these data rates all, or almost all, processing
algorithms to implement a matched filter would *have* to be implemented
in an FPGA--that not even the fastest DSPs from TI/Analog Devices could
process data this quickly? Is this a valid viewpoint?
No... real-time SAR systems generating digital data and using DSP to form
images were around before FPGAs... Remember that systems are often
pulse-Doppler radars, and that a rate buffer can follow the ADC to slow
the data rate from the burst rate of the ADCs.
Post by Bo
I don't know the length/types of coding that will be employed on this SAR
yet--but discussion is leaning toward digital encoding of perhaps length
32 or 64 PN codes. How much (ballpark) would such PN codes spread the
bandwidth of say a nominal 1GHz BW LFM chirp?
*** Why would you use a PN code on top of a LFM chirp? It is not
necessary merely to achieve fine resolution.
Because this is not a single radar-- but rather N radars and we are
contemplating use of PN codes to allow each radar to distinguish the other
radar's returns. Either that or find a way to sync the radars very precisely
so that only one transmits at a given time. The final signal
coding/LFM/combo scheme is very much up for grabs right now. I'm looking
into the +/- of each type and how one can implement the system once the
decision is made.
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
This notwithstanding, a technique known as "stretch" processing for
Linear FM chirps allows 'de-chirping' the echoes for substantial
bandwidth reduction. This is how state-of-the-art radars can achieve
4-inch range resolution (>1.5 GHz of resolution bandwidth). Note that
'de-chirping' the echoes is in fact a partial compression scheme, that
is, a partial implementation of a matched filter in analog RF.
Can you explain what 'partial compression' means in this context? Or
provide any links on the method or available HW for analog RF matched
filter?
*** mixing the received echoes with a local oscillator chirp removes the
chirp characteristic from the received signals, thereby compressing its
bandwidth with no loss of signal. This generates SNR gain in addition to
bandwidth compression. The result is a partial compression along the way
to a matched filter. A matched filter is the ultimate (in a minimum mean
square error sense) compression of the signal, i.e. maximizing the SNR.
Post by Bo
Post by Armin Doerry
Post by Bo
We may be used coded CDMA waveforms as well--which would as I
understand it, even further widen our bandwidth requirements.
*** resolution is the same function of bandwidth regardless of the
waveform used. The system impulse response is the autocorrelation of the
waveform, which is the Fourier transform of the power spectral density of
the waveform, regardless of the exact signal itself.
Check out the appendix in
http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2006/060821.pdf
Is it _possible_ to implement a matched filter for CDMA in an analog
fashion? I'm thinking not. Which leads me to the question-- _must_ a CDMA
signal be directly sampled---or can it be sampled after mix down to IF?
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
Could matched filters be done with analog or RF circuits?
*** Yes... LFM chirp range compression with SAW filters is well known...
And by using these SAW filters for matching, I could then beat the SAW
output signal down to baseband for sampling/processing?
I looked into SAW filters after your reply--but could find nothing available
beyond the 2-4GHz range. The ones I found also had fairly limited bandwidth
as well. Perhaps one existed for X band I have not yet found...(?)
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
and CDMA coded matched filters?
*** I suspect this requires at least some minimal digitization of the
signals, but I don't know...
Thanks again,
Bo
Bo
2006-06-13 14:19:00 UTC
Permalink
Post by rge11x
Yes, it is possible to build matched filters in a purely analogue
fashion. This is how it was done in the good old days, 50 years ago
with delay lines made out of LC circuits (group delay equalized
filters), there is a huge literature exists on how to design that kind
of stuff but the invention of SAW technology quickly made them
obsolete. Bulk Acoustic technology can get you to a few GHz range but
conventional LC is not practical above a few hundred MHz IF.
If I understand your problem then I think you will be better off using
a correlator instead of a match filter. You should be able to do that
at any carrier frequency directly with a homodyne mixer followed by an
integrator.
My understanding of current SAW technology is that it is not yet capable of
working in the X band.

Can you explain to me what you mean when you use the terms correlator and
matched filter? I'm guessing that correlator impies a A/D conversion and
digitally manipulating the signal? and matched filter, as you are using it,
means a pure analog solution? If I'm misinterpreting your meaning, please
try to spell it out for me again.

My current thinking is that the signal will have to be mixed down to an IF
and then sampled and manipulated to create a digital matched filter---my
concern is the mix down process will corrupt any encoding in the actual
signal and thus make the matched-filter portion unable to make a good
match--or 'correlation'.

Thanks,

Bo
Post by rge11x
Post by Bo
Armin,
Thanks for the reply. I've been out a few days and am just getting back to
this--see my replies below.
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.
In particular, I am interested in finding out how matched filters can
be implemented for X band SAR radars.
*** Note that all (at least the useful and used ones) SAR processing
algorithms attempt to implement a matched filter to the scene pixel
locations. Where they fall short is in the approximations employed for
efficient processing techniques - usually some kind of transforms.
The
specific assumptions and approximations made leading to specific
techniques employed then distinguish the various image formation
algorithms.
Post by Bo
I know that one method is through DSP of sampled data---but in our
particular case, needing 500MHz- 1GHz bandwidth, I don't see that
sampling I/Q data for those bandwidths is practical. (is it?).
*** As a matter of fact, this is posible with the latest A/D converters...
I know there are 3GHz 8bit ADCs available--but that leads to further
questions---like
1) will 8 bits provide enough SNR?
*** generally, yes... The image dynamic range is the sum (in dB) of the
processing SNR gain and the ADC dynamic range...
Post by Bo
2) re-iterating the earlier thread questions about I/Q sampling---how
could one use these 8b 3GHz ADCs to perform I/Q sampling?
*** same as any other ADC... Look up quadrature demodulation... For
example
http://members.tripod.com/michaelgellis/mixerscom.html
There are two basic techniques for achieving quadrature (I/Q) data
1) form analog I/Q channels and then sample each channel with separate ADCs
2) Sample the IF with a single ADC and do digital baseband conversion and
formation of I/Q channels
Post by Bo
3) if 8 bit is too low for system SNR, how could this be improved?
*** If you de-chirp (stretch processing) for LFM waveforms you will need
more bits than if you do not de-chirp. The difference is due to the SNR
gain of de-chirping.
Post by Bo
4) I assume that at these data rates all, or almost all, processing
algorithms to implement a matched filter would *have* to be implemented
in an FPGA--that not even the fastest DSPs from TI/Analog Devices could
process data this quickly? Is this a valid viewpoint?
No... real-time SAR systems generating digital data and using DSP to form
images were around before FPGAs... Remember that systems are often
pulse-Doppler radars, and that a rate buffer can follow the ADC to slow
the data rate from the burst rate of the ADCs.
Post by Bo
I don't know the length/types of coding that will be employed on this SAR
yet--but discussion is leaning toward digital encoding of perhaps length
32 or 64 PN codes. How much (ballpark) would such PN codes spread the
bandwidth of say a nominal 1GHz BW LFM chirp?
*** Why would you use a PN code on top of a LFM chirp? It is not
necessary merely to achieve fine resolution.
Because this is not a single radar-- but rather N radars and we are
contemplating use of PN codes to allow each radar to distinguish the other
radar's returns. Either that or find a way to sync the radars very precisely
so that only one transmits at a given time. The final signal
coding/LFM/combo scheme is very much up for grabs right now. I'm looking
into the +/- of each type and how one can implement the system once the
decision is made.
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
This notwithstanding, a technique known as "stretch" processing for
Linear FM chirps allows 'de-chirping' the echoes for substantial
bandwidth reduction. This is how state-of-the-art radars can achieve
4-inch range resolution (>1.5 GHz of resolution bandwidth). Note that
'de-chirping' the echoes is in fact a partial compression scheme, that
is, a partial implementation of a matched filter in analog RF.
Can you explain what 'partial compression' means in this context? Or
provide any links on the method or available HW for analog RF matched
filter?
*** mixing the received echoes with a local oscillator chirp removes the
chirp characteristic from the received signals, thereby compressing its
bandwidth with no loss of signal. This generates SNR gain in addition to
bandwidth compression. The result is a partial compression along the way
to a matched filter. A matched filter is the ultimate (in a minimum mean
square error sense) compression of the signal, i.e. maximizing the SNR.
Post by Bo
Post by Armin Doerry
Post by Bo
We may be used coded CDMA waveforms as well--which would as I
understand it, even further widen our bandwidth requirements.
*** resolution is the same function of bandwidth regardless of the
waveform used. The system impulse response is the autocorrelation of the
waveform, which is the Fourier transform of the power spectral density of
the waveform, regardless of the exact signal itself.
Check out the appendix in
http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2006/060821.pdf
Is it _possible_ to implement a matched filter for CDMA in an analog
fashion? I'm thinking not. Which leads me to the question-- _must_ a CDMA
signal be directly sampled---or can it be sampled after mix down to IF?
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
Could matched filters be done with analog or RF circuits?
*** Yes... LFM chirp range compression with SAW filters is well known...
And by using these SAW filters for matching, I could then beat the SAW
output signal down to baseband for sampling/processing?
I looked into SAW filters after your reply--but could find nothing available
beyond the 2-4GHz range. The ones I found also had fairly limited bandwidth
as well. Perhaps one existed for X band I have not yet found...(?)
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
and CDMA coded matched filters?
*** I suspect this requires at least some minimal digitization of the
signals, but I don't know...
Thanks again,
Bo
Jerry Avins
2006-06-13 14:30:50 UTC
Permalink
Bo wrote:

...
Post by Bo
I'm guessing that correlator impies a A/D conversion and
digitally manipulating the signal? and matched filter, as you are using it,
means a pure analog solution? If I'm misinterpreting your meaning, please
try to spell it out for me again.
...

You don't have to compute the sum of two weights. You can just put them
on the scale at the same time. You don't have to correlate by
calculation. You can just /do/ it. http://www.sss-mag.com/corr.html

Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Bo
2006-06-13 20:06:11 UTC
Permalink
...
I'm guessing that correlator impies a A/D conversion and digitally
manipulating the signal? and matched filter, as you are using it, means a
pure analog solution? If I'm misinterpreting your meaning, please try to
spell it out for me again.
...
You don't have to compute the sum of two weights. You can just put them on
the scale at the same time. You don't have to correlate by calculation.
You can just /do/ it. http://www.sss-mag.com/corr.html
Jerry
--
Engineering is the art of making what you want from things you can get.
¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Thanks for the link Jerry....

Very interesting...now I must find my Tums/Rolaids in order to digest all of
this....

Regards,

Bo
rge11x
2006-06-14 12:01:08 UTC
Permalink
Crudely speaking a matched filter is a correlator for all times. A
correlator is a multiplier followed by an integrator with the
integration being done between fixed time instants. Both matched
filtering and correlation can be done by either analogue or digital
means or some combinations thereof but based on your last questions I
think you should crack and read a book on radar; there are many good
ones, for example, Nathanson's.
Post by Bo
Post by rge11x
Yes, it is possible to build matched filters in a purely analogue
fashion. This is how it was done in the good old days, 50 years ago
with delay lines made out of LC circuits (group delay equalized
filters), there is a huge literature exists on how to design that kind
of stuff but the invention of SAW technology quickly made them
obsolete. Bulk Acoustic technology can get you to a few GHz range but
conventional LC is not practical above a few hundred MHz IF.
If I understand your problem then I think you will be better off using
a correlator instead of a match filter. You should be able to do that
at any carrier frequency directly with a homodyne mixer followed by an
integrator.
My understanding of current SAW technology is that it is not yet capable of
working in the X band.
Can you explain to me what you mean when you use the terms correlator and
matched filter? I'm guessing that correlator impies a A/D conversion and
digitally manipulating the signal? and matched filter, as you are using it,
means a pure analog solution? If I'm misinterpreting your meaning, please
try to spell it out for me again.
My current thinking is that the signal will have to be mixed down to an IF
and then sampled and manipulated to create a digital matched filter---my
concern is the mix down process will corrupt any encoding in the actual
signal and thus make the matched-filter portion unable to make a good
match--or 'correlation'.
Thanks,
Bo
Post by rge11x
Post by Bo
Armin,
Thanks for the reply. I've been out a few days and am just getting back to
this--see my replies below.
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.
In particular, I am interested in finding out how matched filters can
be implemented for X band SAR radars.
*** Note that all (at least the useful and used ones) SAR processing
algorithms attempt to implement a matched filter to the scene pixel
locations. Where they fall short is in the approximations employed for
efficient processing techniques - usually some kind of transforms.
The
specific assumptions and approximations made leading to specific
techniques employed then distinguish the various image formation
algorithms.
Post by Bo
I know that one method is through DSP of sampled data---but in our
particular case, needing 500MHz- 1GHz bandwidth, I don't see that
sampling I/Q data for those bandwidths is practical. (is it?).
*** As a matter of fact, this is posible with the latest A/D converters...
I know there are 3GHz 8bit ADCs available--but that leads to further
questions---like
1) will 8 bits provide enough SNR?
*** generally, yes... The image dynamic range is the sum (in dB) of the
processing SNR gain and the ADC dynamic range...
Post by Bo
2) re-iterating the earlier thread questions about I/Q sampling---how
could one use these 8b 3GHz ADCs to perform I/Q sampling?
*** same as any other ADC... Look up quadrature demodulation... For
example
http://members.tripod.com/michaelgellis/mixerscom.html
There are two basic techniques for achieving quadrature (I/Q) data
1) form analog I/Q channels and then sample each channel with separate ADCs
2) Sample the IF with a single ADC and do digital baseband conversion and
formation of I/Q channels
Post by Bo
3) if 8 bit is too low for system SNR, how could this be improved?
*** If you de-chirp (stretch processing) for LFM waveforms you will need
more bits than if you do not de-chirp. The difference is due to the SNR
gain of de-chirping.
Post by Bo
4) I assume that at these data rates all, or almost all, processing
algorithms to implement a matched filter would *have* to be implemented
in an FPGA--that not even the fastest DSPs from TI/Analog Devices could
process data this quickly? Is this a valid viewpoint?
No... real-time SAR systems generating digital data and using DSP to form
images were around before FPGAs... Remember that systems are often
pulse-Doppler radars, and that a rate buffer can follow the ADC to slow
the data rate from the burst rate of the ADCs.
Post by Bo
I don't know the length/types of coding that will be employed on this SAR
yet--but discussion is leaning toward digital encoding of perhaps length
32 or 64 PN codes. How much (ballpark) would such PN codes spread the
bandwidth of say a nominal 1GHz BW LFM chirp?
*** Why would you use a PN code on top of a LFM chirp? It is not
necessary merely to achieve fine resolution.
Because this is not a single radar-- but rather N radars and we are
contemplating use of PN codes to allow each radar to distinguish the other
radar's returns. Either that or find a way to sync the radars very precisely
so that only one transmits at a given time. The final signal
coding/LFM/combo scheme is very much up for grabs right now. I'm looking
into the +/- of each type and how one can implement the system once the
decision is made.
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
This notwithstanding, a technique known as "stretch" processing for
Linear FM chirps allows 'de-chirping' the echoes for substantial
bandwidth reduction. This is how state-of-the-art radars can achieve
4-inch range resolution (>1.5 GHz of resolution bandwidth). Note that
'de-chirping' the echoes is in fact a partial compression scheme, that
is, a partial implementation of a matched filter in analog RF.
Can you explain what 'partial compression' means in this context? Or
provide any links on the method or available HW for analog RF matched
filter?
*** mixing the received echoes with a local oscillator chirp removes the
chirp characteristic from the received signals, thereby compressing its
bandwidth with no loss of signal. This generates SNR gain in addition to
bandwidth compression. The result is a partial compression along the way
to a matched filter. A matched filter is the ultimate (in a minimum mean
square error sense) compression of the signal, i.e. maximizing the SNR.
Post by Bo
Post by Armin Doerry
Post by Bo
We may be used coded CDMA waveforms as well--which would as I
understand it, even further widen our bandwidth requirements.
*** resolution is the same function of bandwidth regardless of the
waveform used. The system impulse response is the autocorrelation of the
waveform, which is the Fourier transform of the power spectral density of
the waveform, regardless of the exact signal itself.
Check out the appendix in
http://www.prod.sandia.gov/cgi-bin/techlib/access-control.pl/2006/060821.pdf
Is it _possible_ to implement a matched filter for CDMA in an analog
fashion? I'm thinking not. Which leads me to the question-- _must_ a CDMA
signal be directly sampled---or can it be sampled after mix down to IF?
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
Could matched filters be done with analog or RF circuits?
*** Yes... LFM chirp range compression with SAW filters is well known...
And by using these SAW filters for matching, I could then beat the SAW
output signal down to baseband for sampling/processing?
I looked into SAW filters after your reply--but could find nothing available
beyond the 2-4GHz range. The ones I found also had fairly limited bandwidth
as well. Perhaps one existed for X band I have not yet found...(?)
Post by Armin Doerry
Post by Bo
Post by Armin Doerry
Post by Bo
and CDMA coded matched filters?
*** I suspect this requires at least some minimal digitization of the
signals, but I don't know...
Thanks again,
Bo
Eric Jacobsen
2006-05-19 23:49:31 UTC
Permalink
Post by Bo
I'm TOTALLY new to RADAR world and am looking for some pointers/info in
regards to SAR radar.
In particular, I am interested in finding out how matched filters can be
implemented for X band SAR radars. I know that one method is through DSP of
sampled data---but in our particular case, needing 500MHz- 1GHz bandwidth, I
don't see that sampling I/Q data for those bandwidths is practical. (is
it?). We may be used coded CDMA waveforms as well--which would as I
understand it, even further widen our bandwidth requirements.
Could matched filters be done with analog or RF circuits?
SAR (and radar in general) has been around for a long time and the
original processors were analog. In the range dimension dispersive
filters were sometimes used which matched the FM rate of the chirp
signal. Optical processors (analog optical processing) were all the
rage until the late 70s or 80s. Range and cross-range matched
filters were done with Fourier transform lenses and screens. Motion
compensation was done by slightly adjusting the lenses as the signal
was processed. Pretty cool stuff...
Post by Bo
and CDMA coded
matched filters? Can anyone point me to some good tutorials/ references,
websites, or mfrs of these type filters?
As was mentioned, SAW filters are often used for this sort of thing.
There are probably other methods, too. Digital processing at these
rates is certainly possible, though.
Eric Jacobsen
Minister of Algorithms, Intel Corp.
My opinions may not be Intel's opinions.
http://www.ericjacobsen.org
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