Forums

SAR matched filters?

Started by Bo May 19, 2006
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.

Bo wrote:
> Armin, > > Thanks for the reply. I've been out a few days and am just getting back to > this--see my replies below. > > > >>> "Bo" <bo@cephus.com> wrote in message > >>> news:c349f$446e2ab4$18d6ec55$24115@KNOLOGY.NET... > >>>> 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. > >>> > >>>> 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... > > > >> 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 > > > >> 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. > > > >> 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. > > > >>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. > > > > >> > >>> 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. > > > >>> > >>>> 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? > > >>>> 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...(?) > > >>>>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" <rge11x@netscape.net> wrote in message 
news:1149295625.854907.62580@i39g2000cwa.googlegroups.com...
> 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
> > Bo wrote: >> Armin, >> >> Thanks for the reply. I've been out a few days and am just getting back >> to >> this--see my replies below. >> >> >> >>> "Bo" <bo@cephus.com> wrote in message >> >>> news:c349f$446e2ab4$18d6ec55$24115@KNOLOGY.NET... >> >>>> 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. >> >>> >> >>>> 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... >> > >> >> 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 >> > >> >> 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. >> > >> >> 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. >> > >> >>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. >> >> > >> >> >> >>> 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. >> > >> >>> >> >>>> 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? >> >> >>>> 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...(?) >> >> >>>>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 wrote:

   ...

> 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. &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
"Jerry Avins" <jya@ieee.org> wrote in message 
news:TsydndQmI62DURPZnZ2dnUVZ_ridnZ2d@rcn.net...
> Bo wrote: > > ... > >> 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. > &#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;&#2013266095;
Thanks for the link Jerry.... Very interesting...now I must find my Tums/Rolaids in order to digest all of this.... Regards, Bo
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.

Bo wrote:
> "rge11x" <rge11x@netscape.net> wrote in message > news:1149295625.854907.62580@i39g2000cwa.googlegroups.com... > > 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 > > > > > > > > > Bo wrote: > >> Armin, > >> > >> Thanks for the reply. I've been out a few days and am just getting back > >> to > >> this--see my replies below. > >> > >> > >> >>> "Bo" <bo@cephus.com> wrote in message > >> >>> news:c349f$446e2ab4$18d6ec55$24115@KNOLOGY.NET... > >> >>>> 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. > >> >>> > >> >>>> 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... > >> > > >> >> 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 > >> > > >> >> 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. > >> > > >> >> 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. > >> > > >> >>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. > >> > >> > > >> >> > >> >>> 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. > >> > > >> >>> > >> >>>> 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? > >> > >> >>>> 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...(?) > >> > >> >>>>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 > >