HelpmaBoab wrote:> "Mr M" <blobb@bredband.net> wrote in message > news:4408965c$0$15784$14726298@news.sunsite.dk... > >>Hello >> >> >> >>I need to sample and demodulate an AM-signal that is between 30 kHz to 100 >>kHz. The signal has a bandwidth of about 10 kHz. >> >> >> >>What approach should I use? >> >> >> >>I'm thinking of using a 16 bit ADC with a maximum sampling frequency of > > 500 > >>ksps. Maybe using an FPGA to control the ADC. The FPGA can also be used to >>do the signal processing. >> >> >> >>How high sampling frequency should I use? >> >> > > Why not just use a diode and a capacitor - !!And then what: digitize the result? Maybe. Peak detection is one of the more difficult tasks in critically samples systems. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Approach to demodulating AM-signals?
Started by ●March 3, 2006
Reply by ●March 4, 20062006-03-04
Reply by ●March 4, 20062006-03-04
Joerg wrote:> Hello Tim, > >> >> WDF? >> > > Well, yes, I thought that he'd like to get rid of switcher noises and > stuff below 30kHz and talk radio in the AM band. Personally I'd use LC > though. >Let's try that again. What in the world does 'WDF' stand for?> >> Getting the absolute value and smoothing should be better -- you'll >> still have some aliasing products getting into the baseband (I think), >> but it'll be better. >> > > If you use a tracking filter there shouldn't be much getting into > baseband. Doesn't have to be WDF, as long as it is nice and steep and > most of all stable against the occasional thunderstrom crackle (that's > where WDFs are pretty good). >Forget the aliasing from sampling, I was talking about the 30kHz carrier plus 10kHz modulation intemodding in the absolute value thingie. You start with energy at 20, 30 and 40 kHz, then run it through a whopping big nonlinearity. You end up with 20 mixing with 30 down to 10 (good), 30 mixing with 40 down to 10 (also good), but you also end up with the 40 from the bottom end of the second harmonic mixing with the 30kHz carrier (bad), etc., etc. I think you'd have to simulate it to really see if it's going to have issues. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/
Reply by ●March 4, 20062006-03-04
On Fri, 03 Mar 2006 22:12:03 -0800, Tim Wescott <tim@seemywebsite.com> wrote:>Let's try that again. What in the world does 'WDF' stand for?http://www.acronymfinder.com/af-query.asp?Acronym=WDF WDF Washington Department of Fisheries ***** WDF Web Design Factory **** WDF Web Design Forums **** WDF Washington Department of Fish **** WDF World Diabetes Foundation *** WDF World Development Federation *** WDF Wave Digital Filters *** I think Wave Digital Filter sounds likely. Allan
Reply by ●March 4, 20062006-03-04
"Jerry Avins" <jya@ieee.org> wrote in message news:VqCdnY6efNbQgpTZRVn-qg@rcn.net...> HelpmaBoab wrote: > > "Mr M" <blobb@bredband.net> wrote in message > > news:4408965c$0$15784$14726298@news.sunsite.dk... > > > >>Hello > >> > >> > >> > >>I need to sample and demodulate an AM-signal that is between 30 kHz to100> >>kHz. The signal has a bandwidth of about 10 kHz. > >> > >> > >> > >>What approach should I use? > >> > >> > >> > >>I'm thinking of using a 16 bit ADC with a maximum sampling frequency of > > > > 500 > > > >>ksps. Maybe using an FPGA to control the ADC. The FPGA can also be usedto> >>do the signal processing. > >> > >> > >> > >>How high sampling frequency should I use? > >> > >> > > > > Why not just use a diode and a capacitor - !! > > And then what: digitize the result? Maybe. Peak detection is one of the > more difficult tasks in critically samples systems. > > Jerry > -- > Engineering is the art of making what you want from things you can get. > �����I was of course being cynical - why spend a fortune sampling anything when there is a perfectly good analogue result costing pennies - an angineer can do for pennies what any fool can do for pounds? Tam
Reply by ●March 4, 20062006-03-04
Allan Herriman wrote:> On Fri, 03 Mar 2006 22:12:03 -0800, Tim Wescott <tim@seemywebsite.com> > wrote: > > >>Let's try that again. What in the world does 'WDF' stand for?Woomera Defense Force?> http://www.acronymfinder.com/af-query.asp?Acronym=WDF > > WDF Washington Department of Fisheries ***** > WDF Web Design Factory **** > WDF Web Design Forums **** > WDF Washington Department of Fish **** > WDF World Diabetes Foundation *** > WDF World Development Federation *** > WDF Wave Digital Filters *** > > I think Wave Digital Filter sounds likely.What distinguishes a wave filter from other kinds of signal filters? Is there a wave analog filter? I'm unfamiliar with the jargon. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 4, 20062006-03-04
HelpmaBoab wrote: ...> there is a perfectly good analogue result costing pennies - an angineer can > do for pennies what any fool can do for pounds?It was a perfectly good question. I think the only reasonable answer is that samples have pizazz. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 4, 20062006-03-04
Tim Wescott wrote: ...> Forget the aliasing from sampling, I was talking about the 30kHz carrier > plus 10kHz modulation intemodding in the absolute value thingie. You > start with energy at 20, 30 and 40 kHz, then run it through a whopping > big nonlinearity. You end up with 20 mixing with 30 down to 10 (good), > 30 mixing with 40 down to 10 (also good), but you also end up with the > 40 from the bottom end of the second harmonic mixing with the 30kHz > carrier (bad), etc., etc. I think you'd have to simulate it to really > see if it's going to have issues.If the sample rate is high enough, absolute value (aka full-wave rectification) will suppress the carrier and odd harmonics. There'll be less to alias (aka intermodulate). Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●March 4, 20062006-03-04
Jerry Avins wrote:> Tim Wescott wrote: > > ... > >> Forget the aliasing from sampling, I was talking about the 30kHz >> carrier plus 10kHz modulation intemodding in the absolute value >> thingie. You start with energy at 20, 30 and 40 kHz, then run it >> through a whopping big nonlinearity. You end up with 20 mixing with >> 30 down to 10 (good), 30 mixing with 40 down to 10 (also good), but >> you also end up with the 40 from the bottom end of the second harmonic >> mixing with the 30kHz carrier (bad), etc., etc. I think you'd have to >> simulate it to really see if it's going to have issues. > > > If the sample rate is high enough, absolute value (aka full-wave > rectification) will suppress the carrier and odd harmonics. There'll be > less to alias (aka intermodulate). > > JerryI just checked this -- in simulation at least, with a two-tone input, it's just fine, with _no_ untoward artifacts lower than 50kHz. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/
Reply by ●March 4, 20062006-03-04
HelpmaBoab wrote:> "Jerry Avins" <jya@ieee.org> wrote in message > news:VqCdnY6efNbQgpTZRVn-qg@rcn.net... > >>HelpmaBoab wrote: >> >>>"Mr M" <blobb@bredband.net> wrote in message >>>news:4408965c$0$15784$14726298@news.sunsite.dk... >>> >>> >>>>Hello >>>> >>>> >>>> >>>>I need to sample and demodulate an AM-signal that is between 30 kHz to > > 100 > >>>>kHz. The signal has a bandwidth of about 10 kHz. >>>> >>>> >>>> >>>>What approach should I use? >>>> >>>> >>>> >>>>I'm thinking of using a 16 bit ADC with a maximum sampling frequency of >>> >>>500 >>> >>> >>>>ksps. Maybe using an FPGA to control the ADC. The FPGA can also be used > > to > >>>>do the signal processing. >>>> >>>> >>>> >>>>How high sampling frequency should I use? >>>> >>>> >>> >>>Why not just use a diode and a capacitor - !! >> >>And then what: digitize the result? Maybe. Peak detection is one of the >>more difficult tasks in critically samples systems. >> >>Jerry >>-- >>Engineering is the art of making what you want from things you can get. >>����� > > > I was of course being cynical - why spend a fortune sampling anything when > there is a perfectly good analogue result costing pennies - an angineer can > do for pennies what any fool can do for pounds? > > Tam > >Sometimes there's other things going on that make the processor essential -- then you save pennies by doing the work there, rather than in the outside world. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/
Reply by ●March 4, 20062006-03-04
Joerg <notthisjoergsch@removethispacbell.net> wrote in news:F77Of.18244$rL5.2282@newssvr27.news.prodigy.net:> Hello Al, > >> >> You could do this on an FPGA or you could use a programmable DSP. A >> SHARC could interface the ADC gluelessly and there are enough MIPs >> > > No pun intended but that's going to be expensive and power hungry. My > first thought would be a nice mixer from the analog world. But Mr.M > would have to tell us whether he knows where in the given range the AM > signal will show up. > > Regards, Joerg > > http://www.analogconsultants.com >The choice of a DSP versus FPGA is usually influenced by many other factors. I would agree that if all you needed was the demodulator, that an FPGA might be a better choice. I have designed lots of circuits both analog and digital over many years. I usually start from a broader top down approach of the whole design requirements and then consider which circuits or technologies make the most sense for my situation. Sometimes, the choice is time to market or my own special knowldege. For example, I could program a SHARC to do this application much faster than I could do it in an FPGA. I have used both, but my skills are better with the DSPs. Another engineer, might be very well grounded in FPGAs and choose that technology for the same reason. Analog guys might think we're all crazy and just implement the whole thing with a more traditional analog approach. I brought up the DSP choice to broaden the possibilities. DSPs have all gotten both cheaper and faster over the years. Design tradeoffs are always a changing dynamic. I can certainly attest to this in my career, because I used to be an analog engineer before I started using DSP. -- Al Clark Danville Signal Processing, Inc. -------------------------------------------------------------------- Purveyors of Fine DSP Hardware and other Cool Stuff Available at http://www.danvillesignal.com
