Vladimir Vassilevsky wrote:> > > Jerry Avins wrote: > >>>> A customer's hi-fi system distorted and overheated, but only when he >>>> listened to AM. It turned out that his 25-watt power amplifier was >>>> pumping 22 watts (equivalent voltage) at 456 KHz into the >>>> loudspeaker. (The tweeter got warm too.) > > Oh, come on, Jerry. That can't be due to inductance of the tweeter. The > typical value is about 0.5uH. > > I fixed the post-detection >>>> filter and all was well. So there may be reason to care about the >>>> inaudible stuff. >>> >>> Who woulda' thunk the power amp had a usable response out to 456 kHz??? >> >> >> I thought it was strange too, so I measured the response. It was about >> 30 dB down. > > ???? > > The small signal bandwidth of the decent audio amplifier should be well > above 1MHz. If this 30dB fall is due to the slew rate limitation, then > the full power bandwidth is only about 70kHz. A fair result, not HiFi by > any means.I assume quite a lot of the rolloff was in the output transformer. We did pretty decent HiFi with 6SL7s or KT66s, but they were a lousy impedance match to loudspeakers unless transformer coupled. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
AM demodulation using DSP
Started by ●June 3, 2007
Reply by ●June 4, 20072007-06-04
Reply by ●June 4, 20072007-06-04
Jerry Avins <jya@ieee.org> writes:> [...] > Randy Yates wrote: >> Jerry Avins <jya@ieee.org> writes: >>> You don't even need an explicit filter if you don't care about the >>> inaudible stuff. >> Well it would be audible in this case due to aliasing. > > There would be no aliasing in the demodulated signal. Where would it > come from?I know you know this, Jerry - you must have something else in your head. Take a simple example: Let's say your carrier frequency is 1 MHz and the signal bandwidth is 100 kHz (+/- 50 kHz around a 1 MHz carrier). Then if you bandpass filter the signal at 1 MHz using a filter with a 100 kHz bandwidth, and then sample the signal at 100 kHz, you're done. If you don't use a filter and sample the signal at 100 kHz, then you'll get all the garbage between -950 kHz and 900 kHz down in your baseband as well as the stuff from +/- 950 to 1050 kHz. So you need to filter. -- % Randy Yates % "So now it's getting late, %% Fuquay-Varina, NC % and those who hesitate %%% 919-577-9882 % got no one..." %%%% <yates@ieee.org> % 'Waterfall', *Face The Music*, ELO http://home.earthlink.net/~yatescr
Reply by ●June 4, 20072007-06-04
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes:(snip)>>That's called "synchronous demodulation", no?> I don't know, Jerry. I've heard of that term but haven't seen it defined. > Rick calls this "bandpass sampling" in [lyonsnew].I believe that is the term for demodulating AM-suppressed carrier, such as the FM stereo subcarrier. Well, I suppose there are two ways. One is to recreate the carrier, add it to the received signal, and then demodulate like ordinary AM. The other is the synchronous demodulator, also called a doubly balanced mixer. -- glen
Reply by ●June 4, 20072007-06-04
On Jun 4, 5:36 am, Jerry Avins <j...@ieee.org> wrote:> c1910 wrote: > > hi! > > i want to demodulate AM signal digitally... > > i want to use a digital filter to separate the carrier and the informat=ion> > signal. i've searched a lot...and i haven't found the answer. > > can anyone help?please... > > thx... > > The last time you asked this question, you were told that you can't > demodulate with a filter. The laws of physics haven't changed in th last > week or so. > > Jerry > -- > Engineering is the art of making what you want from things you can get. > =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF==AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF= =AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF=AF One day in the distant future some bright spark will invent the diode and all software people will sit back with awe!
Reply by ●June 4, 20072007-06-04
<gyansorova@gmail.com> schrieb> One day in the distant future some bright spark will > invent the diode and all software people will sit back with awe!def diode(x): if x > 0.7: return x else: return 0.0 Remember, you saw it here first! (Rushing off to patent office) Martin
Reply by ●June 4, 20072007-06-04
Martin Blume wrote:> <gyansorova@gmail.com> schrieb > >>One day in the distant future some bright spark will >>invent the diode and all software people will sit back with awe!It really doesn't make the things any simpler.> > def diode(x): > if x > 0.7: return x > else: return 0.0Totally wrong :) Diode: I = Io (exp(V/Vt) - 1) VLV
Reply by ●June 4, 20072007-06-04
Randy Yates wrote:> Jerry Avins <jya@ieee.org> writes: >> [...] >> Randy Yates wrote: >>> Jerry Avins <jya@ieee.org> writes: >>>> You don't even need an explicit filter if you don't care about the >>>> inaudible stuff. >>> Well it would be audible in this case due to aliasing. >> There would be no aliasing in the demodulated signal. Where would it >> come from? > > I know you know this, Jerry - you must have something else in your > head. > > Take a simple example: Let's say your carrier frequency is 1 MHz and > the signal bandwidth is 100 kHz (+/- 50 kHz around a 1 MHz carrier). > Then if you bandpass filter the signal at 1 MHz using a filter with > a 100 kHz bandwidth, and then sample the signal at 100 kHz, you're > done. > > If you don't use a filter and sample the signal at 100 kHz, then > you'll get all the garbage between -950 kHz and 900 kHz down in your > baseband as well as the stuff from +/- 950 to 1050 kHz. > > So you need to filter."Phase locked to the carrier" would mean sampling at 1 MHz, bringing the upper sideband down to DC to 50 KHz and -50 KHz to DC properly phased to add. Without a filter, you also get other garbage outside that range, inaudibly. Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●June 4, 20072007-06-04
Vladimir Vassilevsky wrote:> > > Martin Blume wrote: > >> <gyansorova@gmail.com> schrieb >>> One day in the distant future some bright spark will invent the diode >>> and all software people will sit back with awe! > > It really doesn't make the things any simpler. > >> >> def diode(x): >> if x > 0.7: return x >> else: return 0.0 > > Totally wrong :) > > Diode: > > I = Io (exp(V/Vt) - 1)A thermionic diode accounting for contact potential? There are more things in heaven and earth, Vladimir, ... :-) Jerry -- Engineering is the art of making what you want from things you can get. ¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Reply by ●June 5, 20072007-06-05
On Mon, 04 Jun 2007 00:14:22 -0400, Randy Yates <yates@ieee.org> wrote:>Jerry Avins <jya@ieee.org> writes: >> [...] >> Randy Yates wrote: >>> Jerry Avins <jya@ieee.org> writes: >>>> You don't even need an explicit filter if you don't care about the >>>> inaudible stuff. >>> Well it would be audible in this case due to aliasing. >> >> There would be no aliasing in the demodulated signal. Where would it >> come from? > >I know you know this, Jerry - you must have something else in your >head. > >Take a simple example: Let's say your carrier frequency is 1 MHz and >the signal bandwidth is 100 kHz (+/- 50 kHz around a 1 MHz carrier). >Then if you bandpass filter the signal at 1 MHz using a filter with >a 100 kHz bandwidth, and then sample the signal at 100 kHz, you're >done.That's not really a synchronous demod, since there's no carrier lock that way. Unless, of course, you do something to appropriately lock the sampling to the carrier. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org
Reply by ●June 5, 20072007-06-05
On Mon, 04 Jun 2007 22:20:34 -0400, Jerry Avins <jya@ieee.org> wrote:>Randy Yates wrote: >> Jerry Avins <jya@ieee.org> writes: >>> [...] >>> Randy Yates wrote: >>>> Jerry Avins <jya@ieee.org> writes: >>>>> You don't even need an explicit filter if you don't care about the >>>>> inaudible stuff. >>>> Well it would be audible in this case due to aliasing. >>> There would be no aliasing in the demodulated signal. Where would it >>> come from? >> >> I know you know this, Jerry - you must have something else in your >> head. >> >> Take a simple example: Let's say your carrier frequency is 1 MHz and >> the signal bandwidth is 100 kHz (+/- 50 kHz around a 1 MHz carrier). >> Then if you bandpass filter the signal at 1 MHz using a filter with >> a 100 kHz bandwidth, and then sample the signal at 100 kHz, you're >> done. >> >> If you don't use a filter and sample the signal at 100 kHz, then >> you'll get all the garbage between -950 kHz and 900 kHz down in your >> baseband as well as the stuff from +/- 950 to 1050 kHz. >> >> So you need to filter. > >"Phase locked to the carrier" would mean sampling at 1 MHz, bringing the >upper sideband down to DC to 50 KHz and -50 KHz to DC properly phased to >add. Without a filter, you also get other garbage outside that range, >inaudibly. > >JerryYeah. that'd work pretty well, as long as there's not significant energy within +/-50kHz of any higher integer multiples of 1MHz. That wouldn't be too hard to accomplish at all, though. And you still have to lock the sampling to the carrier, but that could be done by limiting the carrier itself and using it as the sampling clock. Slick! I like that approach, it's wickedly simple in concept. And it'd work at any IF frequency and the only worry about alias energy is whatever is at integer multiples of the carrier. The limiting amp would still need a reasonable IF filter, though, but that could be a resonator or something else simple. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org
