Forums

AM demodulation

Started by luke_ November 6, 2006
Hi there!


I'm finding hard to get information about "switched gain AM demodulation"
and how to implement it.

Apparently this demodulation  techique involves synchronization with the
signal carrier..

Can anybody provide some hints?

Regards,
Luke



On Mon, 06 Nov 2006 09:24:25 -0600, "luke_" <ltrizio@tiscalinet.it> wrote:

>Hi there! > > >I'm finding hard to get information about "switched gain AM demodulation" >and how to implement it. > >Apparently this demodulation techique involves synchronization with the >signal carrier..
Well, I don't know what switched gain means here, but I do know that AM can be demodulated by developing a sine wave locked to the carrier (using a phase-locked loop). Then this sine wave (and its quadrature cousin, cosine) can multiply the signal over short periods of time. CosSum = sum of cosine * sig SinSum = sum of sine * sig Then you take (CosSum^2 + Sinsum^2) ^ (1/2) as the demodulated signal for that short time period. If the phase lock is good enough, you don't need to use the cosine at all. Just take the SinSum as the demodulated signal, because if the PLL lock was good, the CosSum will be zero anyway. I think this method of demodulation has certain advantages over simple envelope detection, but I don't know what they are. Robert Scott Ypsilanti, Michigan
luke_ wrote:

> Hi there! > > > I'm finding hard to get information about "switched gain AM demodulation" > and how to implement it. > > Apparently this demodulation techique involves synchronization with the > signal carrier.. > > Can anybody provide some hints? > > Regards, > Luke > > >
It sounds like that is someone's term for a particular kind of synchronous AM demodulation. Searching on that will certainly get you a lot of hits, and may lead you to what you're looking for. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html
Robert Scott wrote:

> On Mon, 06 Nov 2006 09:24:25 -0600, "luke_" <ltrizio@tiscalinet.it> wrote: > > >>Hi there! >> >> >>I'm finding hard to get information about "switched gain AM demodulation" >>and how to implement it. >> >>Apparently this demodulation techique involves synchronization with the >>signal carrier.. > > > Well, I don't know what switched gain means here, but I do know that AM can be
-- snip --
> > I think this method of demodulation has certain advantages over simple envelope > detection, but I don't know what they are. >
AM signals can suffer from a variety of degradations that make simple envelope demodulation less than optimal. The worst offender is selective fading, where one sideband (and possibly the carrier) is severely attenuated; this can distort the signal severely. Even severe noise can cause envelope demodulation to be worse than other methods. Synchronous demodulation of AM, and it's close cousin exalted carrier demodulation, both work on the principal that if the carrier is much bigger than the sidebands then the demodulated signal will be less sensitive to selective fading and noise. Fixing this on the transmit side would involve transmitting way more power for the actual signal transmitted, so it's fixed in the receiver, either by synthesizing a carrier signal that's locked to the actual signal and adding it in before demodulation (synchronous detection), or by running the carrier through a very narrow band filter and adding it back in before demodulation (exalted carrier). -- Tim Wescott Wescott Design Services http://www.wescottdesign.com Posting from Google? See http://cfaj.freeshell.org/google/ "Applied Control Theory for Embedded Systems" came out in April. See details at http://www.wescottdesign.com/actfes/actfes.html

> > I think this method of demodulation has certain advantages over simple envelope > detection, but I don't know what they are. > >
I believe one of the advantages of synchronouse demodulation is the elimination of the threshold effect where the demodulated baseband signal SNR degrades very rapidly when the carrier CNR drops below a certain (threshold) value around 10 dB. Therefore a synchronous AM demod should work better under poor CNR conditions. It also has an advantage in VSB type AM TV detection, but probably no one is working on a new design for one of these... Mark
Mark wrote:
> >> I think this method of demodulation has certain advantages over simple envelope >> detection, but I don't know what they are. >> >> > > I believe one of the advantages of synchronouse demodulation is the > elimination of the threshold effect where the demodulated baseband > signal SNR degrades very rapidly when the carrier CNR drops below a > certain (threshold) value around 10 dB. Therefore a synchronous AM > demod should work better under poor CNR conditions. > > It also has an advantage in VSB type AM TV detection, but probably no > one is working on a new design for one of these...
Sideband fading can cause severs distortion even if the CNR remains high. When one sideband is completely missing, an envelope detector produces the square root of the original modulation. When the percentage modulation is very small, the square root is nearly linear over its small range. Exalting the carrier reduces the effective modulation percentage that the detector "sees". Synchronous detection is the injection of a strong signal in phase with the carrier. That does the same thing. Think of demodulating SSSC with BFO injection. Jerry -- "The rights of the best of men are secured only as the rights of the vilest and most abhorrent are protected." - Chief Justice Charles Evans Hughes, 1927 &#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 many suggestion I got so far.

Does anybody know how to implement a analogue mixer (basically a building
block that multiplies its two inputs, and sends the analogue result to the
output) ??

I can't buy a demodulator chip or use any digital technique for my
project...so I have to implement AM synchronous demodulation with discrete
components (MOS, BJT, resitors, caps...)? 

Regards,
Luke


>Mark wrote: >> >>> I think this method of demodulation has certain advantages over simple
envelope
>>> detection, but I don't know what they are. >>> >>> >> >> I believe one of the advantages of synchronouse demodulation is the >> elimination of the threshold effect where the demodulated baseband >> signal SNR degrades very rapidly when the carrier CNR drops below a >> certain (threshold) value around 10 dB. Therefore a synchronous AM >> demod should work better under poor CNR conditions. >> >> It also has an advantage in VSB type AM TV detection, but probably no >> one is working on a new design for one of these... > > >Sideband fading can cause severs distortion even if the CNR remains >high. When one sideband is completely missing, an envelope detector >produces the square root of the original modulation. When the percentage
>modulation is very small, the square root is nearly linear over its >small range. Exalting the carrier reduces the effective modulation >percentage that the detector "sees". Synchronous detection is the >injection of a strong signal in phase with the carrier. That does the >same thing. Think of demodulating SSSC with BFO injection. > >Jerry >-- > "The rights of the best of men are secured only as the > rights of the vilest and most abhorrent are protected." > - Chief Justice Charles Evans Hughes, 1927 >&#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; >
luke_ wrote:
> Thanks for the many suggestion I got so far. > > Does anybody know how to implement a analogue mixer (basically a building > block that multiplies its two inputs, and sends the analogue result to the > output) ?? >
Google for "balanced mixer". Google for "four-quadrant analog multiplier"
John Hadstate wrote:
> luke_ wrote: >> Thanks for the many suggestion I got so far. >> >> Does anybody know how to implement a analogue mixer (basically a building >> block that multiplies its two inputs, and sends the analogue result to the >> output) ?? >> > > Google for "balanced mixer". > > Google for "four-quadrant analog multiplier"
That's all you need for a modulator, but you also need a source of in-phase carrier to demodulate synchronously. A phase-locked loop (PLL) can be the source of that. 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;