FM Demodulation
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From: Henry VIII on 5 Jul 2008 01:33 I have an application where I need to detect an unmodulated tone at a fixed baseband frequency on an FM carrier. The mod index of the tone is quite low, such that only the first Bessel pair of FM sidebands is significant. Can I demodulate the tone by directly detecting its Bessel components (the first two sidebands) at RF without first passing the composite FM signal through a discriminator? Any references to this would be appreciated. The purpose of this is to gain detector sensitivity, as it would eliminate the conventional discriminator threshold.
From: Vladimir Vassilevsky on 5 Jul 2008 05:40 "Henry VIII" <donteventhink(a)emailingmehere.com> wrote in message news:lIDbk.14309$mh5.5367(a)nlpi067.nbdc.sbc.com... > I have an application where I need to detect an unmodulated tone at a fixed > baseband frequency on an FM carrier. The mod index of the tone is quite > low, such that only the first Bessel pair of FM sidebands is significant. Is there any other modulation over FM carrier besides that tone? > Can I demodulate the tone by directly detecting its Bessel components (the > first two sidebands) at RF without first passing the composite FM signal > through a discriminator? Any references to this would be appreciated. If the modulation signal is just one pure tone, the solution is straightforward. If the modulation contains other stuff plus a pure tone, this can also be done although not very trivial. > The purpose of this is to gain detector sensitivity, as it would eliminate > the conventional discriminator threshold. Any apriori knowledge about the signal allows lowering the threshold. Vladimir Vassilevsky DSP and Mixed Signal Consultant www.abvolt.com
From: Henry VIII on 5 Jul 2008 15:22 "Vladimir Vassilevsky" <antispam_bogus(a)hotmail.com> wrote in message news:YdIbk.13152$uE5.1822(a)flpi144.ffdc.sbc.com... > Is there any other modulation over FM carrier besides that tone? Yes, there is other information in the baseband besides the tone, but that other information is never at or near the tone frequency. Also, all that information is random while the tone is the only continuous unmodulated signal in the baseband. So the baseband consists of random information such as audio, and the CW tone is separated in frequency from it. > If the modulation signal is just one pure tone, the solution is > straightforward. If the modulation contains other stuff plus a pure tone, > this can also be done although not very trivial. Hints to get me started on the second case please? > Any apriori knowledge about the signal allows lowering the threshold. The only thing known a priori is the unmodulated tone, and its frequency is precise. My main objective here is to lower the threshold as much as possible, and preferably to eliminate the discriminator knee by doing direct detection of the tone's spectrum. We don't need to handle the entire Carson's Rule bandwidth, we only need to deal with the spectral components of the pure tone. Would appreciate suggestions on how to pursue this..
From: Jerry Avins on 5 Jul 2008 18:49 Henry VIII wrote: > "Vladimir Vassilevsky" <antispam_bogus(a)hotmail.com> wrote in message > news:YdIbk.13152$uE5.1822(a)flpi144.ffdc.sbc.com... > >> Is there any other modulation over FM carrier besides that tone? > > Yes, there is other information in the baseband besides the tone, but that > other information is never at or near the tone frequency. Nevertheless, that means that the signal can't be treated as NBFM, which would be easier if it were possible. > Also, all that > information is random while the tone is the only continuous unmodulated > signal in the baseband. So the baseband consists of random information such > as audio, and the CW tone is separated in frequency from it. ... Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
From: Vladimir Vassilevsky on 6 Jul 2008 08:53 Henry VIII wrote: > "Vladimir Vassilevsky" <antispam_bogus(a)hotmail.com> wrote in message > news:YdIbk.13152$uE5.1822(a)flpi144.ffdc.sbc.com... > > >>Is there any other modulation over FM carrier besides that tone? > > > Yes, there is other information in the baseband besides the tone, but that > other information is never at or near the tone frequency. Also, all that > information is random while the tone is the only continuous unmodulated > signal in the baseband. So the baseband consists of random information such > as audio, and the CW tone is separated in frequency from it. If I remember correctly, you inquired about this problem at some time ago. It was about FM stereo pilot tone, is that right? Can you describe the problem as a whole; i.e. what are you trying to accomplish. >>If the modulation signal is just one pure tone, the solution is >>straightforward. If the modulation contains other stuff plus a pure tone, >>this can also be done although not very trivial. > > Hints to get me started on the second case please? Short term correlation. > >>Any apriori knowledge about the signal allows lowering the threshold. > > > The only thing known a priori is the unmodulated tone, and its frequency is > precise. My main objective here is to lower the threshold as much as > possible, and preferably to eliminate the discriminator knee by doing direct > detection of the tone's spectrum. We don't need to handle the entire > Carson's Rule bandwidth, we only need to deal with the spectral components > of the pure tone. Would appreciate suggestions on how to pursue this.. Before getting into the minor technical details, will you please give a full picture of the problem. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
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