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.
FM Demodulation
Started by ●July 5, 2008
Reply by ●July 5, 20082008-07-05
"Henry VIII" <donteventhink@emailingmehere.com> wrote in message news:lIDbk.14309$mh5.5367@nlpi067.nbdc.sbc.com...> I have an application where I need to detect an unmodulated tone at afixed> baseband frequency on an FM carrier. The mod index of the tone is quite > low, such that only the first Bessel pair of FM sidebands is significant.Is there any other modulation over FM carrier besides that tone?> Can I demodulate the tone by directly detecting its Bessel components (the > first two sidebands) at RF without first passing the composite FM signal > through a discriminator? Any references to this would be appreciated.If the modulation signal is just one pure tone, the solution is straightforward. If the modulation contains other stuff plus a pure tone, this can also be done although not very trivial.> The purpose of this is to gain detector sensitivity, as it would eliminate > the conventional discriminator threshold.Any apriori knowledge about the signal allows lowering the threshold. Vladimir Vassilevsky DSP and Mixed Signal Consultant www.abvolt.com
Reply by ●July 5, 20082008-07-05
"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message news:YdIbk.13152$uE5.1822@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..
Reply by ●July 5, 20082008-07-05
Henry VIII wrote:> "Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message > news:YdIbk.13152$uE5.1822@flpi144.ffdc.sbc.com... > >> Is there any other modulation over FM carrier besides that tone? > > Yes, there is other information in the baseband besides the tone, but that > other information is never at or near the tone frequency.Nevertheless, that means that the signal can't be treated as NBFM, which would be easier if it were possible.> Also, all that > information is random while the tone is the only continuous unmodulated > signal in the baseband. So the baseband consists of random information such > as audio, and the CW tone is separated in frequency from it.... Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●July 6, 20082008-07-06
Henry VIII wrote:> "Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message > news:YdIbk.13152$uE5.1822@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
Reply by ●July 6, 20082008-07-06
"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message news:Ze3ck.12758$jI5.12353@flpi148.ffdc.sbc.com...> If I remember correctly, you inquired about this problem at some time ago. > It was about FM stereo pilot tone, is that right? > Can you describe the problem as a whole; i.e. what are you trying to > accomplish.> Before getting into the minor technical details, will you please give a > full picture of the problem.Yes, that's correct. The goal is to improve FM stereo receiver pilot detection in the presence of high thermal noise and multipath interference. We want to be able to detect the stereo pilot at C/N levels well below conventional discriminator threshold, and at the same time eliminate the limiter which causes small-signal suppression problems when multipath is present. This will give more balanced receiver performance in environments where the audio can be recovered but the pilot, due to its much lower injection level, can't.
Reply by ●July 6, 20082008-07-06
Henry VIII wrote:> "Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message > news:Ze3ck.12758$jI5.12353@flpi148.ffdc.sbc.com... > > >>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.> Yes, that's correct. The goal is to improve FM stereo receiver pilot > detection in the presence of high thermal noise and multipath interference.What hardware is available? I.e. how big of the DSP you can afford? The practical solution would be optimizing the discriminator and using the narrowband filter with a good PLL to pick up the pilot at the discriminator output. There could be an improvement of ~10dB over the trivial solution just by doing that.> We want to be able to detect the stereo pilot at C/N levels well below > conventional discriminator threshold, and at the same time eliminate the > limiter which causes small-signal suppression problems when multipath is > present. This will give more balanced receiver performance in environments > where the audio can be recovered but the pilot, due to its much lower > injection level, can't.It is certainly possible to do the near-optimal detector for the analog FM composite signal. This would be a tracking frequency estimator based on the prediction of the signal. However I doubt if this has any practical value, especially as the broadcasting changes to digital. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
Reply by ●July 6, 20082008-07-06
Vladimir Vassilevsky <antispam_bogus@hotmail.com> writes:> [...] > estimator based on the prediction of the signal. However I doubt if > this has any practical value, especially as the broadcasting changes > to digital.We've heard a lot lately about the conversion of television to digital, but I haven't heard of a similar thing with broadcast audio (AM/FM). Of course there are things like Ibuiquity's IBOC high-definition radio, but I haven't heard that analog modulations are being retired any time soon. What am I missing? -- % Randy Yates % "Remember the good old 1980's, when %% Fuquay-Varina, NC % things were so uncomplicated?" %%% 919-577-9882 % 'Ticket To The Moon' %%%% <yates@ieee.org> % *Time*, Electric Light Orchestra http://www.digitalsignallabs.com
Reply by ●July 7, 20082008-07-07
"Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in message news:xPack.1005$zv7.194@flpi143.ffdc.sbc.com...> What hardware is available? I.e. how big of the DSP you can afford?Will be all-new design from scratch. There is no specific DSP cost limitation at this time.> The practical solution would be optimizing the discriminator and using the > narrowband filter with a good PLL to pick up the pilot at the > discriminator output. There could be an improvement of ~10dB over the > trivial solution just by doing that.Yes, but can the discriminator be eliminated altogether to achieve even better C/N performance? The great majority of the FM Carson's Rule bandwidth is due to the high modulation index audio. Why deal with all that pre-detection bandwidth if we don't care about the audio? So is it realistic to simply detect the pilot's spectral sidebands at RF - no discriminator - and demodulate the pilot from them? Say we can achieve filters of a few tens of Hz bandwidth centered on the pilot's Bessel components. That's a noise bandwidth improvement of roughly 30 dB or so over conventional discriminator. Does this approach make sense?
Reply by ●July 7, 20082008-07-07
"Henry VIII" <NoBodyHome@SSSpamavoidance.com> wrote in message news:C1hck.14800$N87.6262@nlpi068.nbdc.sbc.com...> "Vladimir Vassilevsky" <antispam_bogus@hotmail.com> wrote in > message news:xPack.1005$zv7.194@flpi143.ffdc.sbc.com... > > Why deal with all that pre-detection bandwidth if we don't > care about the audio? So is it realistic to simply detect > the pilot's spectral sidebands at RF - no discriminator - and > demodulate the pilot from them? Say we can achieve filters > of a few tens of Hz bandwidth centered on the pilot's Bessel > components. That's a noise bandwidth improvement of roughly > 30 dB or so over conventional discriminator. Does this > approach make sense?Not to me. The pilot's spectral components are all mixed up with components that belong to other features of the signal (as well as noise). Detection by analyzing the raw RF seems like a non-starter. However, if you feel you must pursue this, you might consider multi-tone filtering, followed by envelope detection of the filters, followed by a neural network to recognize the pilot's spectral characteristics. I probably wouldn't try this because one of the problems with very narrow filters is that when excited by noise, they tend to ring at the frequency they are designed to detect.> >
