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FM PLL Demodulation

G

George

Jan 1, 1970
0
I have a wideband FM signal primarily modulated by random voice and music, but the baseband also contains one pure sinusoidal tone at a known fixed frequency and at a relatively low modulation index. The tone is the only thing I want to recover from the FM signal.

The total RF signal occupied bandwidth is much greater than needed for the tone alone. I'd like to narrow the receiver BW for improved threshold. Can I do this using a PLL optimized for the tone frequency?
 
T

Tim Williams

Jan 1, 1970
0
Speaking of FM, I heard rumor that it was possible to deconvolve the message
from the spectrum alone, using Bessel functions -- just not that you would
want to, since it's so much more complicated. I've never seen a reference
to this, anyone got a hint?

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms

I have a wideband FM signal primarily modulated by random voice and music,
but the baseband also contains one pure sinusoidal tone at a known fixed
frequency and at a relatively low modulation index. The tone is the only
thing I want to recover from the FM signal.

The total RF signal occupied bandwidth is much greater than needed for the
tone alone. I'd like to narrow the receiver BW for improved threshold. Can
I do this using a PLL optimized for the tone frequency?
 
M

miso

Jan 1, 1970
0
Your PLL isn't the PLL in the original question. You are talking about
PLLing the demod signal, while the original poster wanted to PLL the
tone right from the FM signal itself. As pointed out, FM in not linear.
[BTW if you do the math, FM bandwidth is infinite.]

I think the CTCSS detection these days is just LPF and a counter. Easy
to do since CTCSS is SF, not MF.
 
G

George

Jan 1, 1970
0
Your PLL isn't the PLL in the original question. You are talking about
PLLing the demod signal, while the original poster wanted to PLL the
tone right from the FM signal itself.



Exactly. I got that and tried to answer what I think he was trying to

ask, instead of what was actually written. I'm guilty of reading

between the lines as charged. If the OP ever returns, we might find

out if my guess is correct.



I see no reason why he would want to demodulate the entire FM

bandwidth in order to recover a single discrete tone. He probably

already has an FM receiver for dealing with the "random voice and

music" which presumably has a functional FM demodulator. Unless he's

building the entire receiver from scratch, which is unlikely because

he's only asking about an FM demodulator, I don't see any reason to

add a 2nd demodulator to the existing receiver. Decoding the audio

output should be sufficient.


As pointed out, FM in not linear.
[BTW if you do the math, FM bandwidth is infinite.]



To avoid having to measure to infinity, we use an occupied bandwidth

measurement. For FCC NBFM, it's Carson's Rule where the occupied

bandwidth is out to where 2% of the total power is in the sidebands

(i.e. 1.0% per sideband):

<http://en.wikipedia.org/wiki/Carson_bandwidth_rule>

For 3GPP (TS 34.121 5.8) it's out to where 1% of the total power is in

the sidebands (i.e. 0.5% per sideband).


I think the CTCSS detection these days is just LPF and a counter. Easy
to do since CTCSS is SF, not MF.



Well, CTCSS is essentially obsolete, replaced by various digital PL or

DCS (digital coded squelch) mutations.

<http://www.n8lhg.com/gmrs5.htm>

These daze, the PL (CTCSS) decoders are all done with a low pass

filter and a period counter. It can be in a PIC

<http://www.cstech.co.uk/pdfs/CTCSS_display_details.pdf>

or as part of uP that controls the radio. Most radios offer PL and

DCS decoding, using the same chip and methods. Fancy radios will also

encode and sometime decode DTMF, MDC1200, 2 tone paging, POCSAG, and

whatever else moves the marketting department.

OP here. I was hoping to extract just the tone from the original FM signalwithout having to demodulate the whole composite baseband first. That would avoid thresholding of the main carrier and allow detection of the tone much lower into the noise.

Conceptually, imagine the carrier modulated by just the tone alone at first.. The mod index is only around 0.3, so the tone is represented essentially by just the first Bessel pair and the carrier. If I could recover just those components at RF I should be able to demodulate the tone. In principle that takes three narrowband receive channels rather than one wasteful wideband one.

Now imagine adding the high-index random audio. That's going to smear the tone components across the entire Carson's Rule BW. But if I could track the original three components (they are the only coherent components in the band) then I should still be able to demodulate the tone. That's what I was hoping to do with the PLL demod with loop filter that is tuned to the tone frequency.

Or am I looking at this wrong?
 
Speaking of FM, I heard rumor that it was possible to deconvolve the message
from the spectrum alone, using Bessel functions -- just not that you would
want to, since it's so much more complicated.  I've never seen a reference
to this, anyone got a hint?

If the tone is really low-level, most of its energy is going to be in
a single pair of RF sidebands anyway. It's reasonable to recover a
narrowband tone from a WBFM signal in many cases, IMHO, if you aren't
pressed for every last dB of SNR.

-- john, KE5FX
 
G

George

Jan 1, 1970
0
Some numbers and details would be helpful.

1. Do you have an existing receiver?

2. If so, what is the IF frequency and bandwidth?

3. Is there an IF output you can grab?

4. What is the approximate tone frequency?

5. If the FM modulation analog, PWM, FSK, BPSK, etc? If it's NOT

analog, then please ignore my suggestions.

6. Any fast response requirements?

7. What do you mean by tracking three components? I thought from

your original description that there was only one tone and that it

didn't move. If you're trying to follow a moving tone around, please

ignore my suggestions.






You're looking at it correctly if you want a universal solution. What

I see if a conventional FM receiver, with a fairly wideband IF and

demodulator. A common FM broadcast receiver would be a good example.



What I would do is borrow some of the IF signal, and filter it through

a narrow band IF crystal or LC filter that's approximately 5 times the

tone frequency (based on Carson's Rule). For example, if the tone is

100Hz and the IF is 10.7Mhz, the filter bandwidth would be about

500Hz. That's a Q of 21.4 so LC should work.



Presumably all the random voice and music is above 100 Hz. That may

be a bad assumption if the random voice and music extends into the

frequency range of the tone. I suspect that's unlikely as nobody

wants to listen to a tone mixed into their music.



After the crystal or LC IF filter, add your favorite FM demodulator,

crystal slope detector, or PLL demodulator for the 10.7MHz. You

should see mostly 100Hz audio coming out which you can detect with

either a 100Hz PLL, period counter, or commercial PL decoder. Don't

bother with de-emphasis if you're using pre-emph and de-emph. It will

just emphasis any jitter, hummmm, and noise below 100Hz.
The signal is standard commercial analog FM broadcast with 15 kHz audio channel cutoff. The tone is 19 kHz. The receiver hardware will be custom designed to whatever is necessary. There may be a lot of receivers built eventually in production.

I'm hoping the PLL demod can be tuned to respond to just the 19 kHz tone and lock to it in the presence of the random audio modulation. (Although thetone is above the audio modulation, at RF it will be buried in the higher-order Bessel spectral components.) The narrow noise BW of the loop filter (~100 Hz?) should give a better overall receive threshold than that of the conventional ~150 kHz FM receiver.

I'm afraid I must be missing something here.
 
T

Tim Williams

Jan 1, 1970
0
Vladimir Vassilevsky said:
Bessel functions apply to simple case of pure sine wave modulation. They
won't help to demodulate arbitrary signal. However it is quite simple to
do
optimal FM demodulation in frequency domain. This could be a good
exercise
in DSP for beginners.

Care to elaborate?

If you track the frequency a little bit at a time, of course, you can get
excellent accuracies from the individual FFTs (or whichever method you
prefer of determining the frequency for that period), or do a sliding FFT,
or a sliding average of the results, or whatever. But true "frequency
domain" means the entirety of the signal (which could be millions of
samples for a practical case, without downconversion that is).

Tim
 
P

Phil Allison

Jan 1, 1970
0
"Jeff Liebermann"
Ok, It's now a standard FM receiver, probably with a 10.7MHz IF.
What you're looking at is something like this:

<http://upload.wikimedia.org/wikipedia/commons/e/ec/Frequency_Spectrum_FM-Radio.gif>

Note the -6dB/octave rolloff from the de-emphasis network.


** Huh ??

There is a +6dB/oct pre-emphasis applied to the audio modulation, comes in
at around 3 to 4 kHz and the receiver un-does that with a matching
de-emphasis in the audio circuit.

Nothing of it becomes visible on that graph.

By time
you get to 19KHz, there's not much left from the original modulation
to trash 19KHz. Note the highly visible 19KHz line. Looks like about
-6dB down from peak audio level. That should be easy to decode.


** The 19kHz pilot tone is modulated at close to 10% of peak audio level or
about +/- 7.5 kHz deviation.

PLLs can be made to lock onto signals that are below noise - so if the OP
filters the signal from the FM detector to a narrow band cantered on 19kHz,
he should have no problem locking onto it with a PLL.


.... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Jeff Liebermann"
I did some Googling and found that that 19KHz pilot tone is suppose to
be about -26 dBV(rms) or 10% of the modulation. I took a quick look
at the local FM stations on my antique HP8554L spectrum analyzer.
Seems about right.

** Ya don't say .....

Also, I screwed up a little in the above paragraph. The -6dB/octave
roll off is due to transmit pre-emphasis.


** Huh ???

There ain't such thing shown.

The generally downward slope of the audio spectrum is due to another cause
entirely.


.... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Jeff Liebermann"
Yeah, I know. I goofed. Thanks.


I think (not sure) that the spectrum plot is directly from the
discriminator output, before de-emphasis, from an FM receiver. The
-6dB/octave is from the transmitter pre-emphasis giving a boost at low
frequencies, and attenuating the highs.


** Fraid that is all wrong.

The audio pre-emphasis of broadcast FM has a time constant of 50uS (or
75uS) in the USA - this means it kicks in only at high audio frequencies.
The turnover ( + 3dB) frequency is 3.2 kHz ( or 2.2 kHz in the USA).

It peaks at about +15dB before disappearing into the steep roll of that
begins at 15kHz.



..... Phil
 
P

Phil Allison

Jan 1, 1970
0
"Jeff Liebermann"
Grumble. I just hate it when I'm wrong (and you're right). It's
neither pre-emphasis or de-emphasis. The -6dB/octave rolloff is some
accidental characteristic of whatever happend to be playing on the
station when the photo was taken.


** That sort of long term, spectrum is typical of classical music.

Bach organ concerto, plus choir maybe ......

I decided to use my IFR-1500 service monitor as a receiver.
<http://802.11junk.com/jeffl/crud/FM-19Khz.jpg>
<http://802.11junk.com/jeffl/crud/FM-19Khz-02.jpg>
That's what it looks like with directly from the discriminator (or
whatever the IFR-1500 uses for an FM demodulator. The 19KHz pilot
tone is clearly visible, but no slope.

** Nice data.


.... Phil
 
T

Tauno Voipio

Jan 1, 1970
0
The signal is standard commercial analog FM broadcast with 15 kHz audio channel cutoff. The tone is 19 kHz. The receiver hardware will be custom designed to whatever is necessary. There may be a lot of receivers built eventually in production.

I'm hoping the PLL demod can be tuned to respond to just the 19 kHz tone and lock to it in the presence of the random audio modulation. (Although the tone is above the audio modulation, at RF it will be buried in the higher-order Bessel spectral components.) The narrow noise BW of the loop filter (~100 Hz?) should give a better overall receive threshold than that of the conventional ~150 kHz FM receiver.

I'm afraid I must be missing something here.


There is a strong smell of stereo pilot tone here.

For a FM stereo, the modulating signal goes well over 15 kHz,
as the difference information is a DSB signal centered on
double the pilot tone frequency, 23 to 53 kHz, so there
may be more mess to handle.

IMHO, you need first to decode the whole multiplex from
the FM before looking for the tone.
 
G

George

Jan 1, 1970
0
Ok, you're not going to answer my questions and prefer to spoon feed

us information while we keep guessing. With all due respect, I don't

have the patience to interrogate you for basic information, especially

when you won't even bother to confirm my guesses.



I think I've mentioned this to you in previous similar exercise, where

you also provided the absolute minimum amount of ambiguous information

in your question and in followups. Please excuse the repetition. If

you want to get sane answer on usenet, you will need to provide:

1. What problem are you trying to solve? A one liner is sufficient.

In this case it would be "I'm trying to design an FM broadcast band

receiver that will provide an indication when a 19KHz pilot tone is

received".

2. What do you have to work with? What existing hardware, software,

programming facilities, devices, do you have available?

3. If for troubleshooting, what have you done so far, and what

happened.









Ok, It's now a standard FM receiver, probably with a 10.7MHz IF.

What you're looking at is something like this:

<http://upload.wikimedia.org/wikipedia/commons/e/ec/Frequency_Spectrum_FM-Radio.gif>

Note the -6dB/octave rolloff from the de-emphasis network. By time

you get to 19KHz, there's not much left from the original modulation

to trash 19KHz. Note the highly visible 19KHz line. Looks like about

-6dB down from peak audio level. That should be easy to decode.









As I previously mentioned, with pre-emphasis, there's no garbage at

19KHz to worry about. If there are any harmonics of the audio, or

intermod mixes, fix your receiver as such distortion will never be

tolerated in a typical FM broadcast receiver.








Correct. The narrower the bandwidth, the higher the sensitivity. See

the typical stereo decoder chip for details. Tell me why you need

extra sensitivity when the above GIF file showing the audio spectra

shows rather fabulous SNR?






Think about an all digital solution. Most FM receivers are all

digital these days, including the cheap receivers found inside

headphones, cell phones, and music players. I'm not really very

familiar with them, but some of them have a 19KHz pilot tone decoder

that lights up a "stereo" light on the front panel. With luck, you

may not need to design very much.





--

Jeff Liebermann [email protected]

150 Felker St #D http://www.LearnByDestroying.com

Santa Cruz CA 95060 http://802.11junk.com

Skype: JeffLiebermann AE6KS 831-336-2558


Gosh fellers ... I've been tryin' to keep this simple ... :eek:)

I'm starting with a standard stereo FM broadcast signal, not with any particular receiver hardware. It probably WILL turn out to be a custom-designedsoftware receiver. But I'm just at the concept stage now. I just want toget the best possible threshold performance for detecting the 19 kHz tone in faded conditions.

If I detect the tone from within the demodulated composite baseband, I willbe limited to the rather high noise floor there which increases 10 dB/dB at threshold. That will be my limit of weak signal performance.

So my objective is to eliminate that wideband noise and detect the pilot directly at RF, taking advantage of the fact that its spectral components will be easy to distinguish (as a sinusoid) from the random program audio surrounding it. Detecting those spectral components (which are just one pair of Bessell tones due to their low mod index) should be doable in a pre-detection noise BW of a couple hundred Hz or so. I hope.

I've never heard of this being done, so maybe I'm way off base, but intuitively it seems valid to me at least.
 
P

Phil Allison

Jan 1, 1970
0
"George is a Wanker "


Gosh fellers ... I've been tryin' to keep this simple ... :eek:)


** You have been taking us on a wild goose chase - and YOU are the lead
goose.


I'm starting with a standard stereo FM broadcast signal, not with any
particular receiver hardware. It probably WILL turn out to be a
custom-designed software receiver. But I'm just at the concept stage now.
I just want to get the best possible threshold performance for detecting the
19 kHz tone in faded conditions.


** There is NO point in that idea whatever !!!

Stereo FM is only of usable quality when an adequate signal level is
available and that signal is free of multipath too.

Below that level, the receiver must switch to mono to improve to s/n ratio
to listenable.

Pilot tone locking is a NON ISSUE

You stupid PITA fuckwit.



.... Phil
 
G

George

Jan 1, 1970
0
"George is a Wanker "





Gosh fellers ... I've been tryin' to keep this simple ... :eek:)





** You have been taking us on a wild goose chase - and YOU are the lead

goose.





I'm starting with a standard stereo FM broadcast signal, not with any

particular receiver hardware. It probably WILL turn out to be a

custom-designed software receiver. But I'm just at the concept stage now.

I just want to get the best possible threshold performance for detecting the

19 kHz tone in faded conditions.





** There is NO point in that idea whatever !!!



Stereo FM is only of usable quality when an adequate signal level is

available and that signal is free of multipath too.



Below that level, the receiver must switch to mono to improve to s/n ratio

to listenable.



Pilot tone locking is a NON ISSUE



You stupid PITA fuckwit.







... Phil


Hi Phil. So nice to hear from you again. If you had read my OP you would have seen "The tone is the only thing I want to recover from the FM signal."

Thanks for contributing.
 
I just want to get the best possible threshold performance for detecting the
19 kHz tone in faded conditions.

If you only want to ride through the fade, why not use a suitable
crystal oscillator (VXCO), derive 38/19 kHz from it. Use the 19 kHz to
lock to the pilot tone and put a sample/hold circuit between the phase
detector and VXCO. When the signal strength drops too low, put the S/H
in Hold mode and the VXCO should remain on the correct frequency. When
the RF returns after thee fade, the VXCO is still at correct frequency
and nearly correct phase, thus relocking would happen immediately,
without any bad phase discontinuities.

These days it might even be possible to decode a _strong_ differential
(L-R) DSB signal (23..53 kHz) without the pilot tone using a Costas
loop.
 
P

Phil Allison

Jan 1, 1970
0
If you only want to ride through the fade,


** The OP said "faded conditions" - so he could sound clever.

God and Satan might know what the posturing, code scribbling ASS really
means.



..... Phil
 
J

josephkk

Jan 1, 1970
0
The signal is standard commercial analog FM broadcast with 15 kHz audio channel cutoff. The tone is 19 kHz. The receiver hardware will be custom designed to whatever is necessary. There may be a lot of receivers built eventually in production.

I'm hoping the PLL demod can be tuned to respond to just the 19 kHz toneand lock to it in the presence of the random audio modulation. (Although the tone is above the audio modulation, at RF it will be buriedin the higher-order Bessel spectral components.) The narrow noise BW ofthe loop filter (~100 Hz?) should give a better overall receive threshold than that of the conventional ~150 kHz FM receiver.

I'm afraid I must be missing something here.

It could well be. The 19 kHz tone is the stereo pilot frequency and it
would be nuts to FM it. However there are Costas loop modulators and
demodulators on that pilot sometimes. Perhaps you are looking for
something to demodulate SCA in the 60 kHz to 75 kHz range of the bare
demodulated audio. Or some other component of the signal?

?-)
 
J

josephkk

Jan 1, 1970
0
Gosh fellers ... I've been tryin' to keep this simple ... :eek:)

I'm starting with a standard stereo FM broadcast signal, not with any particular receiver hardware. It probably WILL turn out to be a custom-designed software receiver. But I'm just at the concept stage now. I just want to get the best possible threshold performance for detecting the 19 kHz tone in faded conditions.

If I detect the tone from within the demodulated composite baseband, I will be limited to the rather high noise floor there which increases 10 dB/dB at threshold. That will be my limit of weak signal performance.

So my objective is to eliminate that wideband noise and detect the pilotdirectly at RF, taking advantage of the fact that its spectral components will be easy to distinguish (as a sinusoid) from the random program audio surrounding it. Detecting those spectral components (whichare just one pair of Bessell tones due to their low mod index) should bedoable in a pre-detection noise BW of a couple hundred Hz or so. I hope.

I've never heard of this being done, so maybe I'm way off base, but intuitively it seems valid to me at least.

I can see one way of doing it digitally, but it won't be cheap. Basically
you need a "comb" filter. 100 MSPS ADC and FPGA to process its output.
Then since you already have the FPGA use it to detect (perhaps even
keeping AM or Costas synchronous LF PM intact).

?-)
 
P

Phil Allison

Jan 1, 1970
0
"josephkk"
What good will it do to improve the 19 KHz recovery when it is still
usable long after the stereo information is long gone? here have been
PLL based stereo decoders since the '80s. National Semicondutors'
LM1800 was one that I saw back then.

<http://pdf1.alldatasheet.com/datasheet-pdf/view/93704/NSC/LM1800.html>

It was used in a lot of car radios back then.

That wasn't even the first IC type. I was seeing them in the early 1970s,
RCA IIRC.


** The first PPL stereo decoder was National Semiconductor's LM1310.

Appeared in about 1975.

It used a simple switching matrix to get L and R outputs.

Earlier ones were full of horrible tuned circuits.



..... Phil
 
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