FM PLL Demodulation

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?

 

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?

 

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.

 

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?

 

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

 

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.

 

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

 

"Jeff Liebermann"

** 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.

** 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

 

"Jeff Liebermann"

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

** Huh ???

There ain't such thing shown.

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


.... Phil

 

"Jeff Liebermann"

** 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

 

"Jeff Liebermann"

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

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

** Nice data.


.... Phil

 

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.

 

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

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.

 

"George is a Wanker "


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


** 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.

 

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.

 

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

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



..... Phil

 

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?

?-)

 

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).

?-)

 

"josephkk"

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