[OZAPRS] on pilot tones and freq references

[Glen English VK1XX]

In general, I would say >95%, the 19kHz pilot tones of a FM radio
station are free running, derived from a nothing special crystal. Don't
expect better than +/- 50 ppm.

>From 1970 to ~ 1992 stereo generators were analog type , xtal derived
+/- 100ppm, but usually within +/- 1Hz (+/- 52 ppm)

When digital signal processing based audio processing began to appear,
the 19kHz pilot was being derived from the sample rate clock. This is
usually better than at least +/-25ppm.

In the 2000s there has been a gradual move in the very most advanced and
newest cap city stations to fully integrated audio chains from digital
consoles to digital modulation FM transmitters, with the audio processor
being fed by AES/EBU, however there is still an asynchronous sample rate
converter and the 19kHz clock is likely to be still supplied by a +/-
25ppm oscillator.
In some cases the FM modulator/transmitter is locked to a GPS, however
this is generally asynchronous with respect to the input sample rate
(and hence 19kHz pilot) because of the restrictions on SR in the modulator.

There is no required accuracy for 19 kHz pilot to function, as receivers
have a VCO in their FM demodulator that gets locked to the incoming
19kHz tone, and used to demodulate the DSB 38kHz stereo signal (which is
AM- being DSB).

There are no easy good off air sigs. If you can demodulate (despread)
the CDMA signal of 3.84Mcps this is probably the best source.

But that's if you want Hz, but most people with their radio probably are
just happen with 100Hz at 100 MHz, which is 1ppm. You can get that with
a half decent off the shelf reasonably aged TCXO or OCXO.

The DVBT  (digital TV) signals that are in single frequency networks are
locked to the GPS, if you can figure out how to demod those, they will
be excellent.

The GPS locked ham beacons are an excellent option.

-Glen.

On 19/11/2017 10:39 PM, Matthew Cook wrote:
> Gavin,
>
> You can go and find the stereo pilot tones in your commercial FM radio
> station those are GPS locked rubidium.   We're lucky here in VK5 that
> our VHF/UHF beacons are GPS locked, so I typically go and use these to
> zero in RTL-SDR's to a few hertz... you might find the ones in Perth
> are of a similar nature, never hurts to ask.
>
> 73
>
> Matthew
> VK5ZM
>
> On 19 November 2017 at 14:56, Gavin Rogers
> <grogers at vk6hgr.echidna.id.au <mailto:grogers at vk6hgr.echidna.id.au>>
> wrote:
>
>     Maybe different in other states, but in Perth the only carrier
>     with GSM still is Vodadone, and their coverage isn't good enough
>     where I am to get a good calibration.
>
>
>
>     On 19 November 2017 11:21:49 am AWST, Peter VK2MPJ
>     <vk2mpj at yahoo.com.au <mailto:vk2mpj at yahoo.com.au>> wrote:
>
>         I’m curious why you say that using kal and a GSM signal is not
>         possible anymore?
>         I did this locally just last weekend…
>
>         2MPJ
>
>
>>         On 19 Nov 2017, at 12:42 pm, Gavin Rogers
>>         <grogers at vk6hgr.echidna.id.au
>>         <mailto:grogers at vk6hgr.echidna.id.au>> wrote:
>>
>>         On 19/11/2017 9:30 AM, vk4tec at tech-software.net
>>         <mailto:vk4tec at tech-software.net> wrote:
>>>
>>>         Or, as i've mentioned before, just link to the existing
>>>         Direwolf SDR+APRS guide, which covers off setting up an
>>>         igate pretty
>>>         well: https://github.com/wb2osz/direwolf/blob/master/doc/Raspberry-Pi-SDR-IGate.pdf
>>>         <https://github.com/wb2osz/direwolf/blob/master/doc/Raspberry-Pi-SDR-IGate.pdf>
>>
>>         In this guide, there's a link to a script that will tune to a
>>         NOAA frequency and re-calibrate an SDR TV stick
>>         ( https://github.com/khaytsus/direwolf-init
>>         <https://github.com/khaytsus/direwolf-init> )
>>
>>         What would be a good high-stability signal source here in VK
>>         for the calibration? I used to use the kal program to
>>         calibrate against GSM transmitters, but that's not possible
>>         any more. I'd think the trunking control channel for an
>>         emergency services' channel would be stable, strong and
>>         always on? Or DAB+ transmitter, or..?
>>
>>
>>         73
>>         Gavin.
>>
>>
>>         -- 
>>
>>               Gavin Rogers                     |  Amateur radio station VK6HGR     
>>             http://www.livingwaters.com/good
>>         <http://www.livingwaters.com/good>   |     http://vk6hgr.ampr.org/
>>                        MSN/Skype/Email: grogers at vk6hgr.echidna.id.au
>>         <mailto:grogers at vk6hgr.echidna.id.au>
>>         _______________________________________________
>>         OZAPRS mailing list
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>>         http://lists.aprs.net.au/mailman/listinfo/ozaprs
>>         <http://lists.aprs.net.au/mailman/listinfo/ozaprs>
>
>
>     -- 
>     Gavin Rogers mobile
>
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