[time-nuts] GPSDO recovery from holdover

Azelio Boriani azelio.boriani at screen.it
Sat Dec 1 14:17:29 UTC 2012

Here in Europe the use of GPSDOs has dramatically increased with the
transition to the digital TV broadcasting. The single frequency network
(SFN) mode of transmission requires the presence of a time and frequency
reference. The ETSI has a Technical Report (TR101-190) where the maximum
time and frequency error is stated: + or - 1uS for the time (PPS) and 1.1Hz
for the highest carrier frequency. The 1.1Hz max error for the frequency
binds the maximum recovery speed for the PPS. The UHF highest carrier is
858MHz so that the 10MHz max frequency error is 0.01282Hz that is 1.282nS
for every second. To recover, say, 1uS, you need 12.8 hours. Recently
(April 2012), our national broadcaster (RAI TV) research lab (RAI CRIT) has
published an article in his technical magazine (Elettronica e
Telecomunicazioni http://www.crit.rai.it/eletel/ ) about how to test GPSDOs
for the SFN use. They state that the maximum time error, above which the
PPS recovery can be made with a phase jump, is 5uS but they forgot that to
recover 5uS without phase jumps at 1.282nS/S speed is 2 days and 16 hours!

On Sat, Dec 1, 2012 at 11:17 AM, Charles P. Steinmetz <
charles_steinmetz at lavabit.com> wrote:

> Hal wrote:
>  I can see two ways to recover.  One is to jump the 10 MHz clock by 10
>> cycles.
>> The other is to adjust the frequency so that the PPS slews back to
>> on-time.
>> The first approach gives you a second with the wrong number of cycles.
>>  The
>> second approach has your clock frequency off for a while with a trade off
>> between how far off and how long it's off.
> Both the TBolt and HP38xx default to the second method you describe, and
> both can be manually forced to jump to the nearest cycle (TBolt = "jam
> sync," HP = ":SYNChronization:IMMediate"), which gets the PPS within 50 nS.
>  At that point, they revert to the first method and do the last <=50 nS by
> adjusting the oscillator frequency.
> The TBolt allows you to program a "maximum frequency offset," which seems
> as if it should establish a limit on how fast it can correct the PPS, but I
> have never seen one come anywhere near the default maximum offset.  The
> TBolt also allows you to set a "jam sync threshold," which seems as if it
> ought to make the unit jam sync automatically when the threshold is
> exceeded -- but I've never seen one do that, either, even when set to "fast
> recovery."  So far as I have seen, jam sync only occurs manually.
> Best regards,
> Charles
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