[time-nuts] GPSDO recovery from holdover

[Magnus Danielson]

Azelio,

On 12/01/2012 03:17 PM, Azelio Boriani wrote:
> 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.

ETSI TR 101 190 V[1] only suggests +/- 1 us in the theoretical case, the 
actual requirement is much slacker, typically +/- 1.5 us to +/- 3 us is 
being used in real networks. It even suggests +/- 5 us if you read 
clause 5.2.4 [1]. An ETSI TR (Technical Report) never specifies 
anything, but provides guidance, which is why it suggest GPS receiver as 
uses, but do not require it.

Many of the european networks is GPS-free, so there is not as many 
GPSDOs as you would think.

Recall that 1 us time-shift, would be equivalent of moving the 
transmitter-tower by 300 m. Timing requirements is to handle 
interference between transmitters that a reciever can see at the same 
time, so time-differences in the air is what counts. Recall that +/- 5 
us is just +/- 1.5 km.

I can't find the 1.1 Hz carrier frequency error. However, looking at 
clause 8.3.1 [1] indicates that the frequency error should be around 
1/1000 of the carrier distance delta-f, which in EN 300 744 [2] Table 
E.4 is listed as 978,563 Hz for the typical 8k mode system.

Clause 8.3.2 on time synchronisation gives a good

> 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.

The frequency error is of less concern than the drift of phase, i.e. how 
long does it take to drift outside of the allocated time-alignment and 
then back again.

> 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 have a nice test-rigg in there and a R&S XSRM rubidium for local 
reference.

Could you provide a direct link to the article you are refering to?

> 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!

[1] ETSI TR 101 190 V1.3.2
[2] ETSI EN 300 744 V1.6.1

Cheers,
Magnus