[time-nuts] V standards

[Bruce Griffiths]

WarrenS wrote:
> Bruce 
>
> Does this mean that by "using carrier phase disciplining techniques"  
> One could get two orders less noise than  is shown 
> on the SRS page#3  graph http://www.thinksrs.com/downloads/PDFs/Catalog/PRS10c.pdf
>
> Is this also better than you get when using the 1PPS correction sawtooth?
> Do you know of a posted plot that shows the noise you get using the Carrier phase techniques?
>
> Do please understand that my questions about using pots etc, 
> are to get a better understand about what is going on, 
> Not suggestions on how to make one.
>
> Thanks,
> WarrenS
> ************************
>
>   
Warren

Yes the carrier phase technique does have a noise level about 2 or more
orders of magnitude lower for an averaging time of 10 seconds.
At averaging times of around 1 day or so the noise levels are limited by
the SV oscillator instabilities and are comparable with the noise level
of a PPS based system.

Some of the complications are:

1) not all GPS receivers make the carrier phase observable accessible to
the user

2) The GPS receiver local oscillators have to be phase locked to the
OCXO being disciplined as the the carrier phase observables are actually
measures of the GPS LO phase errors.
This introduces some complexity except for receivers like the Novatel
Superstar that use a 10MHz local oscillator.
Most geodetic class GPS receivers require a 5MHz, 10Mhz or 20MHz
external frequency standard input (the input frequency depends on the
receiver model).

3) Extracting timing data from carrier phase observables is somewhat
complex due to an unknown offset that has to be determined.
Extracting useful information for disciplining the OCXO frequency is
somewhat easier and doesn't need the the extensive post processing and
precise orbits etc required for time and position comparisons when using
a geodetic receiver.

4) GPS carrier cycle slips also have to be accounted for.

5) The GPS reciver LO has to be phase locked to the OCXO unless the
realtive phase of the GPS LO and the OCXO is measured, however this
requires additional hardware.

Some of the advantages are:

1) All the required measurement hardware is built in to the GPS
receiver, no add on phase comparators etc are required.
However a DAC is still required to drive the OCXO EFC input.

2) The measurement resolution and phase are much lower.

Posted plots are hard to find but some idea of the performance can be
gleaned from the commercial carrier phase disciplined OCXO performance
specs and from data extracted from survey data obtained using arrays of
carrier phase tracking receivers (~ 3E-11 noise for an averaging time of
1 sec when using the TCXO embedded in a Novatel Superstar receiver).

Your questions are useful in that they (or at least the replies
generated in response to them) help clarify various points/issues.
One point that I should have mentioned with pots is that cermet, carbon
and conductive plastic pots are fine when there is no dc current flowing
in them.
However they produce significant flicker noise when significant dc
current is flowing in them.
Wire wound and thin film pots are much quieter.
However precision thin film pots tend to degrade/wear out after a
relatively low number of adjustments.
Wire wound pots are more durable but have limited resolution.

Bruce