[time-nuts] FE-.5680A trimming resolution

[Magnus Danielson]

On 02/02/12 01:17, Azelio Boriani wrote:
> In my opinion the work done locking the VCTCXO of the Oncore is different
> from the TBolt OCXO management: the TBolt steers the OCXO based on the
> received signal instead they locked the Oncore oscillator to a Cs
> reference. Yes, if all the world is the same then there is no difference:
> the Cs locks the VCTCXO frequency where it should be as if steered from
> satellites but how can you drive an OCXO from an Oncore even with hardware
> modifications?. There is an oscillator offset in the @@Ha status message...
> maybe by using it?

Well, what you say is true to some degree, but there is a few things 
which we need to talk separately about not to confuse things.

1) Use of a more stable clock.

The cesium locked Oncore VP and steered OCXO in the Thunderbolt shares 
the same basic aspect of a lower noise level at taus around one and 
longer, compared to most off the shelf GPS units. This gives an 
increased receiver sensitivity which I was pointing at.

2) Steering vs. stable error

A typical off the shelf clock has a fairly OK but still not very great 
TCXO. It's frequency will be off and it will wander around. As a 
consequence the GPS receiver's software will make regular adjustments of 
the receivers time to compensate. With a more stable clock which is 
geared towards a "good" rate, the software routine will just experience 
a very long stable time and do not see very high rate of adjustments.

These adjustments can be found in both books and papers. They are 
obvious in raw-carrier phase measurements among other things.

For GPS receivers which locks up their reference clock, a different 
steering strategy may be employed once the clock have been steered. 
There is by the way far more receivers able to steered their (external) 
clocks if you look around, you will also find the commands to trim their 
loop algorithm, just as the Thunderbolt allows you to.

So, to sum up:

You can get the gain of more stable clock, without locking it, but you 
can use the locking to render better data out of it.

Using rubidiums to steer carrier phase receivers usually suffice to 
provide good data.

Cheers,
Magnus