[time-nuts] Interesting paper: Don't GPSD' your Rb...

Bill Hawkins bill at iaxs.net
Mon May 7 03:12:08 UTC 2012


Thanks for the analysis, Magnus.

A few other time constants might be interesting -

When a step change is made to the control voltage or current,
how long does it take for the oscillator to settle down to a
new value? Is it instantaneous compared to a second?

Do different components in different oscillators affect the
settling time?

It is not useful to make the next change before the last one
is complete, at least for sampled systems. Using counters
filters the change rather than taking a sample.

Bill Hawkins
 

-----Original Message-----
From: Magnus Danielson
Sent: Sunday, May 06, 2012 4:45 PM
To: time-nuts at febo.com
Subject: Re: [time-nuts] Interesting paper: Don't GPSD' your Rb...

On 05/06/2012 09:01 PM, Bill Hawkins wrote:
> Here is a different tactic for disciplining Rb from GPS/TXCO -
>
> Consider the relatively (relative to a second) long stability of
> an Rb oscillator and the not-so-good stability of GPS. Perhaps
> using 1 PPS for a sampling period for stabilizing Rb is way too
> short. Maybe 1000 seconds is better. That's way too long for an
> analog integrator to do, so a microcomputer is required.

Using analog integrator for 1 PPS stuff is hardish and your performance 
will most probably suffer. You want your integrator to be digital as it 
is clearly a better memory over time.

However, this is not to be confused with the time-constant, which should 
be much higher than 1000 s to use the stability of the rubidium where 
the GPS noise is worse. See PRS-10 manual for good illustration.

> Count the Rb and GPS 1 PPS signals for 1000 counts of Rb 1 PPS.
> You'll need to interpolate between 1 PPS GPS intervals to the
> level of accuracy required, so maybe we count cycles of 10 MHz
> from the GPS, using as many registers (cascaded integer counters)
> as required for 1 E10 (or more) counts (2540BE400 hex). At the
> end of 1000 seconds, use the difference between the lowest counter
> reading and 0xEB400 (times an appropriate gain) to tweak the value
> for the DAC doing the fine correction to the control voltage or
> current. Use the upper counters for a sanity check on the reading.

Why wait with the updates of the DAC? By incrementally average for each 
second I think you get a smoother transitions.

You sure want time-constants in excess of 1000 s, but you can achieve 
that by using 1 s updates. Again, read the PRS-10 manual for a fairly 
good description.

> As may be evident, I have never tried to discipline an Rb, but I
> am well aware of the effect of sample time on the control of a
> long time constant loop. True, the effect is strongest when dead
> time dominates the time constant, but that is not the case here.
> Still, I think there is value in using a long sampling time for
> the control action.

I fail to see the benefit, and I have many times learned the hard way to 
see the downside of too low update rate.

> Comments accepted with enthusiasm.

:)

Cheers,
Magnus

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