[time-nuts] Testing a LPRO RB

[WarrenS]

In additional to the standard ADEV answers,
Two important additional performance numbers to consider when testing
and evaluating a RB Osc, are its TempCo, (as in freq change per degC)
and its Ageing rate, (as in drift rate per day).
Anyone with a spare TBolt can measure them.

One simple method to measure the generally elusive Temperature coefficient
and the ageing rate of a high end Osc like the LPRO Rb is by using a
TBolt-Tic and LadyHeather.
Attached is the LH screen dump showing the results of a 7 day run in which
a LPRO is being disciplined using a TBolt that has been modified for use
with a external Osc. see <http://www.ke5fx.com/tbolt.htm>

The Dac Plot, in the attached screen dump, shows what the LPRO's EFC needs
to do to keep the LPRO's freq and phase tracking the GPS over the long term.
>From this Dac plot it is easy to see what the LPRO's tempCo is as well as
what it's ageing rate would be if the unit where allowed to go open loop and
be undisciplined.
With the room temperature changing around +- 1 degC, this units
TempCo is  around 3 e-13/degC, which is measured by adjusting
LH's temperature plot so that it tracks and overlaps as close as possible
the Dac voltage plot.
The 1 week average drift rate shows a very surprising and optimistic
ageing rate of 7e-16/day, over this "special" selected time period,
as verified by measuring the slope of the Dac's trend line.

This is a standard LPRO in the open on a desktop, which is just
sitting on top of a simple heatsink.
As of yet, I have not made any effort to try and get the best from this
LPRO or protect it from changing room temperature.
(the house heater goes on at 7am and cycles somewhat throughout
the day and night)
As can be seen, even over this limited +- 1 deg temperature range,
the change due to temp is by far the dominate error factor.
The first thing this LPRO needs, to bring it up to nut standards,
is the addition of a simple temperature compensating circuit.

At these LH's setting where the temp and Dac plots are made to overlap,
One Dac div = 0.33e-12 freq offset, One Temp div = 1.102 degC,
so TempCo = 0.33e-12 / 1.102degC.
LH's reported aging rate is 6.8e-14 /day which needs to be divided
by 100 because I'm running the unit in the x100 extended TC mode.
(To fool the TBolt's S/W tracking time constant so that it will be slower,
I've told the Tbolt that the Dac gain is 0.36Hz/V when in fact it is only
1/100 of that)

The H/W setup I'm using to disciplined this LPRO's is done by connecting the
LPRO's external C field EFC input thru a 221K 1% resistor, which gives a
measured Dac sensitivity of 1e-10 freq change for a 280mv Dac change.
This equals  0.0035Hz /Volt for this LPRO setup.   28mv for 1e-11,  9,333uv
in the Dac = 3.3e-12 of freq offset per division for the DAC plot.
(resolution is 4e-15 / Dac count)
I set the Tbolt's gain to +0.36 Hz/V to give it the x100 extended TC range
(because the TC needs to be more than the otherwise maximum useable 1000
secs).
The 30 sec TC setting I use causes an effective tracking TC of 3000 sec and
the 8.0 damping factor setting gives an effective 0.8 damping factor.
(1/sqrt_extended_gain)
(The Tbolts tracking TC can be set from 10 seconds (using 0.1 sec) up to
just over one day (using 1000 sec) with this x100 extended configuration.)

Now  set the elevation and AMU, (with this antenna I use 20 deg and 6.5
AMU), and do an extended LH plot until a weeks worth of stable, repeatable
data is obtained.
If the plot shows that the unit is unstable or has not yet stabilized fully,
cleanup the settings where necessary and continue plotting, until you get a
weeks worth of consistent data.


There is another description of how to discipline a LPRO using a Tbolt that
has a bit more details.
http://www.febo.com/pipermail/time-nuts/2011-May/056526.html
http://www.ko4bb.com/dokuwiki/doku.php?id=precision_timing:lpro_disciplining_with_thunderbolt

a 1ms to 1000 sec ADEV plot of a typical LPRO is at:
<http://www.febo.com/pipermail/time-nuts/attachments/20111006/46ca3fbc/attachment-0001.gif>
and
 <http://febo.com/pages/oscillators/rubes/>


ws

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