[time-nuts] Allan variance of Thunderbolt

[John Miles]

I believe the Allan variance graph in Trimble's data sheet was taken before
Selective Availability was turned off.  I'm not sure what impact this would
have; I wouldn't expect any at all at tau << 100s.

A pronounced hump in the ADEV plot could suggest that the disciplining loop
is underdamped.  Ideally you wouldn't be able to tell where the OCXO's
influence ends and GPS's begins.

-- john, KE5FX

> -----Original Message-----
> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
> Behalf Of Mark Sims
> Sent: Friday, July 11, 2008 2:51 PM
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] Allan variance of Thunderbolt
>
>
>
> The Thunderbolt data sheet says that the unit requires 25mA at
> 12V.  That spec would imply it is for a unit without an OCXO
> since the oven heater draws considerable current from the 12V
> power supply.  But then,  the data sheet says it draws 15 watts
> cold, 10 watts steady state.  That implies the unit has a heater.
>  And if you multiply the published  power supply requirements out
> you get 2 watts,  not 10-15 watts.  It appears that some of the
> specs are for an ovenless unit,  others are for the ovenized unit.
>
> The data sheet also talks about a telecom version with improved
> holdover performance...  this has got to be the ovenized unit.  I
> suspect the Allan variance plot is for the low end unit,  but the
> shape of the curve is funny.  It always slopes downward.  Almost
> all GPSDO Allan variance plots show a hump in the graph around
> 100 seconds.
>
> Trimble's numbers and graph shape do seem to relate closely to my
> "auto-Allan variance" plots of the OSC offset data supplied by
> the unit (where the tiiming reference is its own interpretation
> of the GPS time signal).
>