[time-nuts] Information on OVCXO (Don J)

Dave Brown tractorb at ihug.co.nz
Thu Jun 14 08:04:49 EDT 2007

----- Original Message ----- 
From: "Don Johnson" <true-cal at swbell.net>
To: <time-nuts at febo.com>
Sent: Thursday, June 14, 2007 8:41 AM
Subject: Re: [time-nuts] Information on OVCXO (Don J)

> voltage controlled OCXO. I believe that this is the oscillator that
> is used in the SRS SR620 Frequency Counter when it has the 2-ppb
> option-01 timebase (please correct me if wrong info). The unit has
> four terminals simply labled 1-4 on the bottom as well as the case
> studs. The label says it is a +15VDC unit. Can anyone provide the
> pin-out of this oscillator. Short of an actual pin diagram, can
> anyone suggest a way to help identify each pin function, i.e. DC
> resistance, AC impedance, safely applying power with series
> resistance? I do not want to blow this unit as I have high hopes for
> it as a replacement disciplined oscillator in my Trimble Thunderbolt
> GPS timebase. I will be glad to share my conversion experience as I
> proceed with this project. Thanks in advance for any help.
> Don Johnson
> True-Cal Services
So what happened to the original oscillator, Don?
DaveB, NZ

Hi Dave,

I guess you are asking about the original oscillator in the 
Thunderbolt. The symptoms would suggest that the heater has gone bad. 
I use it to track my Rb standard for long term drift. I use an SRS 
SR620 with a Tek display to watch Alan Variance and drift over both 
short and long time intervals. I monitor the rate of time change 
between ch-A (Rb) and ch-B (Thunderbolt) 10MHz signals as they cross 
through 0.0V trigger point. The Thunderbolt has started to do a rapid 
change of >100 ns as my air conditioning cycles in the room. The 
Thunderbolt monitor computer interface program displays only 0.5 
degree change in internal temperature. In the past, I could always 
maintain <20 ns change that resembled a sine wave with about a 
5-minute period (GPS disciplined TC) with little affect from the room 
AC. Now it looks like a sawtooth with the period of my AC.

The original oscillator is an OCXO but not as good as this new double 
ovened SC-cut oscillator. The VC range is the same between the two 
oscillators and the time-constant I can change with the Thunderbolt 



Thanks Don- Yes it was the Thunderbolt oscillator I was interested in- 
I wonder if anyone else has had issues with the oven heater in them? 
Certainly it's not something I have seen reported here recently. 
Seems to me that it would take a complete failure of the heater to 
allow the aircon to have that effect. I don't know what temperature 
the OCXO would have normally operated at but the case surface 
temperature might provide a clue as to what is going on without major 
surgery. If it's cold then I guess your worst fears will have been 

It will be interesting to see how you get on with the replacement 

 DaveB, NZ 

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