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

Don Johnson true-cal at swbell.net
Wed Jun 13 16:41:20 EDT 2007

> I just become the owner of a Precision Crystal EROS-750- SBR -4 
> 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 software.

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