[time-nuts] Re: James Miller/Brooks Shera 10 MHz GPSDO

[Bob kb8tq]

Hi

The typical HP 10811 OCXO has a manual tuning adjust. You can re-tune
the unit over a pretty wide range by playing with a tuning tool. That should
allow you to hit whatever EFC target you need. 

The AD1861

https://www.analog.com/media/en/technical-documentation/data-sheets/ad1851_1861.pdf <https://www.analog.com/media/en/technical-documentation/data-sheets/ad1851_1861.pdf>

Should put out +/- 3V according to the above data sheet. I don’t have
a schematic of the GPSDO here with me so there may be some other 
“stuff” involved. 

Bob

> On Aug 28, 2022, at 7:25 PM, Mervyn Thomas via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi All,
> 
> I'm new to time nuts but here goes.
> 
> I built a GPS disciplined  10MHz osc about 15 + years ago.  It uses an HP 10811 D/Oven Oscillator and the Brooks Shera PCB and software V402N.  The physical construction article was per James Miller G3RUH.
> 
> This unit has worked flawlessly for all these years but now is unable to hold the freq to precisely 10MHz - it is constantly .29 to.60 Hz high.  The DAC output which controls the EFC into the osc outputs a negative only adjustable voltage between -3v down to 0V and is settable by varying a 10K potentiometer.  The Osc. now needs abouit +0.46V to bring it down to 10MHz but the DAC does not seem able to output a positive voltage.  The DAC is an AD1861 (obsolete) and has a +5V applied to pin 16 and a -5V applied to pins 1 and 8.
> 
> My first reaction was the HP osc. had reached the end of it's life but the datasheet states the EFC control voltage can be between a plus 5V and a minus 5V which varies the freq by 1Hz so it seems it is just over half way through it's life.
> 
> I feel the Brooks s/ware would have provided some method to ensure the DAC output could be adjusted to swing the full full 5V or at least 3V?  I have the source code but I'm no programmer so would appreciate any help someone can provide please.
> 
> 73 de Merv  VK6BMT
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