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

[Mervyn Thomas]

Hi Askild,

Unfortunately the HP 10811's I have are in a completely enclosed can - 
no holes to poke anything through.  To get at any tuning capacitor would 
probably entail taking the insides out of the can.

The oscillators like mine must have been fairly rare as I am unable to 
find anything via Google.  They have 2 main power leads coming out of 
the can with 6 pin female connectors to connect to the power Power Board 
which is out of an HP Z3801 GPSDO and 2 thin coaxes with SMB connectors 
on the end - one marked as 10MHz and the other as EFC.

Regards,  Merv

On 29/08/2022 1:09 pm, Askild via time-nuts wrote:
> Hi Merv,
>
> I don't know about this GPSDO design, but the HP 10811 OCXO has a coarse
> frequency adjustment capacitor on top of it.
> You should be able to adjust this, so the EFC tuning voltage are again
> inside the range of the potentiometer/DAC of the GPSDO.
>
> Regards,
> Askild
>
>
>
> On Mon, Aug 29, 2022 at 5:56 AM 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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