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

[André Balsa]

Hi Mervyn,
I would suggest you use an extremely simple op-amp circuit to level shift
the DAC output by +3V or +4V as you see fit, instead of trying to modify
some 20-year old code which is otherwise perfectly functional.
Just my €0.02.
Cheers,
Andrew

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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