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

[Lloyd Blythen]

Hi Merv

I never built the Shera GPSDO but have studied it a bit. There were
numerous published versions of the source, with some mods possibly
better-researched than others. Not sure which one you have, or which PIC
chip you're using. Notes accompanying sourcefile PIC402NE_Final.asm say
it's for the PIC16C73, PIC16F73, PIC16F873A and PIC16F876A. From those
notes: "This is the final version of the GPS Discipline [sic] Controller
code released by Brooks Shera with a reformatting for readability and
corrected Auto_N and limiter routines. This is the final public release
code as of 4/10/13 with all fixes."

If you search that sourcefile for "ASCII input routines", you'll find a
summary of serial command formats which can be used for testing. In
particular you can override the DIP-switch settings and command the EFC
drive directly, which may at least let you see how far you can swing the
DAC and what effect that has on your oscillator. Also there's a command to
"replace [the] current PLL set point", which I haven't looked into but
which may allow some calibration.

Best of luck

Lloyd ZL2LEB

On Mon, 29 Aug 2022 at 15:54, 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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