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

[David]

I breadboarded the Shera GPSDO some 7 years ago.  At that time some of
the ICs were no longer available, so I had to make some modifications. 
As I recall, it wouldn't stay locked very long, perhaps because I didn't
adjust the resistor network between the DAC and the OCXO. 

The AD1861 data sheet says its output voltage range is ~ +-3v. Its pin 8
is "NC", so that doesn't matter.  Are you measuring the output voltage
right at the DAC output (pin 9 , 10), or at the VCXO itself?  The
resistor network between these point changes the voltage range. 

It's been quite a while, so I won't claim to be expert on the software
(in PIC assembly language), but I don't see anything there that would
preclude positive DAC outputs.  I don't see a version number in the
source code, so I don't know if we have the same software. 

I have these notes on the DIP switches: 

DIP switches
("on" means low voltage, ie switch closed)
1-3    mode: 0 = setup, 1 = 30 sec time constant, 2-7 = successively
longer time constants
4    ocxo electronic tuning polarity: "on" if ocxo frequency decreases
with increasing EFC voltage
5    "on" allows changes to DAC (& EFC) voltage 

Can you connect the RS-232 output from U10 ( a MAX233 ) pin 5 to a PC to
observe the ASCII output?  If so, I could send you some info on
interpreting the three numbers that are sent.  

I also have some more info on using the DIP switches that I could send
you, as well as a pdf of the original July 1998 QST article. 

Also, you can put an *analog* voltmeter on PIC pin 2. The voltage range
is 0-2. The value will be about 1 if the measured phase difference is
near the controller's set point. 

Dave  WA8YWQ 

On 2022-08-28 20:25, Mervyn Thomas via time-nuts 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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