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

[ed breya]

Since it appears there's no nice and easy way to access the OCXO coarse 
tuning cap (there almost certainly is one in there, whether it's 
accessible or not), a practical fix is a simple circuit mod (add one 
resistor) to the DAC circuit.

I looked up the AD1861, and see that it has the typical built in opamp 
and feedback resistor for current to voltage conversion. Also, 
apparently, depending on hookup and the data formatting, it can be 
arranged to go between +/- 3V output. The simplest way to fix the output 
range issue, without changing the basic hookup, any software, or VCO 
gain factors, is to just add pure DC offset current at the summing node.

If you need the output voltage to be more plus, then pull some current 
from the summing node into the -5V supply through a resistor. The FS 
current from the DAC seems to be around 1-2 mA, so a value for R in the 
few to tens of k-ohms range can move it significantly. Just figure out 
what the scaling actually is, and set it up for enough offset to keep 
the range good for a while - you can adjust again as it ages. Two things 
that will be different from original is that the external offset R won't 
track the internal feedback R thermally, and using the minus 5V for a 
reference may not be as clean as you'd want. The data sheet I found 
doesn't say much about the internal reference, but I'm guessing it's a 
+1.25 or 2.5V bandgap. Or, it may simply be the +5V supply.

Since it's overall in a closed loop feedback system, the offset R 
tracking is probably irrelevant, but the noise on the -5V may be an 
issue, depending on the PLL characteristics. First prove the concept 
with what you already have, then refine it as needed.

Good luck.

Ed