[time-nuts] time-nuts Digest, Vol 53, Issue 8
Richard Moore
richiem at hughes.net
Mon Dec 1 22:53:23 UTC 2008
On Dec 1, 2008, at 2:33 PM, Stanley Reynolds wrote:
> Message: 6
> Date: Mon, 1 Dec 2008 14:31:33 -0800 (PST)
> From: Stanley Reynolds <stanley_reynolds at yahoo.com>
> Subject: Re: [time-nuts] DAC resolution in GPSDO
>
> The oscillator I think has a manual adjustment. You could be using
> the 5 volt provided on board or the 8 volt provided by the OCXO for
> the ECC op amp. Note the ECC voltage is dependent on the control
> board regulation some what even using an external reference. You
> may want to manually adjust the OCXO closer to zero and set the
> upper limit lower as well. By?using a external voltage divider on
> the ECC voltage or lowering the gain (R7 & R8).
Nope, this Isotemp was made for OFC, labeled OSC092, or maybe OFC/
McCoy sold out to Isotemp. There's no external adjustment available.
I used a 78L05 to make the +8V reference supply for the OCXO and it
works quite well. I don't use the BertBoard's signal output chip at
all, nor the RS-232 chip, so loading hasn't been a problem. Compared
to my recently purchased Thunderbolt, running off the same antenna,
the Zauhar GPSDO moves around by a few parts in 10E-10 to 10E-11.
Using the +8V ref supply to make a larger range of DAC output makes
the problem worse, not better. A 16-bit DAC would help, but this
project has gotten more complicated than I wanted already -- see my
webpage. The best solution now would be a good 10811A...
Dick Moore
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