[time-nuts] ECS ECOC-2522 (was GPS Talking Clock)

Attila Kinali attila at kinali.ch
Tue Mar 6 17:51:58 EST 2018


Hoi Nick,

On Mon, 5 Mar 2018 09:36:01 -0800
Nick Sayer via time-nuts <time-nuts at febo.com> wrote:

> I’ve built two GPSDO units now with this OCXO. For the first one, I fed the 
> DAC (AD5680) from the oscillator’s reference output. This resulted in a very 
> poor (compared to expected) short term ADEV result (1-2 E-11). There was a 
> lot of noise (something like 5 mV P-P) on the reference output. Also, rather 
> oddly, the reference voltage was something like 2.8v. The datasheet doesn’t 
> say what the expected voltage is, but if you were to use that as your DAC 
> reference, you’d be throwing away the top 20% or so of the tuning range.

That sounds weird. Usually the reference voltage is the full scale
of the EFC range. Als, it should be pretty quiet, as it's (again usually)
derived from a internal Zener diode inside the oven. I could understand
a few dozen µV of noise (in a BW of 0.1Hz to 10Hz), but it shouldn't be
more than that. mV level of noise would mean that something is seriously
wrong with the reference.


> For the second unit, I built it with the same circuit I use for the CW 
> OH300, which lacks a reference output. There, instead, I use an NCP51460 
> precision regulator for a reference. There, I achieved a short term ADEV 
> much closer to expectations. It’s just under 6E-12 at tau 1-5s. ECS claims 
> it ought to be closer to 3, but it’s entirely possible that my reference 
> (Thunderbolt) and/or counter (53220A) are contributing error, or that my 
> design has some other noise contribution I haven’t yet found. Still, it’s at 
> least in the ball park. And the oscillator hasn’t even yet been running for 
> 24 hours, so it may get better with some time (I’m a little skeptical about 
> wear-in helping low-tau ADEV though. Doesn’t that usually operate on longer 
> term drift instead?).

For short tau measurements, It is advisable to set the Tbolt into holdover.
If the drift of the Tblot is not negligible, then you can measure its drift
and compensate for it manually after measurement. This will give you more
accurate results for these short tau measurements. 

Also, keep in mind that precision OCXO take a couple of days to settle.
I have a few measurements that a relative high frequency drift up to 
one to three weeks after power up, before the OCXO settles for something
an order of magnitude lower. I don't know the OH300, but they might
exhibit something similar, just at a different timescale. Look at the
datasheet, it should state a number of how many hours/days the OCXO
reaches the stability specs. Another aspect is that you should remove
any apparent phase/frequency drift (and probably other deterministic
effects) before calculating ADEV.



			Attila Kinali
-- 
<JaberWorky>	The bad part of Zurich is where the degenerates
                throw DARK chocolate at you.


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