[time-nuts] Datum TS2100 10MHz frequency drift

Charles Steinmetz csteinmetz at yandex.com
Wed Oct 28 19:03:12 EDT 2015

Philip wrote:

>few weeks ago I acquire TS2100GPS with TCXO. Few days ago receive 
>MTI OCXO and install in device. After replacement change gain and 
>filter constant as described byJason Rabel's post from 9/26/2010

Looking at that post in the list archive, it does not appear to 
describe any particular loop tuning solution -- Jason was asking a 
question regarding (i) whether the tuning parameters available in the 
2100 would support an OCXO, and (ii) if so, what parameters should he 
use for his specific OCXO.  It does not appear that he received any 
responses to his question.

>Unfortunately I'm not happy with results because 10 MHz output 
>signal after GPS lock (both leds on front "Locked" and "tracking" 
>are "ON") has frequency drift (phase shift) with period around one 
>second plus minus ~18 deg (+-5 ns) compared to other 10 MHz.

The PLL control loop is apparently unstable with the parameters you 
have programmed.  You need to determine, first, whether the 
parameters available are sufficient to accommodate an OCXO and, if 
so, what specific parameters you need to program to achieve optimal 
(not just basically stable) operation.  In other words, YOU have to 
answer Jason's question.

Best regards,


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