[time-nuts] GPSDO won't lock: OCXO aged out?

[Bob kb8tq]

Hi

There is both short term and long term noise on the GPS PPS signal 
out of a GPS module. Some of the short term noise can be reduced 
by software using the “sawtooth correction” out. That assumes the
module you are using provides this output. The uBlox M8T is one
of many that provide this information. 

Even with correction, you are still stuck with “many nanoseconds” of
noise on the GPS PPS. A good OCXO is *much* more stable than 
this. A filter in the locking circuit is needed to reduce this noise. 

The lock circuit can be either a PLL (phase locked loop) or a FLL
(frequency locked loop). If it is a FLL, the time output of the GPSDO
will drift. If you want time to stay locked, you need to use a PLL.

Even with a PLL long term accuracy is dependent on an integrator
in your lock circuit. That is what watches the long term problems
and corrects for them. 

Working out the timing / corner frequencies of the proportional and
integrator portions of the loop is highly dependent on your oscillator
and the goals you intend to achieve. A reasonable starting point
for the proportional portion is in the 10 to 100 second range. The
integrator usually is 10 to 100X longer. 

Getting all this working involves comparing it to “something better”.
Often in a basement project, finding something better becomes
a major effort. 

Fun

Bob

> On Jan 16, 2021, at 9:43 AM, 李 <bd4kc at 126.com> wrote:
> 
> 
> If the gpsdo output pps signal does not drift, but the gps pps signal drifts slowly, it is bound that after a period of time, the gpsdo pps signal will have a very large beat in order to follow the gps. How to solve this problem? In tens of nanosconds, I see that the pps of the gps are always drifting.
> 
> 
> 
> | |
> 李
> |
> |
> 邮箱bd4kc at 126.com
> |
> 
> 签名由 网易邮箱大师 定制
> 
> On 01/16/2021 12:46, Scott Newell wrote:
> At 07:49 PM 1/15/2021, Bob kb8tq wrote:
>> then indeed, the temperature sensitivity will go
>> way up. Most of the OCXO’s off these boards
>> are a sub $20 sort of item on eBay. How good are they? Who
> 
> Any suggestions for a SC-cut 5V OCXO? Searching
> for "Trimble 73090" or "73090 OCXO" just brings
> up the ~$80 GPSDO boards. Nearly everything 5V
> I'm seeing is square wave, but maybe that doesn't
> matter in this box? (Or maybe the 73090 is square
> wave; I'll check next time I have it open.)
> 
> --
> newell  N5TNL
> 
> 
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