[time-nuts] Weird GPSDO behavior

Bob kb8tq kb8tq at n1k.org
Fri Sep 29 08:18:20 EDT 2017


> On Sep 28, 2017, at 6:59 PM, Mark Sims <holrum at hotmail.com> wrote:
> I suspect that it is either temperature related (the funkiness starts around when the temperature reaches a minimum) or related to the way the disciplining parameters are hacked to get the extended time constant.  

Like it or not, most of these devices were made back a while ago. 
The CPU’s used were not the ones we have today. the code was tested
over the “expected range” of values. Most (but not all) of the control loop
code was done as integer math. With limited RAM, tossing everything into 
64 or 128 bit integers was not an option. In some cases 32 bit int’s were at
a premium. Multiply this by that, that, and that. Then divide by something, and 
something else. … hmmm…. a few bits just went missing. Could you reorder 
and fiddle to fix some of this? Sure, that’s where we get back to the expected
range stuff. 

Even if it’s not in the “main loop”, GPSDO code is full of checks for this or that.
Like the main math, they are scaled and tested for the normal range of parameters.
Not all of them spurt error messages when they get involved. Some just bump this 
or that and move on. 

Lots of possibilities ….


> Try setting up for say a 10,000 second time constant and see how things change.
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