[time-nuts] Re: Validating GPSDO control loop with simulation and measurement. Is this amount of difference between measurement and simulation to be expected?

[Magnus Danielson]

Hi Erik,

On 2022-02-26 17:36, Erik Kaashoek wrote:
> Magnus, Bob,
> Thanks for the replies.
> @Magnus, I need to study your input a bit longer. What I currently 
> fail to understand is : Should there be a low pass filter with a 1/100 
> s corner frequency in series with the Vtune when the ADEV of the 
> VC-TCXO and the PPS intersect at 100 s? Why not have a Kp of 0.01 
> instead of an additional low pass filter? I understand how the low 
> pass filter can help reduce noise from the DAC if needed or is there 
> another reason?

The reason I did so was that the modulation, which always occurs as you 
step back and forth to interpolate, was intentionally moved to as high 
frequency as possible to make it as easy to filter out variations from 
as possible. This way the filter to remove most of that noise can have a 
bandwidth higher than the loop bandwidth, which is needed for loop 
stability. When they come near each other, this third pole may create 
unwanted pole-splitting moving a pole-pair into the forbiden/unstable 
region (right-hand side of Laplace domain or outside the unity circle of 
the z-transform-domain).

Hope it makes sense.

Cheers,
Magnus

> @Bob, the reason for not converging may be some infrequent 
> disturbances of the control loop caused by something not yet 
> identified. Need to do some investigation into the cause.
> Erik.
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