Lars wrote:
I have wondered what is meant by a proper digital filter below?
Is a proper digital filter something more than the LP-filter +
PI-loop I use in the DIY Arduino GPSDO?
What is used in commercial GPSDOs?
The loops are substantially more sophisticated than a simple LP
filter -- typically 3rd order or higher. Well-designed commercial
GPSDOs also have several time constants so they can achieve basic
lock using a wider loop, then narrow the loop in steps until it is
fully narrow (unless S/N is low, in which case they may not reach
full narrow and should set an alarm). Loop filters in commercial
GPSDOs use algorithms that suppress systematic ripple on the VCO
control related to the comparison frequency.
It is extremely unlikely that someone would stumble upon workable
parameters for this sort of loop by trial and error. One needs to
measure and characterize the open-loop behavior, then carefully
design the loop filter to achieve the desired result.
Best regards,
Charles
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