[time-nuts] Advise on building a DIY GPSDO?

[Charles Steinmetz]

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