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

[Magnus Danielson]

On 04/27/2016 01:12 AM, Charles Steinmetz wrote:
> 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.

The lowpass-filter + PI-loop is third order.

>  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).

Some care needs to be taken in articulating the filter such that you do 
not have to scale the state as you change the parameters. This is 
however fairly simple for both the low-pass filter and integrator (which 
hold state).

With some care you can articulate the P and I in the forms of equations 
relating to the loop parameters you want and compensating for the 
DAC/EFC control gain (exact value not important, but getting bandwidth 
and damping in the right neightborhood is).

The heuristics about when changing parameters takes some learning. One 
of the design criteria is to be able to always capture, but you also 
want it to go as quick as possible. Finding a good balance can be 
problematic. There is tricks to track-in quicker.

>  Loop filters in commercial GPSDOs use algorithms that suppress
> systematic ripple on the VCO control related to the comparison frequency.

It will be there, so you need to manage it one way or another.

> 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.

If you know what you are doing, you can get a good start using analysis, 
knowing the damping and bandwidth you want. It always becomes a measure 
and trim exercise.

Cheers,
Magnus