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

[Magnus Danielson]

Lars,

On 04/27/2016 02:14 PM, Lars Walenius wrote:
> Thanks Magnus and Charles,
>
> Could you please explain a little more about this:
>
> Charles wrote:
>>>   Loop filters in commercial GPSDOs use algorithms that suppress
>>> systematic ripple on the VCO control related to the comparison frequency.
> Magnus wrote:
>> It will be there, so you need to manage it one way or another.
>
> What is the systematic ripple?

On the comparison frequency, whatever it is, there will be measurements 
and adjustments. The overtones of the sample rate it represents will be 
a systematic "tone".

> Is the comparison frequency normally 1Hz (1PPS)?

Yes.

> What is the algorithms used?

There can be many. Oversampling and smoothing that way helps for 
instance. Resolution in adjustment is an indirect way.

> If I sample the difference between the PPS and the divided 10MHz output every second and have a third order loop driving a DAC will I get systematic errors?

Yes. Depending on how picky you are you can live with it or you find 
ways to manage it.

Cheers,
Magnus

> Lars
>
> Från: Magnus Danielson<mailto:magnus at rubidium.dyndns.org>
> Skickat: den 27 april 2016 10:02
> Till: time-nuts at febo.com<mailto:time-nuts at febo.com>
> Kopia: magnus at rubidium.se<mailto:magnus at rubidium.se>
> Ämne: Re: [time-nuts] Advise on building a DIY GPSDO?
>
>
>
> 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
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