[time-nuts] Re: Timestamping counter techniques : dead zone quantification

[Bob kb8tq]

Hi

Without knowing the ultimate “target use” for this system, it is 
very hard to guess what will or will not be a problem. 

As an example, lets say we’re “building an engine”:

Ok, engines go into chain saws. Engines go into dump trucks.
Engines also go into airplanes. Some are run in stationary applications
at fixed speeds for their entire life. The target application / use does
matter when you try to work out what will and will not be problematic. 

In a commercial setting, pretty much everybody knows what the
company does and what they are targeting as goals. Thus it
often does not get put onto paper, it’s obvious to everybody at 
the table. 

On a list like this, with the vast range of folks trying to do things, 
we have the opposite situation. There *is* no set of defaults that 
all are the targets for this or that. There is no way to “assume” or
to guess. 

That said, dead bands are layout / cabling / hookup dependent. That
(in most settings) makes them difficult to model or predict.

Bob

> On Feb 3, 2022, at 3:36 AM, Erik Kaashoek <erik at kaashoek.com> wrote:
> 
> To prepare for the implementation of dead zone countermeasures I did some measuring of the dead zone band width versus frequency of the subharmonic
> The test setup use a generator with two outputs, one fixed at 10MHz and one variable to test the dead zone. The fixed 10MHz was send to one input of the timestamping counter. The variable frequency output was send to the other input.
> The reference clock used for the timestamping was set to 200 MHz and, through its VC-TCXO,  locked to the fixed 10MHz using a SW control loop updating the voltage to the VC-TCXO once every 10 seconds.
> As the generator used was only able to set frequency at 0.1 Hz resolution there where some limitations in this assessment.
> The dead zone was observed on the sub harmonics of 200MHz and its harmonics. The size of the dead zone was very much dependent on the used frequency
> Below 1MHz the width of the dead zone was below 0.1Hz and thus not observable
> At 10 MHz the width was about 1 Hz
> At 40 MHz the width was about 2 Hz
> At 80 MHz the width was about 10Hz
> This makes implementing dead zone counter measures doable as with lower frequency subharmonics the width of the dead zone decreased thus putting a limit on the amount of subharmonics to include in the calculations and this makes it unlikely there is a  scenario where the two input frequencies used make it impossible to find a reference frequency avoiding the subharmonics.
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