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

[Erik Kaashoek]

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.