[time-nuts] Sub Pico Second Phase logger

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Dec 5 22:03:18 UTC 2008


Warren
>> Another potential issue is crosstalk between flipflops in the same
>> package, in particular between the 2 clock signals.
>> Such effects will not be evident when using a single clock source to
>> evaluate the system noise.
>> Using fully differential logic such as ECL will reduce such coupling at
>> the expense on increased power dissipation and relatively high logic
>> level tempcos.
>> Dithering one or both of the clocks may help.
>>
>> Bruce
>>
>>   
>>     
> The effect of cross coupling between the clocks will be more significant
> as the phase shift between the clocks approaches 0 or 180 degrees (for
> 50% duty cycle clocks).
> Dividing the input frequencies by 4 to produce quadrature phase outputs
> wont help.
> You would need to actually use a pair of phase shifted clocks from one
> of the input sources feeding separate dividers so that clock transitions
> of the 2 clocks within a chip do not coincide.
> Either a selected length of coax or a quadrature hybrid would suffice
> for producing the 2 clocks with a near 90 degree phase shift.
>
> Bruce
>
>   
A much simpler solution to the clock cross coupling problem is to use
separate chips for the 2 dividers and live with the less accurate delay
tracking.
One then has to ensure that significant cross coupling via the power
supply and ground systems doesnt occur.

Bruce




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