[time-nuts] ? phase comparison or other device

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed May 9 19:42:31 EDT 2007


Bill Janssen wrote:
> I thought that someone was designing a circuit that could be used to compare
> two oscillators.
>
> What happened to that project?  I now have a HP 5370A so I have 
> something, but
> I would like to make simultaneous measurements on three or four "precision"
> clocks.I am not qualified to design a "state of the art" device, so I am 
> looking for others
> to do that.
>
> Thanks
> Bill K7NOM
>
>
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>
>   
Bill

Ulrich and I have designed and Ulrich is currently testing a CPLD 
implementation of the improved version of the HP K34-5991A linear phase 
detector.
It includes programmable prescalers (1-256) so that frequency like 10MHz 
and 5MHz for example can be compared. The maximum input frequency is 
about 50MHz.
It has 2 quadrature phase outputs. The prescalers also allow the phase 
detector gain to be adjusted. The phase detector has a triangular wave 
characteristic with a period of 4 cycles of the input frequency to the 
phase detector (ie at the built in prescaler output).

Preliminary results using a very crude kitchen table "breadboard" 
indicate that instabilities of a few parts in 1E12 are easily seen 
within an hour or so.
Sensitivity is likely to be much better than this but a 10X prescaler 
was used on each 10MHz input.

Comparing 3 or 4 standards requires using a set of distribution 
amplifiers plus a set of linear phase comparators to achieve the desired 
configuration.
This is more flexible than trying to anticipate exactly how many 
channels a user may want, it also has less crosstalk than an 
implementation with more than 2 input frequencies to a single board or CPLD.

With external prescalers the maximum input frequency can be extended to 
100MHz or more.

Bruce




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