[time-nuts] Divider pecking order ?

[Magnus Danielson]

Bruce,

Bruce Griffiths wrote:
> Magnus Danielson wrote:
>> Bruce Griffiths wrote:
>>> The performance of the sine to square wave conversion clock shaper 
>>> circuit may dominate the divider performance.
>>> Thus an evaluation of the jitter performance of sine to square wave 
>>> conversion circuits would also be informative/useful.
>> We should standardise some test for estimating the input contribution.
>> I would prefer using various amplitudes, preferably with a 
>> measurement of achieved slew-rate.
> The input circuit will have an intrinsic jitter plus contributions due 
> to the input signal slew rate at the threshold crossing.
> A resultant measured performance of the form:
> trigger jitter~ SQRT( 1E-22 + (1E-4/S)^2) plus other terms
> where the actual numbers depend on the particular input clock shaping 
> circuit may be expected.
Indeed. This is the basic formula and summation of errors I had in mind.
>>> If one is using a digital divider in a narrow band PLL the close in 
>>> phase noise performance of the divider may be more important than 
>>> its phase noise floor, in which case an ECL or CML divider (or at 
>>> least a divider using a CML or ECL resynchronising flipflop) may be 
>>> a better choice than a TTL or CMOS divider, especially if a low 
>>> noise diode mixer is used as a phase detector is used.
>> I have reason to beleive that pulse-width factor has great importance 
>> in the produced result for mixer-based measurements. I have not had 
>> the time to make any conclusive measurements or theoretical work for 
>> it, it just looks like that.
> Yes, NIST have shown that the mixer input signal should either be a 
> relatively low distortion sinewave or the duty cycle of a square wave 
> should be close to 50%.
> This may make it difficult to measure the phase noise of those 
> dividers with outputs that don't have a 50% duty cycle.
> Achieving accurate quadrature between the mixer/phase detector inputs 
> can be critical in classical mixer based systems.
An indication has been that following a "high jitter" divider by a 
divide-by-2 provides a lower jitter than anticipated. Asymmetries in 
input clocks have also caused some problems for some dividers again 
being improved by the divide-by-2. All according to NIST measurements. 
Thus, care should be taken in ensuring this property when measuring.
> Divider jitter will also have contributions due to aliased noise from 
> the input signal.
Indeed. I have been pondering over that aspect myself.
>>> Thus characterising divider phase noise may also be useful, although 
>>> it may be difficult to do this for divider output frequencies much 
>>> below 1Mhz or so.
>> Indeed.
>>
>> It's a pitty that the SIA-3000 stops doing meaningful measurements at 
>> 11 MHz... and lower. If someone could tell me how to overcome that, I 
>> would be very greatful.
>>
> The TSC5120A allows phase noise measurements on signals with 
> frequencies down to 1MHz.
> Classical diode mixer based systems can go somewhat lower in frequency.
You should realize that I have reasons to believe that the limit is one 
of software parameters rather than anything else.

Lacking a TSC5120A or sufficiently similar in my lab...
>> Cheers,
>> Magnus
>>
> For those dividers that have a buffered 10MHz sqare wave output 
> measurement of the jitter of lower frequency outputs with respect to 
> the buffered 10MHz output may provide a better measure of divider 
> performance as the jitter due to the clock conditioning/shaping 
> circuit should be largely common mode.
Indeed, but for most uses it doesn't help as it is the output absolute 
jitter which is being seen by the measurement application, so for most 
uses it needs to be included in the measurement, where as we of course 
is curious in figuring out at which stage the jitter performance is 
significantly impaired.

Cheers,
Magnus