[time-nuts] ? phase comparison or other device

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Sun Jun 24 20:16:10 EDT 2007

Pete wrote:
> Bruce,
> This idea is NOT intended to rival the JPL results. Instead,
> it's intended to be cheap, easy to replicate & allow rather
> low cost instruments to be used to compare good sources
> to parts in 1E12, quickly. The 1KHz heterodyne frequency
> makes life much easier than 1Hz. Noisy components &
> ground loops are still of concern, but not so hard to fix.
> ADA4899-1 overload recovery is <50ns (per data sheet).
> I've attached a rather poor schematic which doesn't show
> power supply decoupling or the need to pull the disable pin high. The 
> ADA4899-1 uses 14mA per part, but it's
> quiet & fast. Metal film resistors are fine for this low
> noise application & all are low values to keep noise down.
> The inductors are easy to wind, but I found materials other
> than moly permalloy powder to be too noisy. Even with
> MPP material, cores with u>200 are prone to field induced
> shifts which are unacceptable.
> Regards,
> Pete Rawson
> ------------------------------------------------------------------------
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Even so, it pays to use a well designed circuit instead of something 
thrown together with little understanding of what you are doing.
The JPL design is not expensive and doesn't require particularly exotic 
wideband components or high resolution counters.
There is still a noise advantage in using a 1Hz beat frequency, suitable 
opamps are readily available.

Magnetic shielding of the inductors and/or the entire circuit is 
probably advisable for the best performance.

The circuit diagram is sufficient to confirm my suspicions.

The input stage noise gain will be high at frequencies away from the 
1kHz frequency of interest.
This is a very poor design.
It is very easy to do much better with the same components.
A 50ns overload recovery will be somewhat problematic when you are 
attempting 1ns or less timing jitter.
A well designed and simple feedback bound circuit will be much faster.
Using an inverting amplifier input stage is not optimum for noise.

In fact the input stage doesn't need to use such a wideband opamp, a low 
noise opamp with a more modest gain bandwidth configured as a non 
inverting stage with gain followed by a bandpass filter will have far 
better performance.
Only the final limiting stage needs to be fast.

Also since you are using a 1kHz offset frequency it may be advantageous 
to use a transformer to couple the mixer output to the input stage, a 
stepup transformer will improve the equivalent input noise significantly 
even when using a somewhat noisier slower and cheaper opamp for the 
input stage.

A low pass filter with a lower cutoff frequency than  the several  MHz  
of the  BLP 1.9 is desirable between the mixer and the input amplifier, 
a tuned bandpass filter would be optimum but don't forget to terminate 
the mixer IF port in a suitable impedance at frequencies other than the 
beat frequency. It should be possible to combine the tuned bandpass 
filter and the stepup transformer.

Try reading the JPL article to gain an understanding of how to do it 
Although their design uses cascaded low pass filtered amplifiers with 
feedback bound circuits, the same technique can be used with bandpss 
Since you use a 1kHz beat frequency it is advantageous to AC couple the 
various stages to reduce the effective output dc offset.
Low frequency earth loops will limit the performance unless a different 
mixer with dc isolated RF. LO and IF outputs is used.
Suitable mixers are available.

Your claimed performance is comparable with that which can be achieved 
using a linear phase comparator which neither requires a mixer (other 
than the implicit mixer built into the phase comparator) nor a high 
resolution counter.


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