[time-nuts] What is the best counter for a Time Nuts?

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Oct 11 21:53:50 UTC 2008


Mike Monett wrote:
>   The Allen deviation is used to describe the performance of  a stable
>   clock. Measuring the performance of a good clock requires  a counter
>   with resolution  down  to picosecond levels. As  Dr  Griffith points
>   out, some  modern counters may have internal signal  processing that
>   makes them unsuitable for this task.
>
>   Another thread  discussed using a mixer to  generate  the difference
>   frequency between  two oscillators, then measuring the  stability of
>   the resulting beat note:
>
>   http://www.febo.com/pipermail/time-nuts/2005-July/019006.html
>
>   The basic  principle  is sound. If the oscillators  were  running at
>   10MHz, and  the  frequency difference was 1 Hz, then  the  beat note
>   would be 1 Hz.
>
>   This represents  one  part in 10 million, or  1e-7  of  the original
>   frequency. If  the  beat   note   is   measured  with  1 microsecond
>   resolution, the  overall resolution is 1e-7 / 1e-6 = 1e-13.  This is
>   beyond the capability of most commercial counters.
>
>   The difficulty  with this approach is the output of a mixer is  at a
>   fairly low  level, perhaps 50 millivolts or so. The  frequency would
>   also be very low, perhaps 1 Hz. This means the counter would have to
>   trigger accurately on a very slow-rising, low amplitude signal.
>
>   
Not true, if both the RF and IF ports are saturated as usually 
recommended for phase detector operation, the output can be as high as 
2V pp (e.g. Minicircuits RPD-1).
The solution to the triggering problem with low slew rate input signals 
is simple, build a slope amplifier.
A slope amplifier (in optimised form) consists of a set of cascaded 
limiting amplifiers with gradually increasing gain and bandwidth.
Oliver Collins showed how to optimise the gain and bandwidth 
distribution to minimise the output noise.
I have since generalised his results to include the case where the input 
(self) noise for all amplifiers are not identical.
For further details/references see:
http://www.ko4bb.com/~bruce/ZeroCrossingDetectors.html 
<http://www.ko4bb.com/%7Ebruce/ZeroCrossingDetectors.html>

I have some spreadsheets for calculating the amplifier parameters both 
for the the restricted and general cases.
>   This is  a very difficult measurement problem. The accuracy  will be
>   degraded by  noise,  such  as the 60Hz  AC  line  frequency  and its
>   harmonics, switching noise from the pc power supplies  and monitors,
>   radiation from nearby fluorescent lighting, plus thermal  noise from
>   the mixer and input stage of the amplifiers.
>
>   This low-level noise is very difficult to eliminate, especially when
>   coax cables are needed to transfer the desired signal from one place
>   to another.  The result is the measurement system is not as  good as
>   it could be.
>   
Not if you use the built in mixer RF transformers to eliminate low 
frequency ground lops at the mixer input and use optical isolation for 
the output of the zero crossing detector comparator.
In other words a PCB using surface or through hole mount mixers is far 
better than using a packaged mixer with a common low frequency ground 
for all inputs and outputs.
It also pays to use a capacitive IF port termination for low beat 
frequencies (<100kHz) as this reduces the noise significantly.
>   There is a solution to this problem. Another kind of mixer  called a
>   "digital mixer"  is ideally suited for this application.  It  uses a
>   d-flop, with one signal going to the clock pin, and one going to the
>   "D" input.  The resulting signal on the "Q' output is  the frequency
>   difference between the two signals.
>   
And this isnt affected by low frequency ground loops?
>   The output signal is a full logic level swing, perhaps 5 Volts, with
>   a risetime  of a couple of nanoseconds. This is an  ideal  signal to
>   pass on  a terminated coax cable to the counter.  The  schematic and
>   waveforms are shown in the attached GIF.
>
>   The output  of  the  first d-flop is passed to  a  second  d-flop to
>   eliminate glitches due to metastability in the first stage. This can
>   occur when  the signal on the "D" input is exactly on  the switching
>   threshold when the clock transition occurs. The resulting glitch can
>   severely disrupt the following logic stages.
>
>   In practice, it might be difficult to offset two  stable oscillators
>   by 1  Hz.  In this case, the frequencies can be  multiplied  to some
>   higher value. For example, the frequencies could be multiplied  by a
>   factor of 10 to 100MHz, and offset by 1 Hz.
>
>   There may be some jitter in the leading edge of the beat  note since
>   the d-flop  may  or may not catch the transition as  it  crosses the
>   threshold on  the  "D" input. Instead of the  standard  +/-  1 clock
>   ambiguity in  digital circuits, the output could  be  several clocks
>   late. However,  if the counter had a resolution  of  100 nanoseconds
>   (10MHz clock),  the  extra  delay  is  much  less  than  the counter
>   resolution and should have no effect.
>
>   The overall resolution in this example would be 1e-8 / 1e-7 = 1e-15.
>
>   
What about the noise?
With a gate array is typically around 4E-11/tau.
The noise with dual flipflops may be a little lower but not stellar.
>   This is  achieved in one second, which is an impossible  task  for a
>   counter. This means the Allen deviation can be measured  much faster
>   than before, and with much higher accuracy.
>
>   A simple LTspice analysis is included in the attached ZIP.
>
>   Best Regards,
>
>   Mike Monett
>   
>
Mike

You've obviously never tried this, in practice its noise is a lot higher 
than you think, perhaps 2 orders of magnitude worse than a a double 
balanced mixer.
You need to breadboard this and do some tests.

Bruce




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