[time-nuts] ? phase comparison or other device

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jun 27 00:51:02 EDT 2007

Pete wrote:
> Bruce,
> A few final thoughts.
> 1. Thanks for the critical view; it does help.
> 2. Like many time-nuts I have a reasonably good 10MHz source &
> sometimes need to check out a newly acquired OXCO to ensure
> it can muster 1E9 or 1E10 performance (with 10x headroom).
> An SR620 would be ideal, but it's just too many $$ ;even used.
> I expect "casual" participants of time-nuts already have a basic,
> decent counter e.g. HP5335A & a basic decent synthesizer
> e.g. PTS040, Fluke6060(?), HP3335x or 6x. Also, I assumed a
> coaxial level 7 mixer & suitable lowpass filter would be available.
> 3. I read the JPL paper (more than once) & developed the first three
> stages (modified for 1KHz bandpass) per their process. At that point
> the measured jitter was well under 1ns rms; which was enough to enable
> 1E12 resolution for 10MHz sources. I deliberately choose the ADA4899-1
> opamp since it's characterized for 5V operation, low noise, fast & cheap
> enough ($4.30/ea). It was apparent that even with 2 stages the ZCD
> was still under 1ns jitter; the risetime wasn't blazing, but it was 
> obviously
> good enough.
> 4. Without PCB capability (at home & now retired) even this simple
> circuit is tough to build; each part adds significantly to the effort 
> when
> doing 1-up. So I examined the need for every part in an effort to
> minimize parts count, but retain jitter performance. I found that the
> opamp overload recovery was more than fast enough to discard the
> limiting without measurable deterioration in jitter. Lots of parts went
> away; construction became easy.
> 5. I went TOO FAR. The opamps I had exhibited such low offset that I
> DC coupled without thinking about it. WRONG answer (as you noted),
> Rookie mistake. I have shown the AC coupling & 2nd stage feedback
> resistor in the revised circuit.
> 6. The ZCD costs <$20 for parts & about 2 hours to build/check out.
> It performs well enough to look at stable sources to 2 parts in 1E12
> in 50 to 100 seconds and be confident in the data. The noise floor is
> easy to measure & verifies functionality.
> Is it "well designed" ? NO. Could it be (much) better? Certainly.
> Does it work well for it's intended purpose? Yes.
> My assumptions about equipment may be out of line. In my case, eBay
> supplied everything, except the mixer, filter & ADA4899-1s, so this
> effort didn't require much in the way of extra $$. It does what I wanted.
> As previously observed, the mixer should have a diplexer between it and
> the filter for the mixer higher order products to be terminated properly.
> I examined the filter input Z, as terminated, and found it to be from 150
> ohms inductive to 1200 ohms inductive from 10 to 30 MHz. This suggests
> the use of a feedthrough termination of around 100 ohms as a first order
> fix. Using a 93 ohm feedthrough, no improvement, or degradation in 
> results
> was noted. This could use more study.
>> From your earlier response, I suspect you have a cheaper, better method
> in mind to achieve the same results. Would you detail it?
> Regards,
> Pete Rawson
> ------------------------------------------------------------------------
> _______________________________________________
> time-nuts mailing list
> time-nuts at febo.com
> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts

Try connecting the input stage inductor and capacitor in parallel with 
the 6190 ohm feedback resistor, but before you do this replace the first 
opamp with a lower gain bandwidth (audio??) device that is unity gain 
stable. This will produce a first gain stage that amplifies the signal 
of interest as well as the noise within the tuned circuit bandwidth 
without unduly amplifying the noise not within the tuned circuit bandpass.
The other thing you could do since you've chosen a 1kHz beat frequency 
is to use an audio transformer to step up the output of the mixer before 
amplifying it. NB dont forget to connect the transformer to ground 
through a capacitor that has a low impedance at 1kHz (this ensures that 
the dc load current at the mixer IF port is low)..

The mixer IF port should be terminated with a 10nF capacitor and a 
simple low pass filter consisting of say a 100uH inductor and a 1nF 
capacitor substituted for the 1.9MHz bandpass filter.
This, as shown by the NIST paper alluded to by Magnus, will increase the 
mixer sensitivity considerably. You should also run the mixer with both 
the RF and LO ports saturated ie more than 7dBm for both of these ports.

The mixer output noise at the 1KHz beat frequency will be somewhere in 
the vicinity of 100nV/rtHz, so if you have say a 1V peak output then the 
inherent jitter due to mixer noise will be around 160ps rms for a tuned 
circuit noise bandwidth of 100Hz. With a suitable amplifier choice you 
shouldn't degrade this by more than 5% or so. Achieving a resolution of 
better than 1E-13 in 1 second with a 10MHz input and a suitable counter 
is easy, provided you dont rely on the counters input circuitry to 
trigger on the amplified mixer output you need a signal chain like that 
used by JPL. With a suitable comparator and well designed limiting 
stages one can easily achieve this resolution.

With a relatively low resolution counter you will get better results if 
you use a 1Hz beat frequency like JPL did, then a resolution of around 
1E-14 in 1 second with a 10MHz input and a cheap 100ns resolution 
counter is easily achieved.

You can do 1E-12 resolution in a minute or so with nothing more advanced 
than a high resolution chart recorder (perhaps not a physical one but 
use an inexpensive ADC) and a linear phase comparator, you don't even 
need a counter.

Ulrich and I are currently working on phase comparator that promises 
higher resolution than you have achieved, however the JPL approach is 
capable of a resolution several orders of magnitude better than this. 
The resolution offered by the JPL approach is required when you want to 
compare a couple of hydrogen masers or other sources with equivalent or 
better stability. For less stable sources a simple phase comparator 
should suffice for most purposes.

A project to produce an equivalent of the JPL zero crossing detector is 
Inexpensive audio grade opamps will be used, wideband opamps are 
The final stage will be a relatively slow comparator (AD790).
The largest contribution to phase instability will be the temperature 
dependent phase shift of the 1Hz low pass filter.

Just as JPL did a mixer with separate isolated grounds for all 3 ports 
will be used (eg Minicircuits RMS-1, HP10534B etc)

Standard synthesizers and DDS circuits are far too noisy for generating 
the 10MHz -1Hz signal, so an offset generator equivalent to the one JPL 
developed will be required for optimum performance.


More information about the time-nuts mailing list