Martyn,

2 parts in 10E-12 in a second's gate time would require to measure the
time interval with an 2 ps resolution. Since the SR620 (at least mine)
features a 20 ps single shot resolution, i fear your number is more
likely "20 parts in 10E-12". Or what you are doing is to make >= 10
measurements / s of phase and computing the arithmetic mean of them.

There are not really many companies to offer you direct sub-picosecond
measurement resolution and if at all the devices may in the same price
reigion as your company's boxes. There ARE some measurement schemes that
enable you kind of "artificial magnification" of the time differences to
be measured, for example the Dual Mixer Time Difference Method. In
principle they all look VERY EASY and may give you the impression that
you can make measurements with that on the kitchen table. I have tried
out a few of them just to find out that they do indeed work (in
principle) but that their technical realisation has some strange
requirements that are not too easily given on standard workbench.
Nevertheless I am still working on the problem using a not so well known
proposal.

Best regards
Ulrich Bangert

At 10:20 AM 5/7/2008, Ulrich Bangert wrote:

For example, the implication is that the SR620's timebase is accurate
to parts in 1E-12 or 1E-13, so that it can count off that 1 second
with suitable accuracy.

Ed, k1ggi wrote:

A little imprecision in the nomenclature, but it all should
mean the same thing.

1 part in 10E12 is the same as saying 1 part per 10E12.  It is
the fraction:

1/[10E12], or  1.0 x 10E-12

...

That would be 2 parts per 10E-12, which would be 2/[10E-12], a
very big number!

I am sure that the author of that snippet actually meant
2 parts per 10E12, or 2 x 10E-12.

Sort of mixing your metaphors

-Chuck Harris

Hi Ulrich:

The SR620 has a 1 kHz Reference output that's derived from the 10 MHz external
clock input.  It also has a Gate/Arm function.

Appendix B in the PRS10 Manual explains how to use these two features to make
1,000 time interval measurements per second thus averaging the 20 ps one shot
way down.  http://www.prc68.com/I/PRS10.shtml

The time interval armed 1,000 times per second and the difference between zero
crossings of a pair of 10 MHz signals are measured.  So the range of
measurement is 100 ns before rollover.  I think at least one of the test
signals needs to be 10 MHz to drive the external clock input to get the 1 kHz
reference.  But you could use Toms PIC divider to generate a 1 kHz signal from
about any test signal and use the same method.
http://www.prc68.com/I/FTS4060.shtml#SR620Fast
http://www.prc68.com/I/TandFTE.shtml#SR620

Have Fun,

Brooke Clarke
http://www.prc68.com/P/Prod.html  Products I make and sell
http://www.prc68.com/Alpha.shtml  All my web pages listed based on html name
http://www.PRC68.com
http://www.precisionclock.com
http://www.prc68.com/I/WebCam2.shtml 24/7 Sky-Weather-Astronomy Web Cam

Ulrich Bangert wrote:

Martyn Smith wrote:

Martyn

To achieve this you need:

1. A frequency standard that has an ADEV < 1x10E-12 for Tau = 1 sec.
   Such sources tend to be rare and expensive. (eg hydrogen maser, BVA OCXO
   etc).
   The frequency of the standard also has to be known to better than 1x10E-12.

2. Heterodyne techniques can easily achieve the required resolution but
   achieving the low noise and drift isnt easy:

   a) The zerocrossing detector shaping the mixer beat frequency output
   has to be designed to amplify the slope whilst minimising the noise.
   The naive design approaches often advocated are inadequate.
   Low frequency ground loops can easily corrupt the measurements.

   b) The temperature of the mixer has to be held constant to better
   than 0.05C.

   c) The zero crossing detector temperature has to be held to constant
   to better than 1C.
   A naive zero crossing detector design may require temperature
   control to 0.01C or better.

   d) A low noise stable offset frequency source of accurately known
   frequency is required

Even when all of the above conditions are met the measurement range may
only be around 0.1Hz or so.

Bruce

Bruce Griffiths wrote:

Brooke Clarke wrote:

Brooke

What about the effect of the phase modulation of the 1kHz output by the
100Hz, 10Hz, 1Hz outputs due to simultaneous switching effects from such
dividers?

Bruce

To all, regarding measurement a 10 MHz signal to parts in E-12.

Here is one way which I believe I had sent a while ago:

You will need the following test equipment:
Traceable 10 MHz Frequency Standard (GPS disciplined Rubidium, Cesium, etc.)
40 GHz Synthesized Signal Generator
40 GHz Frequency Counter with 12 digits of resolution at 40 GHz (1/10 Hz
resolution).

Setup. Connect the equipment as follows. The 10 MHz signal from the
traceable Frequency Standard is connected to the Synthesizer's external
reference input. Set the output of the Synthesizer to 40 GHz (or the highest
frequency that the test counter can count). Connect the Synthesizer output
signal to the microwave input of the test counter. Connect the '10 MHz Out'
signal from the DUT to the 'External Reference Oscillator Input' on the test
counter. Set the resolution on the test counter to display the greatest
resolution possible.

This method effectively multiplies-up the traceable Frequency Standard's 10
MHz output to a traceable 40 GHz signal into the test counter's microwave
input (with the accuracy of the frequency standard). The accuracy of the
test counter's readout of this 40 GHz signal is now dependant on the
accuracy of the 10 MHz in the DUT.

To interpret the counter's readout of the 10 MHz DUT, a reading of:
40.000 000 000 1 GHz is equivalent to 2.5 parts in 10E-12
40.000 000 003 GHz is equivalent to 7.5 parts in 10E-11
40.000 000 023 GHz is equivalent to 5.8 parts in 10E-10
40.000 000 040 GHz is equivalent to 1 part in 10E-9
40.000 000 800 GHz is equivalent to 2 parts in 10E-8
40.000 006 000 GHz is equivalent to 1.5 parts in 10E-7; etc.

Tom
Tom Duckworth
510-886-1396

-----Original Message-----
From: time-nuts-bounces@febo.com [mailto:time-nuts-bounces@febo.com] On
Behalf Of Bruce Griffiths
Sent: Wednesday, May 07, 2008 5:13 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Measuring 10 MHz accurately: correction

Bruce Griffiths wrote:

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