[time-nuts] Dual Mixer

Bob Camp lists at rtty.us
Tue May 11 21:04:33 UTC 2010


I had not realized they had *finally* published that. They've been using
parts of it for at least 10 years now....


-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Bruce Griffiths
Sent: Tuesday, May 11, 2010 4:27 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Dual Mixer

As long as one is aware that your method (as implemented by you) doesn't 
actually measure Allan variance, it may be useful for comparing the 
relative stability some sources for small Tau (unfortunately the range 
of Tau for which the method may produce useful results depends on the 
phase noise characteristics of the sources being compared).
To measure AVAR the technique has to have the same response to all phase 
noise spectral components as does AVAR.
Since you do not integrate/average the frequency measures the phase 
noise response of the method is not identical to that used in 
calculating AVAR.
This technique probably works best when white phase noise dominates the 
phase noise spectral region of interest (usually for small Tau).

For those who can follow the theory, the following paper shows how the 
above method is affected by aliasing etc:

The paper also shows how the required integration (needed to actually 
measure AVAR) can be approximated from the discrete sample sequence.
Alternatively one could avoid the numerical integration by replacing the 
ADC with a zero deadtime (ie not a dual slope converter. A multislope 
algorithm like that used in the 34401A (but not the 3458A) should work 
as the signal is integrated continuously) integrating ADC. One 
possibility is to use a VFC as NIST did when they used this technique 
some decades ago.

Of course, the classical DMTD setup undersamples the phase noise 
spectrum and thus may suffer from aliasing artifacts.
Such aliasing artifacts have no significant effect when the phase noise 
spectrum is flat.


WarrenS wrote:
> For the Really cheap time nuts,
> It sounds like Bert Kehren has done a great Job building a Dual Mixer 
> tester.
> There are other simpler, less standard ways to get good data for Allan 
> Variance and small frequency differences.
> My VERY simple $10.00 analog tight PLL Tester BB (Previoulsy posted) 
> pretty much accomplishes the same goals as his,
> and it can do 1e-13 in a second, and 1e-11 in 10ms  (limited of course 
> by the single reference Oscillator used)
> A simple test that most can do at home, and still challenges the best 
> high end testers out there is Tom's the swinging Oscillator test.
> http://www.leapsecond.com/pages/10811-g/
> (The results from my PLL tester is attached)
> ws
> ******************
> ----- Original Message ----- From: <EWKehren at aol.com>
> To: <time-nuts at febo.com>
> Sent: Tuesday, May 11, 2010 7:02 AM
> Subject: [time-nuts] Dual Mixer
>> The Dual Mixer project is nearing completion.
>> Let me refresh every ones memory as to my goals.
>> a)  Total cost less than $ 200
>> b)  1 E-13 with a one second offset
>> c)  use parts attainable by every one
>> d)  easy to assemble only a few surface mount parts
>> e)  a five channel counter that yields 1 E 15 resolution and  interfaces
>> directly to a PC via  RS232 or USB
>> f)   A counter that also gives you instant frequency  difference at the
>> sample rate, not only Allan Variance
>> g)  Modular so one can use only the Dual Mixer
>> h)  Modular so one can use multiple units to do simultaneous  
>> comparison of
>> more than two oscillators.
>> i)   Isolation between D/M and counter so that the counter can be  
>> powered
>> by the PC USB port
>> I am happy to report that all goals have been accomplished, attached 
>> is a
>> picture of the D/M, limitation of the file size does not allow me to 
>> attach
>> an  actual board picture, but if you contact me direct I will send 
>> you one,
>> the  final board is actually nicer since the first layout had to 
>> accommodate
>> several  variances.
>> The D/M part leans heavy on the original NIST unit with a few 
>> substitutions
>> and recommendations from Bob Camp. Also beside Opto Couplers SN65LVDS1's
>> have  been included for those that want to use other counting methods.
>> Selection of  filter capacitors allow the use at other offset 
>> frequencies such as
>> 10 and 100  Hz. The D/M fits in a standard 74 X 111 X 20 mm Euro case 
>> and
>> the counter can be  stacked below or next to it using the Opto 
>> Isolators as
>> the inter connect. The  SYPD-1's fit right on the board but 
>> connections are
>> included to use the HP 10514  A. As a matter of fact removing the HP 
>> mixer
>> board from its housing   fits it nicely on the board and every thing 
>> is still
>> inside the housing.
>> The counter will handle 1 an 10 Hz offset with a 1 E 14 resolution at 10
>> Hz. Thanks to Richard Mc Corkle we have great drawings and code, 
>> available to
>> every one.
>> Code, drawings, list of material and PC board layouts and its  file, 
>> will
>> be available to every one once the project is completed.
>> I need help in the following areas
>> a)  help me create a nice set of drawings that are computer generated
>> something I am not able to do
>> b) create the computer program that takes the output of the counter 
>> board
>> and allows Allan Variance plots, frequency difference and dual 
>> temperature
>> readings and plots using RS232 and USB.
>> c) an independent test by a third party.
>> As I said previously, I am not getting in the business of supplying 
>> parts
>> but will work with people that will help achieve the three points listed
>> above.  Presently I have boards on order and will have two 
>> uncommitted board
>> sets and  probably also component kits.
>> Please contact me directly.
>> Again thank you Corby Dawson, Richard Mc Corkle and Bob Camp.
>> Bert Kehren   Miami
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