[time-nuts] Checking accuracy of Rubidium standards

[J. L. Trantham]

I have been enjoying this discussion.  

Since the original question was the desire to 'compare' the frequency of an
LPRO to a Z3801, it seems that you could consider that from two (at least)
perspectives.

Before I begin, I confess that I am a novice in this arena and please
correct me in any area that needs it.

The first perspective is the issue of frequency.  That seems to me to be the
issue of the average frequency of the LPRO versus the average frequency of
the Z3801.  Assuming that there is no gross difference of the 10 MHz
signals, a lissajous figure (X-Y display) on a scope with the appropriate
bandwidth amplifiers would be a reasonable initial approach.

Assuming that they are both near 10 MHz and you do not know which is the
most accurate (although the Z3801 would seem to be the default standard), if
it takes 10 minutes for a single cycle of the lissajous figure to complete,
then it is 1 cycle per 600 seconds difference between the two and therefore
the two are within 1/600 Hz or 1.67 mHz of each other.  If we assume that
they are both close to 10 MHz, then that is 1.67 parts in 10E-10 difference
between the two.  Is my logic faulty?

The other perspective is the issue of 'purity'.  That is to say, what is the
'frequency modulation' of the source?  This, I think, is the issue of phase
noise.  Correct?

That is something that I have not yet had a chance to contemplate as far as
how to measure.  It would appear to require a particularly stable (pure)
source as a reference though.  Various multiplying or dividing protocols
would seem to introduce a host of other variables that would seem to be
difficult to account for though they might accentuate an impurity in the
signal in question.  I have read Bruce's comments and I still do not
understand the basics of time stamping or how a sound card might provide
this.

I would appreciate any direction for further reading regarding this and I
would appreciate any direction/correction/etc. in the thoughts above.

Joe

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of wa1zms at att.net
Sent: Saturday, November 08, 2008 6:59 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Checking accuracy of Rubidium standards

This sounds to be a very similar method that my Tracor 895A uses.
Does that sound correct?

-Brian, WA1ZMS

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com]On
Behalf Of Neville Michie
Sent: Saturday, November 08, 2008 5:09 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Checking accuracy of Rubidium standards


Hi,
I have a plan which involves the dividing down of the 10MHz of a GPSDO
and a rubidium (LPRO) to about 1MHz or 100kHZ and applying them to a
XOR or D latch to get a PWM signal that can be averaged for a strip  
chart recorder or
12 bit analogue data logger. The DC output gives a range of 5 volts  
for one
microsecond or 10 microseconds phase difference and folds back if  
this difference is exceeded.
The data from the datalogger is in a format that a spreadsheet can use.
With time and phase measurements I wonder how hard it is to get Allen  
variance.
I realise the PWM method requires a low pass filter and this will  
prevent short period
variances from being calculated.
cheers, Neville Michie
and follow the instructions there.

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