[time-nuts] Calculating frequency differences using Lissajoufigures

[Bill Hawkins]

Well, it's 5 PM in Minneapolis, so I joined my wife on the deck to toast
the vanishing sun. Didn't sing any Jimmy Buffet songs, though.

But, to the matter at hand. If your Lissajous pattern scope has a Z input
for intensity modulation, drive it with one of the 10 MHz sources. The
direction of rotation will be obvious, as the intensified portion chases
its way around the pattern.

Then again, maybe you need to phase shift one of the signals 90 degrees,
apply the straight and shifted versions of the same signal to X and Y so
you get a circle, and then apply the other signal to the Z input. That
would not be a Lissajous figure, though.

You would still need to observe any changes, unless you are certain that
the frequency difference will maintain the same sign. I guess that's what
drives people to buy GPIB interface cards and spend hours making the whole
thing work correctly.

Bill Hawkins


-----Original Message-----
From: J. L. Trantham
Sent: Saturday, November 07, 2009 5:46 PM
To: 'Discussion of precise time and frequency measurement'
Subject: Re: [time-nuts] Calculating frequency differences using
Lissajoufigures

And you watched it the whole time to see the 'reversal' of the rotation.  

Once going, it is not possible to tell 'which way' it is rotating.  In other
words, can you tell if it is rotating clockwise or counter clockwise?  

I would suggest making the GPSDO the 'trigger' for the scope and watch the
LPRO on one of the input channels of the scope.  If I recall correctly, if
it moves left, the LPRO is high and if it moves right, it is low in
frequency.  Please do not quote me on this as I have had a glass of wine.
After all, it's 5 o'clock somewhere.

Joe

-----Original Message-----
From: Hal Murray
Sent: Saturday, November 07, 2009 3:50 PM
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] Calculating frequency differences using Lissajou
figures


>>> So your frequency drift in this example is 1.7e-11 / month.

>> Not quite, you need to take the sign of the frequency difference into 
>> account.

> But the sign of the difference would be obvious from the rotation
> direction of the Lissajous.   

Only if you remembered to include the direction when you took the first 
measurement and if you connected the scope up the same way a month later.


-- 
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