[time-nuts] Noob question on measuring Allan Deviation on 10 MHz source

John Ackermann N8UR jra at febo.com
Wed Dec 14 16:19:09 EST 2011

Hi George --

You can feed frequency data into Stable32, but the documentation doesn't 
clearly explain that you need to scale the readings into fractional 
frequency using the scaling function in the File/Open dialog.  To get 
fractional frequency, you divide the results by the nominal frequency, 
except that the scaling model in the Stable32 input box allows 
multiplication only.

So, for a nominal 10 MHz (or 1e7 Hz) source where the data is in Hz 
format (10,000,000.xxx Hz), you would multiply by 1e-7.

But if your counter outputs in MHz format, (10.xxx MHz), that's already 
effectively scaled by 1e-6.  So you end up using 1e-1 as the multiplier.

I have lost much hair trying to keep this straight; as wonderful as 
Stable32 is, the documentation is aimed at people who already know what 
they are doing. :-)



On 12/14/2011 3:29 PM, George Dubovsky wrote:
> List;
> OK, I need to measure the stability of a 10 MHz sine-wave source. After
> reading a lot of background info on this list and some of the sources that
> were referenced, I thought I could get away with a frequency measurement. I
> now think I was wrong.
> What I have is an Agilent 53230A counter (a pretty capable box - claims 20
> ps one-shot resolution in TI mode), a Trimble Thunderbolt, the 10 MHz
> oscillator to be measured,  and a copy of Stable32. My first effort
> involved feeding the Trimble 10 MHz into the counter as its Ext Reference.
> I then fed the Trimble 1pps into the Ext Trigger input of the counter and
> fed the sinewave 10 MHz signal to be measured into Ch 1 of the counter. I
> then captured the frequency reading of the counter every second and stuffed
> those numbers into a file. I collected about 20 hours of frequency
> readings, but when I imported that into Stable32 and attempted to do an
> Allan Dev plot, it didn't look very good - specifically, the sigma numbers
> were in the region of 10e-2 to 10e-4.
> So, I grabbed another Thunderbolt and attempted to do the same measurement
> on it, figuring that everyone (but me) has taken data on a T'bolt, so I
> could just look on tvb's site or some such to find proper data on a Tbolt.
> Again, the plot didn't look like it should.
> Am I going to have to go to time interval measurements to do what I want?
> And does this mean I will have to square up my 10 MHz signal to have real
> edges?
> geo
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