[time-nuts] Measuring gpsdo vs itself

[Tom Van Baak]

Ah, I see what you mean now. Yes, that setup can give you a rough estimate of the counter's noise floor.

I can't give you specific numbers but one danger with this sort of test is that the input and the timebase are artificially locked together (i.e. fixed phase relationship) through the common reference. Your measurements may thus show artificially less noise than a real-life case of independent input(s) and reference.

This can happen if your sub-ns counter is based on interpolators. Because the input and the timebase are locked in phase, the counter lands near the same point of the interpolator scale on every single measurement, rather than experiencing the noise (and non-linearity) of the entire scale.

/tvb

----- Original Message ----- 
From: "Bill Dailey" <docdailey at gmail.com>
To: "Tom Van Baak" <tvb at leapsecond.com>; "Discussion of precise time and frequency measurement" <time-nuts at febo.com>
Sent: Sunday, November 04, 2012 8:33 PM
Subject: Re: [time-nuts] Measuring gpsdo vs itself


I guess what I am saying is if I discipline the counter with 10MHz and then measure the same 10MHz.  Just making sure we are on the same page.

Doc

Sent from my iPad

On Nov 4, 2012, at 10:22 PM, "Tom Van Baak" <tvb at LeapSecond.com> wrote:

> Bill,
> 
> This is usually a good idea, since the counter then has both a good short- and long-term stable/accurate timebase, inherited from the GPSDO. It means the internal timebase of the counter is no longer a factor in measurement stability or accuracy. There are exceptions to this, but I'll guess your setup is not one of them. This configuration is especially good for 1PPS TI measurements since it means a short 100 ns TI measurement is just as accurate as a long 0.999999900 s measurement.
> 
> I'm not sure I'd call this a "reference independent" system; it's simply using a GPSDO as the reference instead of the internal XO timebase of the counter.
> 
> /tvb
> 
> ----- Original Message ----- 
> From: "Bill Dailey" <docdailey at gmail.com>
> To: "Time Nuts" <time-nuts at febo.com>
> Sent: Sunday, November 04, 2012 7:19 PM
> Subject: [time-nuts] Measuring gpsdo vs itself
> 
> 
>> If I use a gpsdo as my reference and feed the same 10MHz into a counter does that yield the reference independent noise floor of the measuring system? Seems to me it would look like an ideal reference with respect to the measuring system.  Thanks,
>> 
>> Doc
>> KX0O
> 
> 
> 
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