[time-nuts] ADEV noise floor vs counter gate time

[jpbridge at aol.com]

Hi All,

I'm in the process of getting a better counter, but at present I'm using my TTi TF930 counter.

For those who don't know it, it is a reciprocal counter which should be continuous, it counts periods in terms of its internal 50MHz clock which I've locked to an external 10MHz reference.

There are 4 gate times available, 0.3 secs, 1 sec, 10 secs and 100 secs.

These correspond to 7, 8, 9 and 10 digits.

I've been experimenting with using a single mixer (mini circuits ZAD+) along with a 1MHz low pass filter and appropriate attenuators to measure Alan Deviation (using my own software).

My set up is a 10MHz reference source (MV89A which I've approximately set using a 10kHz GPS signal).

The reference is used as the external reference for an Agilent 33522A arbitrary waveform generator.

The 33522A generates an 9.999910 MHz (10MHz - 90Hz) sine wave at 300mVpp to the mixer and the mixer is also fed by the 10MHz reference output of the 33522A via an attenuator to get it to roughly the same level.

The second output of the 33522A generates a 10MHz square wave as a reference for the counter (the counter requires quite a high reference signal and the reference out of the 33522A is too low a voltage to be used directly).

I initially ran this with a gate of 1 second and the LOG10(ADEV) curve drops linearly vs LOG10(tau) but then curves back up again. (I tried many variants such as using period rather than frequency and so on.)

But when I set the gate time to 10 seconds or 100 seconds then I get both lower curves and ones that no longer curve upwards.

The attached pdf shows the three curves on the same graph.

What puzzles me is that the counter at longer gates is only averaging to get more digits so the difference must come down to quantization in terms of the number of digits that are passed to the computer over the USB/RS232 link.

I find it rather puzzling.

James

 



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