[time-nuts] GPS: ADEV or MDEV? (Tom Van Baak)
brooke at pacific.net
Mon Apr 23 19:25:27 EDT 2007
That's one sweet meter. Years ago at work I used them and we had both flavors.
One used the old definition of the volt and the other used the new volt.
Although our cal lab was not good enough to calibrate them we could feed them
signals and measure their readings. They differed by the difference in the
definition. Quite amazing.
They also can be used as a kind of sampling scope. As I remember it the data
is arranged in a strange way. There might be a driver available from Agilent
to do the scope thing.
Colin Bradley wrote:
> My 3458A can make measurements down to the 10nV level. At 7 1/2 digits it can do 60 readings
> a second. At 8 1/2 digits it slows down to 6 readings a second. The real question is the frequency
> of the noise. I have yet to interface it to HP-IB bus. I have several Z3801's I could try it on.
> Good. Thanks for the cool plots. OK, my eyeball sees:
> 5 uV peak-peak, maybe 2 uV rms.
> 10 uV peak-peak, maybe 4 uV rms.
> < 1 uV peak-peak, about 0.2 uV rms
> 4 uV peak-peak, guess 1 uV rms
> 3.5 uV peak-peak, < 1 uV rms
> 10 uV peak-peak, 1 uV rms
> 3 uVpp per spec
> This suggests that one could get Vref noise down to about
> 1 uV (or better if you measure rms). Does anyone on the list
> have actual experience with this issue?
> Anyway, assume a full 1 uV of noise and assume a 10811A
> with an EFC sensitivity of 1e-6/volt. Then Vref noise translates
> to 1e-12 in frequency -- which is worth considering since it's
> in the same ballpark as good OCXO noise. Maybe not the
> dominant factor, but a potential contributing factor.
> For example, below is a 10-minute frequency plot of a typical
> hp 10811A which has a low ADEV of about 2e-12 (tau 1 to 10
> seconds). The scale is 1e-11 / division.
> So you can imagine that Vref noise on the order of 1e-12 might
> just start being noticeable in the plot, meaning that if you want
> short-term performance down in the low 12's and if you have
> an OCXO that even gets that low, then you want to keep your
> Vref (and DAC, filters, etc.) clean to microvolt levels.
> It's just a back of the envelop calculation so I'm not really sure.
> Someone correct me if you see something wrong.
> Related - does anyone have equipment in your home/lab that
> can directly measure, at uV or sub-uV levels, noise on the
> EFC line? I wonder how clean a Z3801A EFC is, for example.
> Also an idea - is there an easy way to artificially increase Vref
> noise, by say 10x, so one could see if that change made any
> measurable change in OCXO output ADEV or phase noise?
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