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Message ID: 497     Entry time: Tue Feb 15 01:02:31 2011
Author: Frank 
Type: DailyProgress 
Category: BEAT 
Subject: calibration problems? 

looks like a fu**** up the data aquisition with the short cable, but i don't know how.

When trying to calibrate the data taken i realized that something was totally wrong as i got some khz/rHz but i couldn't find the mistake.
So i thought the gain setting if the preamps must be different than i wrote down but they are not.
So i checked the calibration again, this time in 1MHz steps all the way through the system but everything was/is OK.
Got almost exactly the same mixer response vs frequency tuning and also the dc-coupled signal was what i measured before.

Here the problem:

The VCO feedback signal gives us a rough idea what frequency we have, which i checked over and over again and matches the frequency counters we have.
The problem is that the DC signal (already amplified) for pi in phase change goes from something like 2.3V to -2.4V.
The same signal from end to end of the VCO goes from -1.7V to 1.4V, see
here.
I measured this several times, last week and today and this is fact whatever equipment i use (scope, multimeter, DAQ).

So now the funny part:

The DC signal and the VCO monitor signal look almost identical (shape, uncalibrated).
As i checked  the calibrated (fitted) VCO monitor signal reflects the beat signal within 100kHz or so, but 1MHz for 100% sure.
Now, taking the recorded DC-signal from the mixer from 35h of data and using the calibration (which i did several times) the beat frequency is out of range of the VCO, totally different !
The recorded signal does not match the VCO signal at all! Using the same coefficient to calibrate the spectrum recorded with the AC-coupled amplifier the noise is way to high, higher than everything we had before.
So i took the VCO monitor signal, assuming it is right and showing me the absolute beat frequency and calculated the right coefficient for the mixer signal, which is something like 5MHZ/V instead of 170MHz/V.

Done that, comparing both time series looks fine.
Taking this coefficient to calibrate the spectrum then gives, at least for a the features around a couple of Hz the right level we measured several times before.
The lower frequencies are dominated by the noise of the preamp, so no comparison possible.

So i don;t know what's wrong because i can't reproduce what we recorded. Everything looks the same as last week, checked over and over again. But the DAQ shows something different.

RESULT:

As we can't trust whatever we recorded we redo the measurement. After bringing back the 4-way splitter to 40m i'm using 2-way splitters now instead, increased the signals where possible and the gain of the preamp. re-calibrated everything and triple checked, taking another measurement over night. We will see tomorrow....

 

Plots, 32h data stretches.
noise plot shows spectrum of data taken with and without 30mHz high pass correction

frequency_drift.png

 

fnoise.png

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