If other people see the drift, then I suspect the gain of the input stages of the ADC boards drift. If so, then switches will be needed in some of my spectrometer designs which currently lack them. What do you see or think?
> I had reason to return to using the old > spectrometer of CASPER Tutorial 3. This > spectrometer is old enough to be useful > for diagnostics with new designs, but I > noticed a problem when viewing the FFT > spectra. With an input of a 1Vpp sine > wave at 5, 10, 15, 20, 25 or 30 MHz, > the amplitude of the peak in the FFT > spectrum changes in two ways: during a > single run and between runs. I was > trying to determine the source of this > change because I am trying to calibrate > the vertical (amplitude) axis of the > spectrum obtained in the tutorial. > > For example, during a run the amplitude > either rises to almost four times its > original value or drops to a quarter of > its original value. If I wait for a > long time, the amplitude seems to rise > slowly. If I shut down the spectrometer > and all the instruments except the > ROACH board, and then repeat the run at > a later time, the readings may be > different by a factor of five or even > ten. This is making calibration very > difficult. If this is normal behaviour > for the spectrometer of Tutorial 3, > then I may be forced to give up an easy > calibration and use a switch for all > measurements so I can monitor the drift > during a run. > > The apparatus consists of the ROACH 1 > board, an old Bee2 ADC board 1GHz > oscillator for the ADC clock, an old > Wavetek oscillator for the input sine > wave and a blocking capacitor at the > input of the ADC board. The oscillator > does not give a perfect sine wave. Is > the drift normal behaviour for the FFT > spectra? > > >
