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?
