Thank you. Now that I know this is not the normal behaviour I can better search for the cause.
> Hello, > > This behavior smacks of a genuine hardware failure, not "drift" > in the usual analog sense. I'd be looking for bad solder joints, > loose or defective cable connectors, that sort of thing. If you have > series SMT capacitor(s) in your signal path, that's another > possibility. These parts are made of very brittle dielectric > materials and can develop cracks and other failure modes due > to board flexing associated with tightening nearby SMA > connectors, etc. If you can arrange to monitor the levels in > near-real-time, try wiggling the signal cable, prodding parts > on the acquisition board, and that kind of thing. > > Or perhaps there's a failure in your signal generator itself. > > Ordinary component drift is of the character of one (or a few) > percent change in value occurring slowly and fairly smoothly > over a span of several years. > > Dana Whitlow > Arecibo Observatory > > > On 3/22/2013 6:00 PM, [email protected] wrote: >> 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? >>> >>> >>> >> >> >> > > >
