14jun12: glitches gone with bias mon off

01jul12

The alfa dewar monitoring was causing glitches in the receiver. They would show up as multiples of 20 Hz in the p2030 data.
On 14jun12 a test was done to see if the glitches would go away if we stopped reading the bias values, or we had to stop all alfa mon reads (temperature, cals, etc.) to get rid of the glitches. The setup was:

alfa cover was off
mock spectrometers 172 Mhz, 128 channels, 65 usec dumps. Used the 1450 Mhz band.
Software was written to turn off just the alfa bias monitoring. The temperatures, amp status, supply voltages were still being read.
take 300 seconds of data with bias monitoring off, other monitoring on,
take 300 seconds of data with all monitoring off.

The data processing included:

Input each file and compute the total power for each sample and beam.
normalize each total power time series to its median value.
Do a 4 million point fft on the total power data.
Plot the spectra of the total power vs freq.

The plots show the results of the test:

total power spectra with bias mon off and all monitoring off
Spectra bandwidth	Bias Reading off	all readings off
500 Hz bandwidth	(.ps) (.pdf)	(.ps) (.pdf)
50 Hz bandwidth	(.ps) (.pdf)	(.ps) (.pdf)
5 Hz bandwidth	(.ps) (.pdf)	(.ps) (.pdf)

Notes:

500 Hz bw:

60 Hz, 120Hz, 240Hz, 360Hz are strong independent of the bias monitoring state.

The 120,240,260 are harmonics of 120 Hz. This could be detected 60 Hz. Probably from inside each mock box.
Bms 5 and 6 are the cleanest.

350 Hz. This is from the FAA radar.

5 Hz bw.

bm 0 polA shows the crosshead frequency of 1.2 Hz.

These can be compared to the plots taken with the bias monitoring on (from 24may12)

Total power spectra all monitoring on (24may12)
spectral bandwidth	all monitoring on
100 Hz	(.ps) (.pdf)
5 Hz	(.ps) (.pdf)

Summary

The 20 Hz glitches from the alfa dewar monitoring is coming from the bias monitoring. Read the other values (temp,power supplies, etc) do not contribute to these glitches.
bm0 a sees the crosshead frequency.
60 Hz and harmonics of 120 Hz are seen. they are probably in the mock boxes. bms 5 and 6 are the cleanest.

processing:: x101/120614/alfabirdies_xx.pro

home_~Phil