Spectral jumps vs A/D and pshift levels.

08jul08



Introduction

    Spectral jumps were seen in data taken back in 18mar08.   That data had the rms fixed at 15 and changed pshift as well as fixing pshift and changing the rms. It was taken with a fixed fftlen of 8192. Since then most data has been taken with an rms=30 and pshift set to have 9 downshifts and then alternate shifting every other butterfly stage.
    A new set of data was taken with the if2 noise source. The setup was:
The goal is to find a configuration that minimizes the rms while being free from the spectral jumps seen in the spectra when the a/d rms is low.
The tests done were:
Tests performed

butterfly downshifts used
a/d
rms
Relative power levels of spectra vs fftlen
8192 4096 2048 1024 512
0
bandPass shape
80
3.6
7.1
3.6
7.1
3.6
1
9 downshifts then 0,1
30
.50
1
.5
1
.5
2
9 downshifts then 0,1
25
.35
.7
.35
.7
.35
3
9 downshifts then 0,1
20
.22
.44
.22
.44
.22
4
10 downshifts then 0,1
30
.5
.25
.5
.25
.5
5
9 downshifts then 0,1
15
.13
.25
.13
.25
.5

   Plots where made to show the results. For the plots:

8192
4096
2048
1024
512
band pass shape:
  
These plots show the bandpass shape for the various beams and fftlengths. The bandpass with rms=80 is plotted as well as the bandpass with rms=30 counts. You can see that the spectra with rms=30 has more bumps in it than that with 80 counts. I wouldn't expect this to be the case.
  • 4 pages per plot showing: polA_loband, polB_loband, polA_hiband, polB_hiband
    • Each page shows the 7 beams.
    • the brown line is test 0 with rms=80
    • the black line is test 1 with rms=30.
    • dashed vertical lines are at powers of two showing where jumps might be.
plot
plot plot plot plot
Ratio spectra/spectraRms80:
   
The spectra from each test has been divided by the spectra with rms=80 to get rid of the bandpass shape.
  • 8 pages/plot.
    • page 1,2: polA lowBand
    • page 3,4: polB lowBa
    • page 5,6: polA hiBand
    • page 7,8: polB hiBand
  • The ripples in the band pass get larger as the a/d rms level decreases (the attenuator value increases).
    • rms=30 (black and blue line about the same)
    • rms=25 red, rms=20 green, rms=15 purple have progressively larger ripple.
    • I would have expected the ripple to go down as the gain went down (since attenuator value goes up). This must mean that the reflection is not passing through the attenuator.
    • The ripple spacing is about 10.6 Mhz or about 10 meters with the velocity set to .68 c.

plot plot plot plot plot
Ratio spectra/spectraRms80 Blowup:
   
The previous set of plots has been blown up about fftlen/4 and fftlen/2 to see the possible jumps.
  • The left column shows fftlen/4, the right column shows fftlen/2
  • Most spectra have steps on the positive side of fftlen/2 (about dc).

plot plot plot plot plot

 

Conclusions:

processing: x101/080708/nsetst.pro

<- page up
home_~phil