spectral jumps at 2^n channels
Spectral jumps have been seen at power of 2 channels
previously. The jumps correlate with the input power levels. Higher
input levels make the jumps go down. Not doing the first down shift
the butterfly stages also makes the jumps smaller. They may be
underflow in the butterfly processing. Computing the rms values in
counts when the jumps occur seems to leave values that should not be
20may10: high res decimation
for adpwr=30,45,120 and pshift=0x1ff5, 0x0ff5.
- Shows jumps vs decimation 1 to 20.
- The size of the jump decreases with increasing input level.
input level changes with the decimation used (but not by more
factor of 2 in voltage).
- Not doing the first butterfly down shift make a big
jumps vs A/D and pshift levels.
- Shows jumps for adjpwr:15,20,25,30,80 with various pshifts.
jumps with noise source.
Data was taken using the if2 noise source. The
- 8192 channels, 172 Mhz bandwidth.
- integrate for 60 and 300 seconds.
- Take data with adjpwr of 30,45 and 120.
- Use the standard pshift=0x1ff5. Also try pshift=0x0ff5 (where
first butterfly down shift is not done).
- test numbers:
The plots show the spectral jumps for
test (.ps) (.pdf):
- pshift=0x1ff5, adjpwr=30, 60 secs
- pshift=0x1ff5, adjpwr=45, 60 secs
- pshift=0x1ff5, adjpwr=120 , 60 secs
- pshift=0x1ff5, adjpwr=120 , 300 secs
- pshift=0x1ff5, adjpwr=30 , 300 secs
- pshift=0x1ff5, adjpwr=45 , 300 secs
- pshift=0x0ff5, adjpwr=45 , 300 secs
- pshift=0x0ff5, adjpwr=30 , 300 secs
- pshift=0x0ff5, adjpwr=120 , 300 secs
- Page 1: The jumps at channel 2048 for the 9 tests.
- A linear fit was done to channels 2007-2047 (fit1 red) and
(2048-2088 fit2) green.
- The jump was: (fit2 -
- The vertical scale is Tsys close to Channel 2048.
- Page 2: jumps, power vs test number:
- Top: The size of the jump vs test number. Black is polA, red
- Bottom: the A/D rms in counts fore each test.
- Page 3: Size of jump vs a/d rms input level
- polA is +, polB is *
- Black has the standard pshift=0x1ff5, red has pshift=0x0ff5
- The size of the jump decreases as the rms input level
- Not doing the first down shift in the butterfly stages makes
large decrease in the jump size.