birdies close to DC
Narrow band data was taken with the mock
spectrometers to investigate birdies close to DC. The setup was:
The plots show the averaged
spectra (.ps) (.pdf):
- if2 noise source was selected.
- group 0 and group 1 pdev boxes were used: BnSmG0 and BnSmG1
- the lo band (175 MHz lo) and the high band (325 MHz lo) were both
- The spectrometers were setup for:
- 172.032 MHz clock
- decimation =512 (gives bw=172.032/512= .336 MHz)
- 8192 channels (-->41 hz resolution)
- 30 second integration.
- Each color is a different beam or box.
- The top plots are polA, the bottom plots are polB.
- For a given beam/box, groupN band0 and groupN band 1 come from
the same box (but different fpga's inside the box).
- Page 1: group 0 band 0. (this is the 175 MHz lo)
- there is a comb with about 3 khz spacing in both polA and polB
- This comb is not very strong in group 1 band 0.
- It is not seen in group 0 band1.
- Page 2: group 1 band 0. (same data as page 1)
- Page 3: group 0 band 1. This is the 325 MHz lo)
- Page 4: group 1 band 1. Same data as page 3.
- A 3 KHz comb is seen in
group 0 band 0
- It is not seen in group 1 band 0 ( the same data split after
- so it must come after the group 0,group 1 split in the mixer
- It is not seen in group 0 band 1.
- these are in the same box but different fpgas.
- There may be some geometry causing this:
- group 0 boxes are above mixer chassis, below pserv cpu
- group 1 boxes are below mixer chassis, above pserv cpu.
- birdies at 75-80KHz.
- some boxes have them at a common freq.
- When a box doesn't have this birdie, it doesn't have it in both
high and low bands
- see bm 1, band 0 group 0 and bm1, band 1, group 0.
- Probably means the birdie is coming from the box
- I think this was around a freq of the power supply
- 1/2 and multiples of this frequency are also seen.