cband stability (shutter closed)
The stability of the cband receiver/ iflo was
measured on 09feb11 during the day.
The setup was:
Processing the data:
- cband receiver centered at 4745.
- shutter on cband receiver closed (receiver looks at absorber
- 7 iflo bands selected at: 4325,4466,4608,4750,4892,5034, and 5176
- mock spectrometer:
- 172 MHz bw/band with 8192 channels/band.
- 1 second dumps
- full stokes
- Two "scans taken"
- scan1 3600 seconds
- scan2 2878 seconds
- 8 seconds between scans.
The plots show the stability of
the system with the shutter closed (.ps) (.pdf):
- The total power was computed for each sample,pol, and band by
averaging the 8192 channels of each spectra.
- Each set of 6478 total power numbers was normalized to the median
value (this defines Tsys).
- Using the first 3600 seconds of data (contiguous in time) compute
the spectrum of the total power to see if there are any periodic
- From the 6478 normalized total power dataset, remove a 2nd
- Pages 1-2: total power vs time. No polynomial removed.
- black is polA, red is polB.
- Each frame is a separate frequency band.
- The vertical lines mark every 600 seconds (to help align things
in adjacent bands).
- total total power varies 0 to 2% of tsys
- Pages 3-4: total power vs time with 2nd order polynomial removed.
- Same as pages 1-2 but the 2nd order polynomial was removed.
This allows us to blow up the vertical scale.
- Page 5: Spectra of the total power.
- the spectra of the total power was computed for the first 3600
- Top frame: polA, bottom frame polB. Each colored plot is a
totalPower spectra from a different frequency band.
- The black (4325 band) has a set of harmonics with .13 Hz
(7.69sec period). This may be coming from rfi leaking in.
- The total power varies by up to 2% of Tsys over the 6478 seconds.
- Tsys is probably 300 Kelvins with the shutter closed.
- The variation includes the receiver and the iflo.
- Some of the spikes in the total power spectrum may come from rfi
leaking through the shutter. There is occasional strong rfi in the 4325
- Comparing this variation with that seen on the sky.
- Tsys on the sky is about 30K. Tsys on the shutter is probably
- If there is a gain variation, then having Tsysshould not make
- If variations are "extra noise power" added near the frontend,
then the Tsys difference may make a difference.
- The dome did not move during the test.
- If there is a loose connection in the dome, then
tracking a source may cause the gain through this connection to
vary more than when the dome sits still.