Single pixel observing with the mock spectrometer supports up to 7
independent frequency bands. Each band can supply: summed pols
(1spectra) , polA,B (2 spectra), or full stokes (4 spectra) with 16 to
8192 channels per spectra. 12 bit A/D samplers running in complex
sampling mode provide a maximum bandwidth for each spectra of 172.032
MHz. A low pass digital filter allows decimation of the 172.032
MHz spectra from 1 to 1024 in steps of 1. A 4*fftlen polyphase filter
bank improves the channel to channel isolation. The output power
spectrum can use 2 or 4 byte integer data as output. Each
spectrometer band (1,2,or 4 spectra) can record data at about 70
MBytes/second. Running all 7 spectrometers at the maximum i/o would
give an aggregate data rate of 490 Mb/sec. To support this data rate,
each frequency band is connected to a separate 5 terrabyte disc array.
The parameters common to all frequency bands are:
- Number of bits for output
- Spectral integration time
The parameters that are independent for each band
- configuration: summed pols, polA,B, stokes
- number of channels
- bandwidth divisor (decimation)
- Single pixel observing does not
support the winking cal
synchronized to the mock buffers.
- Each mock spectrometer box has two fpga's (sp0 low band, sp1
high band) each capable of processing a complete (all pols) 172MHz
band. Single pixel observing only uses sp1 (the high band) from each of
the 7 mock boxes.
- The baseband mixer before the mock boxes provides:
- to sp0:
- 0 to250 MHz low pass filtered IF.
- IF CFR can be 150 to 250 MHz
- to sp1:
- 0 to 400 MHz low pass filtered IF
- IF CFR can be 250 to 400 MHz.
- Wideband single pixel observing uses the 1 Ghz IF.
- When mixing from the 1st IF (1=2 Ghz) to the 2nd IF (200-400
MHz) 3 Overlapping filters are used:
- 1000 to 1500,1250 to 1750,1500 to 2000
- The 2nd Lo is placed IF2 MHz away from the center freq in
IF1. Since each filter is 500 MHz wide you would like the 2nd IF to be
at least 250 MHz to put the 2nd LO outside the 500 MHz filter.
- If we used the lower band sp0 IF=175, then the 2nd LO would be
inside the 500 MHz filter for the IF1 to IF2 mixing.
- The winking cal synchronized to the mock buffers is only
generated from the lower sp0 fpga (this patch on the board is not
connected to sp1).
- Since single pixel uses sp1 (hi band) there is no winking cal
output signal generated.
- In 32 bit stokes mode, the fpga
drops 1 fft between each
accumulated spectra. If 400 ffts are accumulated, the wall time
each accumulated spectra will be 401 fft times.
- Since it is an fft based spectrometer, the integration times must
be multiples of the time for 1 fft (fftlen/clockFrequency). The
interface will find the integration times closest to the ones requested.
- The single pixel mock
spectrometer currently (24mar10) uses a 325
MHz IF center frequency. This is different than the 250 or 275
frequency used by the wapps. This difference precludes observing with
the wapps and the mock spectrometer simultaneously (this will
eventually be fixed).
- nov10: I've added the clock and losynth as settable parameters
in cima (svnwork version). You should be able to set the 2nd IF to 250
MHz. This is on the edge of the i,q phase generating chip for this
band. We need to check whether this is really going to work before
telling people to use it..
More info on the mock spectrometer can be found here