MPIfR Fast Fourier Transform Spectrometer (FFTS1)
Hiroshige Yoshida
Caltech Submillimeter Observatory
This memo provides brief instructions on using the Fast Fourier
Transform Spectrometer (FFTS), developed for the Caltech Submillimeter
Observatory (CSO) by the Max-Planck-Institut für Radioastronomie
(MPIfR), with the CSO facility heterodyne receivers through the
standard User Interface Program (UIP).
The basic characteristics of the FFTS are summarized below.
| Wideband Mode | High Resolution Mode |
Signal Processing | 4-tap WOLA with Flat Top Window |
Total Bandwidth | 1000 MHz | 500 MHz |
Resolution | 8192 channels |
Channel Spacing | 122 kHz | 61 kHz |
Equivalent Noise Bandwidth | 117 kHz | 58 kHz |
The FFTS can be used either with the new generation 4-8 GHz IF
receivers or the 1-2 GHz IF receivers in the Side Cabin. The FFTS
can process the 4-8 GHz IF of the former directly. For the latter, a
separate IF processing system, developed for the array acousto-optical
spectrometer (AAOS), up-converts the 1-2 GHz IF to 6-7 GHz. The
center of the spectrometer can be positioned anywhere within either IF
passband. The FFTS can be operated simultaneously with the existing
acousto-optical spectrometers (AOSs).
UIP's
AOS command has been replaced with the (more appropriately
named)
SPECTROMETER command, which supports the FFTS. In
addition to all the parameters and options present for the
AOS
command, the
SPECTROMETER command provides two new options,
/FFTS1W and
/FFTS1N. The
/FFTS1W option specifies
that the MPIfR FFTS should be active in the 1000 MHz bandwidth mode,
while the
/FFTS1N option specifies that the FFTS should be
active in the 500 MHz bandwidth mode. They take an optional frequency
offset in GHz as their argument that specifies the position of the
FFTS relative to the IF center. The center IF defaults to 1.5 GHz
(6.5 GHz after up-conversion) if the 1-2 GHz IF receiver is active
when the command is issued, or 6.0 GHz otherwise. Some examples
are shown below.
-
UIP> SPECTROMETER /FFTS1W
will start the FFTS in the 1000 MHz bandwidth mode, while
-
UIP> SPECTROMETER /FFTS1N
will switch the FFTS to the 500 MHz bandwidth mode (use of the
/BANDWIDTH option has been deprecated).
-
UIP> SPECTROMETER /FFTS1 -0.25
will shift the center of the FFTS to the IF of 5.75 GHz when one of
the 4-8 GHz IF receivers is in use, or to the IF of 1.25 GHz when one
of the 1-2 GHz IF receivers is active (the original
/FFTS1
option can still be used for this purpose).
In order for the FFTS to be configured correctly, the receiver
(specifically whether the 4-8 GHz IF receiver or 1-2 GHz IF one) has
to be selected using the
LO command
before the
SPECTROMETER command is issued to change the bandwidth or IF
offset.
The telescope identifier of the FFTS in the CLASS header is
CSO
FFTS 1W when the spectrometer is in the 1000 MHz bandwidth mode, or
CSO FFTS 1N when it is in the 500 MHz bandwidth mode.
The temperature calibration (TCAL) scheme has been changed for the
FFTS to take advantage of its higher dynamic range. The input IF
signal level is auto-adjusted on the
sky before the TCAL scan.
Only for the 230 GHz receivers, an extra 3 dB attenuation is added
while the integrations on the ambient temperature load and sky are
taken. In contrast, the input IF signal level of the AOSs is
auto-adjusted on the
ambient temperature load before the TCAL
scan, then it is
readjusted on the
sky after the
calibration.
One annoying bug, which will not be fixed, is that the FFTS
occasionally gets stuck in a failure mode where it takes
twice
longer to integrate, or it will not integrate at all. To solve this
problem, the server program needs to be restarted. To do so, start
the VNC viewer on the computer
cso-ffts in the AOS lab. as the
user
FFTS. The password can be found in the control room. Then
type
^C in the terminal within the VNC environment to kill the
running server and restart it by typing
./startWOLA (we are
using the WOLA core instead of the regular windowed FFT core).
File translated from
TEX
by
TTH,
version 4.03.
On 25 May 2012, 09:34.