Frequency-switched Observations

Hiroshige Yoshida
Caltech Submillimeter Observatory

Frequency-switched observations for the heterodyne instruments at the CSO was originally implemented by fast-switching the second LO frequency in the 1-2 GHz IF pass using the FPGA-controlled hardware. It is not possible to frequency-switch in the same way with the current spectrometers (AOS5, FFTS1, and FFTS2), since they are designed for the 4-8 GHz IF and have their own IF processor.
A newly implemented FSWITCH command allows frequency-switched observations by "slowly"-switching the first LO frequency. The basic command syntax is as follows:
FSWITCH <Separation(MHz)> [<Cycles>]
For example, to repeat 3 second integration each of ±10 MHz offset pair 5 times, issue the following UIP commands:
UIP> uip\spectrometer 3
UIP> uip\fswitch 10 5
Please note that the shortest possible integration time for the AOS5 and FFTS1 is 0.5 seconds. However, it has been reported that short integration time (1-2 seconds) causes the FFTS1 to crash. Three (3) second and longer integration seems to work without problems.
Scans taken using the FSWITCH command are saved as regular (position-switched) scans in a CLASS data file. In order to be able to "fold" them, a header section used by the FOLD command needs to be added by the MODIFY SWITCH_MODE command. Set the SWITCH_MODE to FREQUENCY and specify the offset frequency in MHz, integration time in seconds, and weight of two phases:
LAS\MODIFY SWITCH_MODE FREQUENCY 2 <Offset_1(MHz)> <Time_1(s)> 1 <Offset_2(MHz)> <Time_2(s)> -1
For example, for the scan taken using the above UIP commands (±10 MHz offset and 5×3 second integration per phase), use the following CLASS commands:
LAS> las\modify switch_mode frequency 2 10 15 1 -10 15 -1
LAS> las\fold
Baselines may need to be taken out before or after folding scans.

File translated from TEX by TTH, version 4.03.
On 25 May 2012, 09:34.