The flexibility in the LO chain (see Chapter 2) means that in many cases more than one combination of LO settings will yield the same sky frequency. To avoid phase jumps, one does not want to change First LO and L6 frequencies for a particular source/velocity combination during an observing run. The user should verify that they remain fixed throughout the run by inspecting the file produced by observe. A concise description of the observe file is given in Appendix B. Generally, observe will select the optimum LO settings for an observation. The program loser, however, allows one to see all of the possibilites so that, if so desired, another setting can be selected from those available. To run loser (again, the program is available on all of the workstations at the AOC) first determine the approximate sky frequencies at which you expect to observe and the total bandwidth (corresponding to the bandwidth code) that you plan to use and run loser with those as inputs. loser will respond with a prompt and will await your input. The command inp prompts loser to display the current input selections; the user has to specify the bandwidth, filter, and observing frequency for each IF pair. The user must also supply an observing mode (e.g. Line Hanning 4IF; see Appendix A for a full listing of the possible observing modes). Once one has entered the necessary inputs, type go and loser will respond by displaying the appropriate LO settings and representations of the LO and FI cards in the observe file filled in with the appropriate values. On-line help is available by typing help.
A typical input and output for loser is shown below. The
LO-card line shows two values for the first LO and two for the
synthesizers, whereas the FI-card line shows two values for the Fluke
synthesizers. The first of each pair of values controls the A and C
IFs, the second the B and D IFs. For a bandwidth of 50 MHz only
certain frequencies can be chosen (
MHz), for 25
MHz some frequencies are inaccessible, while for narrower bandwidths
there often are several choices. One can specify the front-end
filters as well. Use of the front-end filters will reduce the number
of options available; there are set-ups for which no front-end
filter but the default 50 MHz one will lead to a valid LO set-up. If
at all possible, choose a combination of LO settings such that the
first LO and L6 values are not changed during the course of your
observations as re-tuning the LO will cause phase jumps. Use the
settings generated by loser to modify the settings used in observe, if so desired. This is dangerous, though. Always check the
RFI section in the OSS to be sure to avoid internally generated RFI,
such as the 4800 MHz birdie.
Sample output:
LOSER Version 3.5
=====
I calculate the Local Oscillator Settings
*acbandw 3.1MHz
*bdbandw 3.1MHz
*acfreq 1412.9MHz
*bdfreq 1412.9MHz
*obse line hann 4if
*inp
ACBANDWIDTH 3.1250000MHz 1.5625000MHz
ACFILTER 50.0000000MHz
ACFREQUENCY 1412.9000000MHz
ACSTANDARD yes
BDBANDWIDTH 3.1250000MHz 1.5625000MHz
BDFILTER 50.0000000MHz
BDFREQUENCY 1412.9000000MHz
BDSTANDARD yes
OBSERVINGMODE Line Hanning 4IF
OUTFILE DSK:LOSER.DAT
*go
//LO -3.2000-3.2000 3590 3590
//FIS 121.437500 221.437500
//LO -3.2000-3.2000 3590 3610
//FIS 121.437500 201.237500
//LO -3.2000-3.2000 3610 3590
//FIS 101.237500 221.437500
//LO -3.2000-3.2000 3610 3610
//FIS 101.237500 201.237500
The channel separation for AC is 97.656 kHz.
1412.9000000 MHz corresponds to channel 16 of channels 0 to 31.
The channel separation for BD is 97.656 kHz.
1412.9000000 MHz corresponds to channel 16 of channels 0 to 31.
The IFs AC are at 20cm, and BD at 20cm.
The observing bands are therefore 'LL'. (Command BANDS in OBSERV)
The bandwidth codes are: 4 4 4 4. (Command WIDTHS in OBSERV)
WARNING - The two passbands overlap.
*q
Normal termination of LOSER program
Note that some of the terminology still reflects the ``old days'', such as reference to Commands BANDS and WIDTHS. This should be ignored.