### FREQUENCY SWITCHING

FREQUENCY SWITCHING switches between 2 different frequencies, so that there are 2 phases. The (source) signal is calculated as the difference between these 2 phases.

FREQUENCY SWITCHING is normally used with TRACK or OTFMAP.

NOTES. IMPORTANT:

FREQUENCY SWITCHING is very powerful and efficient for some projects, e. g., mapping of narrow spectral lines in cold dark clouds outside the plane of the Milky Way. However, before deciding to use frequency switching one should consider some potential drawbacks:

The target lines should be narrow enough so that line signals from the 2 phases of the switching cycle are well separated.

The spectral baseline will generally be less flat than in other switching modes.

Some spectral lines are also emitted in the earth's mesosphere, e.g., the mesopheric lines from (12)CO are rather strong, and they will be seen in FREQUENCY SWITCHING spectra taken toward astronomical sources with a low Doppler shift. The mesospheric lines will appear at a frequency and velocity that corresponds to the rest frame of the athmosphere, i. e., the observatory. Care must be taken that they are not confused with the lines from the astronomical source. (Information computed by the ASTRO software can help with this decision).

When observing sources near the plane of the Milky Way, line emission from clouds at other velocities than the target source, e. g., other spiral arms, can cause confusion.

In case of doubt, consult the special memo on FREQUENCY SWITCHING or ask an experienced FREQUENCY SWITCHING observer!