The local oscillator LO1 corresponds to the LO used for the first frequency conversion. The keyword lo_config determines whether LO1A or LO1B is used as a tracking LO, with the other unused or operating as a test tone generator. Doppler tracking can be activated by setting ref_frame to anything other than ``local''.
The LO1 can be used in continuous wave (CW) mode, appropriate for total power (TP) observing, or the LO1 can be switched between different frequencies with some pattern as in frequency switching (FSW). The signal frequency (0) is determined by the rest frequency, source velocity, and reference frames if Doppler tracking. There are two reference frequencies (1 and 2) which can be set by the user as offsets from the signal frequency.
The switching mode is defined by the Scan Coordinator keyword sc.switch_mode. When freqency switching is not employed (e.g., TP), the LO1 is fixed at the signal frequency (0) which changes only through Doppler tracking. There are three fixed frequency switching modes (FSW_01, FSW_12, FSW_0102) and a user defined mode (FSW_USER). When using the fixed FSW modes the two reference frequency offsets are determined from ref_freq_1 and ref_freq_2, while the user defined FSW mode offsets are determined directly from fsw_offsets. Below we list the FSW modes.
FSW_01: This is a straightforward frequency switching cycle with only one reference frequency offset (ref_freq_1).
FSW_12: This is a symmetric switching where the two halfs of each spectrum are processed separately and later combined in an average. Typically ref_freq_1 = -ref_freq_2.
FSW_0102: This mode is like FSW_01 except that two reference frequencies are used, presumably to remove a first-order, frequency-dependent gain term in the front end passband. In most cases the two signal spectra and the two reference spectra will be averaged together before further processing.
FSW_USER: This is the user defined mode. The keyword fsw_offsets must be set directly to define the sequence of frequency switched offsets.