SYNTH is a 3 element array of frequency settings for the three 2-16 GHz synthesizers for the VLBA. This is the LO for the first mix. After this mix (or 2 mixes for 1.3cm and 7mm), the signal must be in the 500-1000 MHz IF band.
Harmonic mixing issue:
Internally generated RFI tones can result from mixing of harmonics of the front-end synthesizers on the VLBA. Under some circumstances, these tones can have amplitudes of several hundred in the autocorrelations as measured using a 32 MHz baseband bandwidth and 31.25 kHz spectral channel bandwidth. Such tones cause very strong ringing across the autocorrelation spectrum without smoothing. They are also seen, more weakly, in the cross-correlations because of the concentration of power in the affected channel. All three of the synthesizers are involved in producing these tones whether or not they are actually in use. So it matters how all synthesizers are set, not just the active ones.
The tones are strongest when the frequencies that mix include the nominal output frequency of one of the synthesizers. They are also strongest, at least for the 4-8 GHz "6cm" receiver when the synthesizers are numbers 1 and 2, both of which feed the IF converter. Other harmonics, and synthesizer 3, which usually is used for the high frequency front ends, create tones that are much weaker. A complication is that, above 8.0 GHz, the synthesizer's main oscillator is running at half the output frequency and doubler produces the final output. A birdy is seen at the half frequency.
SCHED tries to warn of choices of front end frequencies (often obtained from the freq.dat file) that could cause problems. When SCHED is allowed to choose the frequencies of the unused synthesizers (Setup parameter SYNTH), it will try exhaustively to find benign frequencies and it should be able to in essentially all cases. So it is best to let SCHED choose.
It is not yet clear which bands can be affected when SCHED is allowed to choose the unused synthesizer settings. Any case where only one synthesizer is in use (most observations below 16 GHz) will be ok as SCHED can find benign settings for the other two. If 2 IF pairs are in use in the new 6cm receiver, it is established that one can get in trouble with this issue - that is where it was found. S/X and high frequencies where synthesizer 3 is used in the front end have not yet been tested.