The extreme tuning range of the receiver is about 315-373 GHz (in sky frequency). Both mixers are tunerless and quite similar in their characteristics. For frequencies within a few GHz of the tuning limits the multiplier output power is marginal, and the automated tuning procedures usually fail. In such cases, manual tuning is required (I recommending adjusting the multiplier bias voltage for maximum mixer current).

Performance is spotty near the limits of the tuning range and cannot be guaranteed. Also, true heterodyne performance (i.e. you should see spectral lines) is not proven near the band edges even though the receiver may tune without trouble.

When tuning near the limits you should place the line frequency in the sideband that minimizes the "stretch" of the LO - i.e. tune LSB at low frequencies and USB at the high end of the range. Remember that the IF (intermediate frequency) is currently 4 GHz and that Sky_freq = LO+IF for the USB and Sky_freq = LO-IF for LSB tunings.