In the frequency switching scheme implemented at the 30m--telescope, a frequency step of the phase lock reference synthesizer (ref. 2) generates a frequency step of the same size at the level of the Gunn oscillator. At 3mm where no additional frequency multiplication is used, is then directly the FSw throw in sky frequency. The 1mm receivers are equipped with triplers, so the FSw throw corresponds to 3. In the way the ref. 2 synthesizers are currently operated they permit a maximum throw of +20 MHz. The phase lock unit can, however, not handle such a large frequency step at all frequencies. Especially at 3mm, a practical maximum step seems to be 18 MHz. Our best estimates of these limits (positive and negative) are used by the OBS FSw command. For the observer it may usually be sufficient to remember that a throw of about 45 kms should not be exceeded. In practice, we expect that the choice of will usually be dictated, as outlined above, by the baseline ripple.