"is the topocentric frequency in Hz corresponding to the current value of the DSBCentre attribute. The value of the IF attribute may be positive or negative: a positive value results in the LO frequency being above the central frequency, whilst a negative IF value results in the LO frequency being below the central frequency. The sign of the IF attribute value determines the default value for the SideBand attribute.
When setting a new value for this attribute, the units in which the frequency value is supplied may be
indicated by appending a suitable string to the end of the formatted value. If the units are
not specified, then the supplied value is assumed to be in units of GHz. For instance, the
following strings all result in an IF of 4 GHz being used:
" 4.0 GHz
" 4.0E9 Hz
When getting the value of this attribute, the returned value is always in units of GHz. The default value for this attribute is 4 GHz.