This attribute specifies the (topocentric) intermediate frequency
in a dual sideband spectrum. Its sole
use is to determine the local oscillator (LO) frequency (the
frequency which marks the boundary between the lower and upper
sidebands). The LO frequency is equal to the sum of the centre
frequency and the intermediate frequency. Here, the "centre
frequency" 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", "4.0 GHz",
"4.0E9 Hz", etc.
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.
