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.