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Minimum
recommended specifications for analogue and digital radio
monitoring receivers are described in the
Spectrum
Monitoring Handbook (Edition of 2002).
One
of these specifications is for example the 3rd order intercept
point level (IP3), a measure of the receiver’s linearity. The
higher the IP3 level (usually expressed in dBm), the better the
linearity of the receiver is. The full importance of this
specification comes out when the receiver operates in an
environment of many high level emissions in the spectra vicinity
to the tuned frequency – that is a frequent scenario in a
monitoring station.
Limited
linearity of a receiver is one of the reasons for the monitoring
operator to get signals that do not really exist in the RF
spectrum. These spurious signals are generated in the equipment
itself. They make the radio monitoring task difficult and
tedious and may conduct to wrong results and false reports.
When comparing the data sheets of similar receivers from
different manufacturers, the IP3 levels indicated are considered
by the users, among other parameters, for the purpose of
receiver quality comparison. To be able to get a fair comparison
between the receivers, however, they all should have been tested
under the same conditions. Otherwise, Receivers from different
makers showing nearly the same IP3 levels in their respective
data sheets, might be of completely different linearity and
performance (and therefore of different suitability as
monitoring receivers) if the test procedure behind the IP3
specification is not defined in such a way that it actually
describes the linearity of the receiver in an unambiguous way. A
uniform measurement procedure is the way to reach this. The same
is true for other receiver data, too.
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