PSM1700 PsimetriQ user manual
2-5
2.4 Measurement Selectivity
In many applications that require frequency response
analysis, it is common for the frequency of interest
(sometimes referred to as the injected frequency) to be
immersed in noise. It is therefore important that the FRA
instrument being used is able to reject frequencies other
than that of the signal of interest. This feature is usually
described as the measurement selectivity of the
instrument and in most cases; selectivity is increased as
the measurement speed is slowed down.
N4L frequency response analysers incorporate analogue
circuits with high common mode rejection and unique ‘real
time’ DFT (discrete fourier transform) analysis to provide
exceptional wideband frequency response measurements
even when the signal of interest is immersed in noise. As a
result of this design technique, users are not required to
concern themselves with the careful choice of selectivity
criteria to achieve stable measurements.
In most applications, the signal of interest is generated by
the FRA itself. When using this normal mode of operation,
PSM units from N4L will automatically analyse the
measurement signal with a DFT algorithm running at
exactly the same frequency as the injected signal. This
process eliminates the problem of signal frequency
detection.
Where an external signal source is used for signal
injection, the PSM units will detect the injected frequency
with a greater level of selectivity as the measurement
speed is slowed down. The user need only select the
measurement speed to achieve the optimum balance of
speed and measurement stability.
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