PSM1700 PsimetriQ user manual
11-1
11 Frequency response analyser
PsimetriQ measures the gain and phase of channel 2
relative to channel 1 using a discrete Fourier transform
(DFT) algorithm at the fundamental frequency.
The DFT technique can measure phase as well as
magnitude and is inherently good at rejecting noise – it is
much more reliable than measuring the rms at one point
relative to another point.
The circuit can be characterised by computing the gain and
phase at a number of points over a frequency range. This
gives results that show the transfer function of the circuit
as a graph on the display.
The DFT analysis yields two components – in-phase and
quadrature, or ‘a’ and ‘b’ values – from which the
magnitude and phase can be derived.
Considering the components at the fundamental
frequency:
The fundamental in-phase and quadrature values of a
periodic waveform, v(), are given by:
2
a
1
= 1/2
v().cos() d
0
2
b
1
= 1/2
v().sin() d
0
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