SFRA45 User Manual
9-1
9
Frequency Response Analyzer
The instrument 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
a1 = 1/
v(
).cos(
) d
0
2
b1 = 1/
v(
).sin(
) d
0
