NF FRA5087, Instruction Manual

Page: 25 / 194

1.4 Principle of operation
FRA5087 1-9
1.4 Principle of operation
1.4.1 Basic principle
The Frequency Response Analyzer (hereafter called FRA) is an instrument that provides
measurement of the frequency transfer characteristics of Systems Under Test (hereunder called
SUTs) with high accuracy/resolution for wide dynamic range. The FRA is equipped with a
sweep oscillator and two channels of analysis input (CH1 and CH2) and calculates, accurately and
with high resolution, vector quantity (amplitude and phase) of each analysis frequency component
from the Fourier coefficient that is obtained through the discrete Fourier transform of the input
signal to be analyzed. The input signal and the output signal of the SUT are entered into
individual analysis input terminals (CH1 and CH2), respectively. Then, vector ratio calculation
(C
．
H1/C
．
H2) is made to produce the gain and the phase of the signals at the angular frequency  for
which the SUT is to be analyzed.
CH1
( t)
OS C
DF T
Frequency
= ω
CH2
( t)
C H 1 C H 2
SUT
FRA
In Out
H
( ω )
DF T
CH2
( ω )
CH1
( ω )
A/D conversion A/D conversion
Ｈ
( ω )
=
CH1
( ω )
CH2
( ω )
= a
( ω )
+ jb
( ω )
=
gain
(
ω )
，
phase
(
ω )
Fig. 1-1 Measurement of frequency transfer characteristics with FRA
At one time of measurement, the gain and the phase only at the analysis frequency  , which is the
oscillator output frequency, will be measured. The frequency characteristics like the Bode
diagram will be obtained by sweeping the analysis frequency and thus accumulating the values of
the amplitude and the phase of CH1 and CH2 at individual frequencies. At respective analysis
frequencies, the preamplifier gain will be readjusted to be most appropriate for the coming
measurement, and therefore, the measurement with a wide dynamic measurement range and also
with the optimum signal-to-noise ratio will be performed, due to addition of the A/D converter
dynamic range and the gain variation range of the preamplifier.