NF FRA51615, Instruction Manual

Page: 143 / 306

4.2 Simple operation for the first use
4
-
9
FRA 51615
■ Cautions concerning loop gain measurements
This instrument measures and displays loop characteristics rather than Aβ characteristics. In the block
diagram presented in " Figure 4-1 Connections for loop gain measurement ", the polarity reversing
part of the feedback on the input side of the amplifier element (A) is included in the measurement. That
is to say, −Aβ is measured. In general automatic control theory explanations, loop characteristics are
usually treated as Aβ, so the phase margin is defined as the difference from 180°. For measurements with
this instrument, however, the phase margin is defined as the difference from 0°. Also, when the Nyquist
plot is displayed, the position that indicates the stability limit is (+1+j 0) rather than (−1+j 0).
Figure 4-3 Nyquist plots obtained from actual measurements
Although the position for indicating the stability limit is shifted, the vector locus is also rotated 180° at
the origin, so the method for determining stability is the same as for an ordinary Nyquist plot.
Real
Imag
+1+j0
Stable
Stability limit
Unstable
Aβ vector locus −Aβ vector locus
(FRA51615 measurement data)
f
0→
∞
Real
Imag
−1+j0
β
Ａ
u y
+
−