NF FRA5087, Instruction Manual

Page: 71 / 194

3.5 Examples of basic operations
FRA5087 3-31

Servo loop characteristics measurement
The servo loop is composed of an amplifier A with high gain and a feedback circuit  . It is
required that you fully understand the characteristics of the amplifier A itself so that you get
improved performance characteristics (e.g., response time, steady-state error characteristic, etc.)
for the servo loop.
The frequency characteristics of amplifiers with feedback circuits incorporated (i.e., closed loop
characteristics :Tc) can relatively easily be measured through the use of a method of “Fig. 3-14
Connection diagram for frequency characteristics measurement (1)”. It is generally difficult to
directly measure the open loop characteristics To of A exclusively or of A
 , due to, e.g., easy
saturation of the amplifier A even with very low input signals. Note that it is required to open
the feedback loop to measure the characteristics of the amplifier A or the open loop A
 .
The FRA5087 can be used to measure of the servo loop; i.e., The oscillator of the FRA5087 is to
be inserted into the servo loop circuit. The measurement is made possible by virtue of such
FRA5087’s features as its wide dynamic range as well as its oscillator output and analyzer inputs
being electrically floated from the cabinet. The oscillator output level is set so low compared
with the signal level of the SUT that the effect of the measurement signal upon the SUT can be
maintained minimal or negligible.
SUT
Ａ
β
+
V
1
V
2
+
+ +
V1
V2
OSC
=V2+OSC
OSC
Fig. 3-16 Connection diagram for loop gain measurement
The open loop gain A
Set the analysis mode at (CH2/CH1) for the FRA 5087 to display the loop gain frequency
characteristics on the LCD screen so that you can directly read them through the display.
The characteristics of the feedback circuit
 can be measured through various methods, e.g., “Fig.
3-15 Connection diagram for frequency characteristics measurement (2)”. You can also obtain
the characteristics of the amplifier A exclusively through dividing the open loop characteristics A

by  . You can calculate A    by using the calculation function of the FRA 5087 .
A 
=
V
2
V
1
A 
=
･
･