User manual

page 48
Analog System Lab Kit PRO
experiment 8
In the front-end electronics of a system, we may require that the gain of the
amplier be adjustable, since the amplitude of the input keeps varying. Such
a system can be designed using feedback. This experiment demonstrates
one such system.
Thereaderwillbenetfromtherecordedlecturesat[25]. Another useful reference
is the application note on Automatic Level Controller for Speech Signals using PID
Controllers [2].
In the signal chain of an electronic system, the output of the sensor can vary
depending on the strength of the input. To adapt to wide variations in the magnitude
oftheinput,wecandesignanamplierwhosegaincanbeadjusteddynamically.
This is possible when the input signal has a narrow bandwidth and the control
system is called Automatic Gain Control or AGC. Since we may wish to maintain the
outputvoltageoftheamplierataconstantlevel,wealsousethetermAutomatic
Volume Control (AVC). Figure 8.1 shows an AGC circuit. The typical I/O characteristics
of AGC/AVC circuit is shown in Figure 8.2. As shown in Figure 8.2, the output value
of the system remains constant at
VV
VVV
2
2
rref
pi rref
=
beyond input voltage
VV
VVV
2
2
rref
pi rref
=
.
Simulate the circuit of Figure 8.1 and obtain the Transfer Characteristic of the
system. Assume that the input comes from a function generator; use a sine
wave input of a single frequency.
Build the circuit shown on Figure 8.1. Plot/print the transfer characteristic
using the oscilloscope and compare it with simulation results.
Transfer Characteristics - Plot the input versus output characteristics for the AGC/AVC.
Design AGC/AVC system to maintain the peak amplitude of the sine wave at 2V.
8.1 Brief theory and motivation
8.2Specication
8.3 Measurements to be taken
8.4 What you should submit
Goal of the experiment
C
R
Vref=
VC
VI =Vpi•sinωt
Vpi•sinωt
2VR
VOP
VR
VC
]
]
VR
VC•Vpi
]
]
VR
1
2
2
Figure 8.1: Automatic Gain Control (AGC)/Automatic Volume Control (AVC)
Figure 8.2: Input-Output Characteristics of AGC/AVC
Table 8.1: Transfer characteristic of the AGC circuit
1
2
3
Plot the output as a function of input voltage. Enter sucient number of
readings in Table 8.2. Does the output remain constant as the magnitude of
the input is increased? Beyond what value of the input voltage does the gain
begin to stabilize? We have included sample output waveform for the AGC
circuit in Figure 8.3.
S.No. Input Voltage Output Voltage
1
2
3
4