# lab 3 – linear integrated-circuit am modulator

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7 Οκτ 2013 (πριν από 4 χρόνια και 7 μήνες)

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LAB 3

LINEAR INTEGRATED
-
CIRCUIT AM
MODULATOR

1.

Introduction

In this experiment
we examine
the operation of the XR
-
2206 monolithic function
generator as an AM DSBFC modulator.
See the data sheet for information on the XR
-
2206
.

2.

Materials Required

Equipment

1

-

protoboard

1

-

Dual dc power supply

(+l2

Vd
c
and

0 to

+
6
Vdc)

1

-

audio signal generator (0 to 20 kHz)

1

-

standard oscilloscope (10 MHz)

1

-

assortment of test leads and hookup wire

Parts List

1

-

XR
-
2206 function generator

1

-

1 k
-
Ohm

variable resistor

3

-

4.7 k
-
Ohm resistors

2

-

0.001

F

c
apac楴潲o

2

-

㄰⁫
-

2

-

F⁣apac楴潲o

1

-

㐷⁫
-

2

-

㄰1

F⁣apac楴潲

3.

Pre
-
lab

Print the XR
-
220
6

data sheet and bring it with you to the lab session. In the
Principles of

Operation
section of the data sheet, find and record the eq
uation for
the
theoretical
frequency of operation,
f
0
.

4.

Circuit 1
-

Output Amplitude

versus

DC
Input Control Voltage

In this
section,

we examine
the output amplitude
-
versus
-
input
Volt
age characteristics of
the XR
-
2206
by applying a
DC
control
Volt
age to
its

modulator
. The schematic diagram
for the function generator circuit used in this section is shown in
Figure
1
.

Procedure

1.

Construct the function generator circuit shown in
Figure
1
.
Set the
DC

control
Volt
a
ge
V
C

to 0
V
dc
.

2.

R
2

until a sine wave with minimum distortion is observed at
V
out
;

then

measure its frequency and amplitude

3.

Calculate the
VCO
free
-
running frequency
from

the formula

in the XR
-
2206 data
sheet
, and compare this to the frequency you mea
sured in step 2.

4.

Increase the amplitude of the control
Volt
age in
0.5
-
V
olt steps for values
from

0
to

+
6

V
dc
, and

m
easure the amplitude of the sine wave observed at
V
out

for

each value of
control
Volt
age.

5.

Using Excel, g
raph the output amplitude
-
versus
-
cont
rol
Volt
age for the control
Volt
ages us
ing the data obtained

in step
5.

On the same graph, plot the theoretical
LAB 3

LINEAR INTEGRATED
-
CIRCUIT AM MODULATOR

2

relation shown in Figure 6 in the XR
-
2206 data sheet.

Figure
1
. Output
Amplitude V
ersus
-
B
ias Voltage.

5.

Circuit
2

A
m
plitude
-
modulated

Double
-
sideband Full
-
carrier
(AM DSBFC)

Modula
tor

In this section
we use
the XR
-
2206
to
generate an AM DSBFC waveform

by

applying a
signal to the

XR
-
2206
modulator that
contains both a dc and an ac component
.

The
schematic diagram for the

linear integrated
-
circuit AM DSBFC modulator is shown in
Figure
2
.
The function generator
circuit of the XR
-
2206 provides

the carrier, and
an
external

audio signal generator
provides

the modulating signal.

IMPORTANT

Do not disas
semble this circuit when you complete the lab; you will use
it again for Lab 5.

LAB 3

LINEAR INTEGRATED
-
CIRCUIT AM MODULATOR

3

Figure
2
.
AM Modulator Using the XR
-
2206 Function G
enerator.

Procedure

1.

Construct the function generator circuit shown in
Figure
2
.

component is the 10
-

F capacitor that connects to the function generator.)

2.

Set

the amplitude of the control
Volt
age
V
C

to 0 V.

3.

Set

the amplitude of the audio signal generator output
Volt
age to 0 V.

4.

C
heck that

the function
generator free
-
running frequency

has not changed
.

5.

Check that the
R
2

a sine wave with minimum distortion is
at
V
out
.

6.

Increase the control
Volt
age to
+
4

V dc.

7.

Set

the
output

of the signal g
enerator output
Volt
age to 3 V
P
-
P

at 1 k
Hz.

8.

Adjust the amplitude of the audio signal generator output
Volt
age until an AM
envelope with

100% modulation is observed at
V
out
.

9.

Sketch the waveform
envelope
observed in step 8
, indicating its amplitude and
frequency
.

LAB 3

LINEAR INTEGRATED
-
CIRCUIT AM MODULATOR

4

10.

Set the amplitude of the audio si
gnal generator output
Volt
age to
1.5
V
P
-
P

and
determine the percent modulation of the output envelope using the following formula.

Percent AM modulation

where

V
max

=

maximum peak
-
to
-
peak envelope amplitude
, Volts

V
min

=

minimum pea
k
-
to
-
peak envelope amplitude
, Volts

(This is most easily done by using the oscilloscope cursors to find the peak
-
to
-
peak
Voltages at the modulation maximum and at the modulation minimum.)

11.

Make the necessary connections between the AM modulator and the osc
illoscope to
display a trapezoidal pattern
, and record this pattern, indicating the Voltages on it that
you need to compute the per cent modulation
.

12.

Vary the amplitude and frequency of the audio signal generator output
Volt
age and
note

what effect varying
them has on the
trapezoidal pattern
.

6. Discussion Questions

1.

Does adjusting the sine wave for minimum distortion affect the VCO frequency
?

2.

In Figure 2, for a fixed function generator amplitude, which components affect the
per cent modulation
?