The frequency modulation
(FM) is one of the widely used modulation technique in wireless
transmission. They are most commonly used in high quality radio
transmission from FM radio stations. The main advantage of the frequency
modulation technique is the very high signal to noise ratio that can be
achieved on the received signals.
In
the technique of frequency modulation the frequency of a carrier wave
is modulated according to the variations in the amplitude of the message
wave. Unlike the amplitude modulation technique the amplitude of the
carrier signal remains same. The receiver tracks for the change in the
frequency of the received signal only and the amplitude variations due
to the noise does not make any changes in the receiver output. Hence
quality of the signal produced at the output of the receiver is very
high compared to other modulation techniques.
This article demonstrates the generation of frequency modulated waveform in the simplest possible way. Here a Wien Bridge oscillator based circuit is used to generate pure sine wave which is then used to modulate the frequency of the waves produced by a 555 timer IC.
DESCRIPTION:
The
555 is an 8 pin versatile timer IC which can be wired to make thousands
of verity circuits. The 555 can be used as monostable, bistable or
astable multivibrator circuits. They are most commonly used as an
astable multivibrator which can continuously produce pulses at constant
intervals of time. The 555 IC requires only few components which can be
connected to their pins and the output pulse waveform can be obtained
from the pin3 of the 555 IC.
The pinout of the 555 IC is shown in the following figure:
The
pin number 1 and 8 are used to supply the ground and VCC to the IC
respectively. The pin number 2, 6 and 7 are used according to the timing
requirements of various pulse generating circuits. The pin number 4 can
be used to reset IC. The pin number 3 provides the output pulse from
the 555 timer IC. The pin number 5 the pin which can change or modulate
the pulse frequency produced at the pin number 3.
In
this project the 555 is wired as an astable multivibrator to produce
continuous square waves with a fixed time period. The sine wave
generated using the Wien Bridge oscillator applied to the pin number 5
of the 555 timer IC to modulate the frequency of the generated pulses.
Since the output of a Wien Bridge oscillator is a pure sine wave having
both the positive and negative half cycles they should not be directly
applied to the 555 which works on positive DC voltage only. The entire
sinusoidal waveform should be clamped to the positive voltage side
before it is applied to the 555 modulator circuit so as to get the
output response for both the half cycles. Hence the entire circuit can
be divided into three separate blocks:
1) Sine wave generator using Wien Bridge oscillator
2) Positive clamper
3) Frequency modulator using 555
The block diagram of the entire circuit is shown in the following diagram:
A
sine wave generator circuit is used in this project which is based on
the Wien Bridge oscillator circuit. The Wien Bridge oscillator circuit
can produce distortion less sinusoidal sweep at its output. The circuit
is designed in such a way that both the amplitude and frequency of the
oscillator can be adjusted using potentiometers.
The circuit diagram of the variable frequency sine wave oscillator is shown in the following:
The
frequency of the above circuit can be varied by simply varying the
potentiometer R2 and the amplitude of the wave form can be adjusted by
varying the potentiometer R. The frequency of the sine wave generated by
the above circuit depends on the components R1, R2, C1 and C2 and the
equation for the frequency is given below:
For
the ease of adjusting the amplitude of the wave to obtain proper
sinusoidal sweep, a coarse and fine adjustment has been implemented
using potentiometers. A low value (1K) potentiometer is connected in
series with the high value (100K) potentiometer so that the coarse
adjustment can be done with the high value resistor and the fine
adjustment with the low value resistor.
Sign up here with your email
ConversionConversion EmoticonEmoticon