Circuit Design: Frequency Modulated Waveform Generation
Table of Contents:
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.
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:
1) Variable frequency sine wave generator
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.