In Wireless communication binary data or bits was transmitted by means of a wireless modem (Modulator and Demodulator) which uses FSK technique to transmit bits from the modulator and the demodulator.In FSK technique the carrier frequency is shifted between two preset frequencies since the carrier frequency is shifted between two preset frequencies it was named as Frequency Shift Keying.Today we are about to see designing of simple design of wireless modems using IC NE555 and PLL 565 which uses the FSK signalling technique.It was really a worthy mini project to do for the Electronics students and even it was given in Anna university Electronics Design Lab syllabus for ECE (Electronics and Communication Engineering) students.There are three main basic components in a wireless modem they are
The above above circuit was the Modulator part of our wireless modem.Here IC 555 was wired as a Astable Multivibrator in addition to a transistor through which the data bits are fed into the circuit. The working of this circuit was very simple to understand as the output frequency of the signal was based on the digital input given to the base of the transistor.
When the input data was logic 1 PNP transistor Q is off and this makes IC 555 to work in normal astable mode giving out series of square wave pulses and thus there will be no change in frequency of the output signal.But when the logic 0 was given as input then the PNP transistor is on and it connects the resistance Rc across Resistance Ra.This makes the charging of capacitor C quicker resulting in high frequency waves as output by this way the FSK was done with this circuit.The output frequency of this circuit will be 1070 Hz or 1270 Hz. For detailed description of this circuit visit Frequency Shift Keying using 555.
The output obtained from the Modulator will be of square wave so in order to convert the square wave use a low pass filter shown in the above circuit diagram.The input was given to the resistor with a capacitor shorted to the ground and the output was taken across the capacitor.By this way you can convert a Square wave into sine wave.
A communication link must be established between modulator and demodulator and here we are using a the efficient IR transmission link which is simple and effective to implement.In this IR LED was used to send the FSK signal when the signal was passed through it emits IR signals and in the detector part Phototransistor was used.The working of the phototransitor was very similar to normal trsnsistor.The infra red signals from the LED falls on the base of the phototransistor and this makes the transistor ON and conducts the signal through the emitter.By this way every IR link wa established we can see this kind of links in remote control appliances.
The demodulator was constructed in means to correctly match the modulator circuit of this wireless modem.The PLL 565 IC locks the input frequency and tracks it between the two possible frequencies with a DC shift at its output.Then a three stage RC filter was used in the circuit to remove the total sum frequency component.The output will be obtained in this way whenever the input frequency was low that is 1070 Hz then the final output will be moved to logic 1 high state and logic 0 low state when the input frequency was high that is 1270 Hz.For detailed description on the above circuit visit FSK DEMODULATOR USING PLL 565.