Study of ASK, PSK and FSK transmitter and receiver

Aim

Study of ASK PSK and FSK transmitter and receiver.

(a) Study of ASK (Amplitude Shift Keying) Modulation and Demodulation.

Apparatus Required:

1. ASK kit (Model No. ME 749-I)
2. Connecting probes
3. CRO

Theory

The binary ASK was one of the earliest forms of digital modulation used in wireless telegraphy. this simplest form of the digital modulation is no longer used widely in digital communication. Nevertheless it serves as a useful model which helps in understanding certain concepts. In an ASK system binary symbol 1 is represented by transmitting a sinusoidal carrier wave of fixed amplitude Ac and fixed frequency Fc for the bit duration Tb seconds whereas binary symbol is represented by switching of the carrier for Tb seconds. This signal can be generated simply by tuning the carrier of sinusoidal oscillator on end of for the prescribed period by the modulating pulse trend. For this reason, the scheme is also known as on-off scheme.

Circuit Diagram

Procedure:-

For Ask Modulation:

1. Connect the output of carrier wave generator (carrier output) to carrier input of ASK modulator & connect the modulating output to modulating input through patchcords.
2. Connect the CRO across output of ASK modulator.
3. Switch on the CRO as well as instrument.
4. Observe the output wave shape on CRO.
5. Change the amplitude & frequency of modulating signal & observe corresponding effect on ASK modulated wave. set the output shape (Ask modulated) through offset potentiometer provided on the front panel.

For Ask Demodulation:-

1. Connect the output of ASK modulator to the input of demodulator through patchcord.
2. Connect CRO probe across output of demodulator.
3. Observe the demodulated (square wave) shape on CRO. We will observe that the demodulated output is of same frequency as the modulated signal with little distortion.

Precautions:

1. Check the cont unity of the connecting probes.
2. Handle the CRO properly.

(b) Study of PSK (Phase Shift Keying) Modulation and Demodulation.

Apparatus Required:

1. PSK Kit (Model No. ME 750-I),
2. Connecting probes
3. CRO

Theory

Digital communication became important with the expansion of the use of the computers and data processing and have continue to develop in to a major industry providing the interconnection of computer peripherals and transmission of data between distance sides. PSK is a relatively new system in which carrier may be phase shifted by 90° for a mark and by minus 90° for a space. PSK has a number of similarities to FSK in many aspects, as in FSK, frequency of the carrier is shifted according to modulating square wave.

Circuit Diagram

Procedure:-

For PSK Modulation:

1. Connect the carrier Output of the carrier wave generator (IC 8038) to carrier input of PSK modulator through patch cords. Also connect any data output from the data outputs of the data generator to the data input of the PSK modulator.
2. Connect the channel 1 of CRO across output of PSK. And channel 2 across data input on a dual trace oscilloscope.
3. Switch on the instrument using ON/OFF toggle switch provided on the front panel.
4. Observe the output wave shape on CRO.
5. Change the data inputs and observe the PSK output on CRO.

For PSK Demodulation:

1. Connect the output of PSK to the demodulator input through patch cord.
2. Connect the carrier to the carrier input of the PSK demodulator.
3. Connect the channel 1 of CRO across output of demodulator.
4. Observe the demodulated output on CRO (it will be same as data input applied in the modulated in the modulator section).

Precautions:

1. Check the cont unity of the connecting probes.
2. Handle the CRO properly.

(c) Study of Frequency Shift Keying.

Apparatus Required:

Data generator, FSK modulation kit, CRO and connecting leads.

Theory

FSK is one of the basic modulation techniques for the transmission of digital data .If the frequency of the sinusoidal carrier is switched depending upon the input digital signal , then it is known as frequency shift keying. As the amplitude remains constant in FSK, so the effect of non-linear ties, noise interference is minimum on digital detection. So FSK is preferred over ASK. Frequency shift keying consists of shifting of frequency of carrier from a mask frequency to a space frequency according to the base band digital signal Frequency shift keying is identical to modulating an FM carrier with a binary digital signal In an FSK system, two sinusoidal carrier waves of the same amplitude Ac but different frequencies fc1 and fc2 are used to represent binary symbols 1and 0 respectively. It can be easily verified that binary FSK waveform is a superposition of two binary ASK waveforms, one with a frequency fc1 and other with a frequency fc2. No discrete components appear in the signal spectrum of FSK signal. The main advantage of FSK lies in its easy hardware implementation.

Generation of FSK signal:-

The PSK signal can be generated by applying the incoming binary data to a frequency modulator. To the other input a sinusoidal carrier wave of constant amplitude Ac and frequency fc is applied. As the modulating voltages changes from one level to another, the frequency modulator output changes its frequency in the corresponding fashion.

Detection of FSK signal:-

FSK can be demodulated by using coherent and non-coherent detector. The detector based on coherent detection requires phase and timing synchronization. Non coherent detection can be done by using envelop detector.

Result:FSK output is obtained on CRO.

Precautions:

1. Do not use open ended wires for connecting to 230 V power supply.
2. Before connecting the power supply plug in to socket, ensure power supply should be switched off
3. Ensure all connections should be tight before switching on the power supply.
4. Take the reading carefully.