1. A MPLITUDE S HIFT K EYING (ASK) & F REQUENCY S HIFT K EYING (FSK)

2. D IGITAL C OMMUNICATION S YSTEMS  Some of the modulation techniques employed by digital communication systems are explained throughout this Lab. Today, Amplitude Shift Keying and Frequency Shift Keying are to be explained.

3. A MPLITUDE S HIFT K EYING (ASK) In ASK, a sinusoidal carrier is gated ON and OFF by the binary sequence to be transmitted. The message signal m(t) is a unipolar NRZ such that: m(t)=0 for logic 0 0≤ t ≤ Tb m(t)= 1 for logic 1

4. ASK M ODULATOR The ASK signal is generated as: Note: the carrier frequency is chosen such that fc=n*Rb ; where n is an integer The transmitted signals are: - S 1 (t)=0 for logic 0 - S 2 (t)=V cos (2*pi*fc*t) for logic 1

5. Illustration of ASK Digital information 1 0 1 1 0 0 1 0 1 0 Carrier wave ASK modulated signal Carrier present Carrier absent Amplitude varying- frequency constant

6. ASK D EMODULATOR The basic block diagram of ASK demodulator ( Non-Coherent Demodulation ): Notes: - The BPF is used to reduce both ; the noise and the interference of other signals through selecting the BW of each signal. - Sometimes a comparator is used instead of the schmitt trigger. ( Threshold = V/2 ). They are used to convert the analog signal to a digital one.

7. S CHMITT T RIGGER A Schmitt trigger is a circuit with positive feedback and a loop gain greater than 1 When the input is higher than a certain chosen threshold, the output is high. When the input is below a different (lower) chosen threshold, the output is low, and when the input is between the two levels, the output retains its value. Comparator Versus Schmitt trigger:

8. D ISADVANTAGE OF ASK The detection process is difficult since the amplitude is not constant. ASK is usable only for Low Data Rate.

9. F REQUENCY S HIFT K EYING (FSK) In a binary FSK, symbols 1 and 0 are distinguished from each others by transmitting one of the two sinusoidal signals that differ in frequency by a fixed amount. 0≤ t ≤ Tb

10. FSK M ODULATOR The direct methods that are used to general FSK signals are: Two - oscillator method ( synchronization problem) Voltage control oscillator:

11. Illustration of FSK Digital information 1 0 1 1 0 0 1 Carrier 1 (frequency #1) FSK modulated signal Carrier 2 (frequency #2) Frequency varying- amplitude constant

12. ASK D EMODULATOR - Non-coherent detection of FSK