DIGITAL DATA TRANSMISSION - II

LAB REPORT EXPERIMENT 6 DIGITAL DATA TRANSMISSION - II SUBMITTED BY PURPOSE The Objectives of this laboratory are: 1. To examine the frequency spectrum of an FSK signal. 2. To examine the correlation between two FSK signals. 3. To implement and investigate a coherent FSK detection system. EQUIPMENT LIST 1. PC wth Matlab and simulink LABORATORY PROCEDURE I. Spectrum of an FSK Signal : FSK involves frequency shifting a carrier between known fixed frequencies to convey digital data. Normally, FSK modulation is achieved using just two frequencies to represent binary data. For example, a frequency of 2000 Hz may represent a binary 1 (mark) and 1000 Hz a binary 0 (space). The energy of the signal alternates between the mark and space frequencies to convey the digital message. FSK describes the modulation of a carrier (or two carriers) by using a different frequency for a 1 or 0. The resultant modulated signal may be regarded as the sum of two amplitude modulated signals of different carrier frequencies: Figure 1a: Frequency shift keying Figure 1b: Frequency shift keying -- frequency domain A. The circuit was set up according to figure 6A shown below. It is a basic assembly to generate a FSK signal. Figure 6a Spectrum of a FSK signal – FSK generation Figure 6bInput signal for generating FSK signal The output of the FSK generator is shown below. The FSK Modulator module consists of f0 and f1 connectors, which are frequency adjustable signals of the form; sx(t) = A cos2Πfxt where x = 0 or 1 and the module is in FREE – RUNNING. However, when the switch is in the SYNC position , a time restriction is placed on the waveforms: sx(t) = A cos2Πfxt for 0 < t < T where T = bit rate = 1/fs (selected by the Master Clock). In this mode, a cosine begins at the start of every data period and proceeds at the frequency fx until the end of the data period. Then the process repeats. Figure 6c The FSk Modulated signal B. Cross – Correlation of the Two FSK Signals. A. FREQUENCY SHIFT KEYING - ERROR PROBABILITY: The Probability of error of an FSK system depends on the separation of “distance” d, between two (or more) signals as given by the equation below: P.E = ½ erfc(√(Eb / N)d). Thus the probability of error is minimum when the distance of separation is maximum i.e larger the d, smaller is the P.E. The distance d is given by: d = (1 - ρ) / 2 where ρ = (1 / E) 0∫T s0(t) s1(t) dt. The integral is the cross-correlation function over the period 0 to T. ρ is restricted to values between +1 and –1. ... - tailieumienphi.vn