DEPARTMENT OF ELECTRICAL & COMPUTER ENGINEERING ECE 4203: COMMUNICATIONS ENGINEERING LAB II SEMESTER 2, 2017/2018 EXPERIMENT NO. 3 FSK Modulation NAME: MATRIC NO: DATE: SECTION:
FSK MODULATION Objective To describe the FSK modulation and demodulation (Frequency Shift Keying) To carry out a FSK connection To examine the noise effect on the connection Material Power unit PSU Module holder base Individual Control Unit SIS1 Experiment module MCM31 Oscilloscope THEORETICAL NOTIONS Frequency Shift Keying FSK In this modulation the sine carrier takes 2 frequency values, determined by the binary data signal (fig.1). The modulator can be carried out in different ways; among the most used we can mention: a Voltage Controlled Oscillator (VCO) a system transmitting one of the 2 frequencies, as function of the data signal a frequency divider controlled by the data signal. The most used demodulation technique is the one using a PLL circuit (fig.2). The FSK signal across the PLL input takes two frequency values. The error voltage supplied by the phase comparator follows such variations, and so, it constitutes the NRZ binary representation (high and low level) of the FSK input signal. The PLL demodulator is followed by a low pass filter, which removes the residual carrier components, and a squarer circuit which forms the proper data signal. The main factors characterizing the FSK are: applications of the data transmission modems (ITU-T V21, ITU-T V23, BELL 103, BELL 113, BELL 202), in digital radio transmission, in the cellular telephone system ETACS (information transmission across the Control Channel) it requires circuits of average complexity high probability of error, but lower than the ASK called Fb the bit transmission speed, the minimum spectrum Bw of the modulated signal is higher than Fb the transmission efficiency, defined as the ratio between Fb and Bw, is lower than 1 the Baud or Baud rate, defined as modulation speed or symbol speed, is equal to the transmission speed Fb. 2
Figure 1: FSK Modulation Figure 2: FSK Demodulation with PLL FSK Modulator The block diagram of the FSK modulator is shown in fig.3. The FSK signal is generated by means of 2 ASK modulators, which outputs are combined together with an adder. The two sine carriers at 1200 and 1800 Hz are applied separately to the 2 modulators. The data reach one of the two modulators in direct form, the other in negated form. In this way a modulator supplies a sine wave when the datum is "1", the second when the datum is "0". The FSK signal is obtained by adding the two outputs (fig.3). The 6dB-attenuator makes the signal amplitude half, and is activated. 3
Figure 3: FSK Modulator FSK Demodulator The FSK demodulator consists in the sections shown in fig.4: an FM detector made with PLL circuit a low pass filter a squarer circuit (with output in TP29) in case of asynchronous data, which are not retimed) a circuit for clock extraction and data re-timing, in case of synchronous data (data output on TP31, clock on TP32). The filter, the clock extraction circuit and the data re-timing circuit are used to demodulate also other kinds of signals. Figure 4: FSK Demodulator 4
PROCEDURE Wave-forms of the FSK Modulator MCM 31 Disconnect all jumpers SIS1 Turn all switches OFF Power the module set the circuit in FSK mode, with 24-bit data source and without data coding (connect J1c-J3a- J4-J5-J6b, set SW2=Normal, Sw3=24 bit, SW4=1800, SW5=1200/0 0, SW6=FSK, SW8=BIT, ATT=min, NOISE=min) set an alternated data sequence 00/11 and push START connect the oscilloscope to TP6 and TP16 and examine the data signal and the FSK signal. Adjust the phase (PHASE) of the 1200Hz carrier to get continuity of FSK signal in the passage between the two frequencies (this kind of modulation is known as Minimum Frequency Shift Keying). Note the two different frequencies are generated at bits 1 and 0, 1800Hz and 1200Hz in this case. Q1 What can you detect across TP 16? 5
Q2 How is the FSK signal obtained? Waveforms of the FSK demodulator Keep the last conditions (connect J1c-J3a-J4-J5-J6b, set SW2=Normal, Sw3=24 bit, SW4=1800, SW5=1200/0 0, SW6=FSK, SW8=BIT, ATT=min, NOISE=min) set an alternated data sequence 00/11 and push START connect the oscilloscope to TP16 and TP20, to examine the FSK signal before and after the communication Channel (Fig. 5) note the effect of the communication channel on the FSK signal. As the communication channel is limited band (the frequency response is low pass), the output FSK signal is slightly attenuated by the component with higher frequency (1800 Hz). Q3 What kind of signal can you detect across TP 23? 6
Figure 5: FSK Waveforms The signal supplied by the FSK demodulator is filtered by low pass filter, which removes the residuals of the FSK frequencies. The detected data signal is obtained across the filter output (TP24, fig. 5) in asynchronous data transmission, i.e. when in reception the clock regeneration and the data re-timing are not required, it is sufficient to square the signal supplied by the filter. The square output (threshold circuit) is detectable across TP29. On the oscilloscope note the correspondence between the transmitted data (TP6) and the received data (TP29). Introduce line attenuation (ATT), and note that the received data are not affected by the amplitude variations of the FSK signal. By inserting the noise (NOISE), too, you cause all alteration on the received data. 7
Q4 Why is the FSK less sensitive than the ASK to the signal amplitude variations? Take the line attenuation and line noise minimum Turn ON switch S15 Q5 The data in TP29 are not correct. Why? 8