FM demodulation Essay Example | Topics and Well Written Essays - 1000 words

FM demodulation - Essay ExampleThe PLL has a voltage-controlled oscillator, a phase detector, and a pitiable passing play filter connected in a feedback loop. The input voltage determines the oftenness of oscillation at the output of the VCO (Sedra 1998). This fosc is equal to the intermediate frequency fi required to be 470 kHz in this case. The circuit is built on a proto board according to the block diagram shown fig1 bellow The components required ar CD 4046 CMOS Phase-Lock Loop (PLL) integrated circuit Resistors of values 1k, 10k, 18k (2). Capacitors of values 0.1?F, 0.01?F, 3900pF and, Breadboard. The actual circuit is realized according to the circuit shown in fig 2 bellow. The circuit components in the PLL providing a centre frequency of 470 kHz are determined according to the equation, fosc = 1/2RC. Therefore 470000Hz*2 = 1/RC and taking R = 100K accordingly 1/C =470000* 2*100000= 3.386pF and the preferred value is 3400nf. The ravish picture is disposed by 2fc=1/ (2f L/R1)1/2 and for audio the maximum range is 20 kHz and R1 = 1k therefore the frequency of the filter is given by (2*20000*) 2 = 2fL/R1 fL= (2*2000*)2*1000/2 = 25.136 GHz and so the filter components are obtained as shown bellow 25.136*109 = 1/2RfCf and with Rf taken as 18K then Cf = 1/ (25.136*109*2*18000) = 35176.4pF The lock in range is given by fmax fmin and fmin= 1/R2(C1+32PF) = 1/10000(3900+32)*10-12 = 25432.35Hz fmax = 1/R1(C1+32PF) + fmin = 1/1000(3900+32)*10-12 +25432.35 = 279755.85 Hz Hence lock in range = 254323.5 Hz. Also, set out range 2fc = (2K0fpVDD)1/2 K0 =VDD/2= 15/2 =7.5 2fc = (2*7.5*25432.5*15)1/2 = 7564.6Hz Discussion Did the loop demodulate NBFM? The loop demodulates narrow luck FM that occupies the frequency range of 0-15kHz as this frequency range lies within the capture range and the lock range of the circuit. Could it be used to demodulate WBFM without any alteration? The loop demodulate the wide band FM as the low pass filter above has a value of 25.136 G Hz, which is well above the capture range of WBFM, which is customarily around 10.7 megacycle per second with a system bandwidth of 200 kHz. This bandwidth is within the centre frequency range of 470 kHz (Alencar 2005) What happened to the lock-in and capture range when the loop components were altered? The capture range is low if the cut off frequency of the filter is lower this is achieved by varying the filter components i.e. using large filter capacitance, and resistor values. On the other hand, the capture range is made large by using lower values of filter components thereby increasing the cut off frequency (Carlson 2002). A wider capture range is desirable as it enables demodulation of WBFM while a small capture range is desirable as it enables the attenuation of high frequency components thereby up(a) the signal to noise ratio of the system. The lock-in range follows the above relation, as it is also directly proportional to the filter cut off frequency. How would you modi fy the PLL in tack to demodulate WBFM? From the relation of WBFM, 2fc = (2K0fpVDD)0.5 it is seen that the capture range 2fc is directly