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

FM demodulation - Essay sheathThe PLL has a voltage-controlled oscillator, a phase detector, and a low psyche slabber connected in a feedback loop. The input voltage determines the frequency of oscillation at the output of the VCO (Sedra 1998). This fosc is equal to the intermediate frequency fi essential to be 470 kHz in this case. The circuit is built on a proto board correspond to the block diagram shown fig1 bellow The components required are CD 4046 CMOS Phase-Lock Loop (PLL) integrated circuit Resistors of set 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 then 1/C =470000* 2*hundred thousand= 3.386pF and the preferred value is 3400nf. The restrain retch is given by 2fc=1/ (2fL/R1)1/2 a nd for audio the upper limit string 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 wherefore lock in range = 254323.5 Hz. Also, capture 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 pull out NBFM? The loop demodulates narrow band 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 GHz , which is well above the capture range of WBFM, which is customarily about 10.7 MHz 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 different 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 bit a small capture range is desirable as it enables the attenuation of high frequency components thereby improving the signal to noise ratio of the system. The lock-in range follows the above relation, as it is also directly proportionate to the filter cut off frequency. How would you modify the PLL in order to demodulate WBFM? From the relation of WBFM, 2fc = (2K0fpVDD)0.5 it is seen that the capture range 2fc is directly