EE390 Frequency Modulation/Demodulation Lab #4

Transcription

1 EE390 Frequency Modulation/Demodulation Lab #4 Objective Observe FM signals in both the time and frequency domain while making basic measurements. Equipment used. The Dual Function Generator: A feature of this generator is the FREQUENCY MODULATION section. Because the FM band is in the VHF band (88108 MHz) we can t directly see FM on our scopes. So we use the function generator to examine this form of modulation. To do this we put our message signal m(t) from channel B into the BNC connector of the Frequency Modulation section. This will create an FM signal on the output from channel A. To set the carrier frequency just set the frequency on channel A while monitoring the LED display. Controls and Connectors Deviation Adjust Knob Allows the deviation caused by a signal at the FM INPUT to be changed. For a 10 V pp signal at the input of the frequency modulator, maximum deviation (50) will be produced with the knob set at MAX. FM INPUT CONNECTOR BNC connector used to input an external signal to frequency modulate generator A output. PreLab4_W2005 Page 1 of 5

2 What s Inside the Black Box? Frequency To Sinewave Triangle Wave Generator FM Input Deviation The FM/PM Receiver RF Tuning: as in AM radio we have a tunable bandpass filter ganged to the local oscillator for mixing the signal to an IF of 10.7 MHz and then an IF filter. For the IF the designers used two ceramic filters to get better selectivity of the channels. Looking at the Quadrature detector. IF Output The Quadrature detector is how we transform a varying frequency into our baseband signal (get the music back). This entire section is built on a single chip, the CA3089 (FM IF System). This chip contains IF Amplifier, Quadrature detector, AF Preamplifier and specific circuits for AGC, AFC, Muting (Squelch), and Tuning meter. To get the signal back we are using a Quadrature detector. There are other methods of retrieving FM such as the Ratio detector, FosterSeely detector and the PLL detector. Σ Audio Recall the basic definition of the derivative: Output d x( t) x( t) x( t τ ) Time PreLab4_W2005 Delay Page 2 of 5 = lim τ 0 τ

3 For a small τ we can write: d x x( t) x( t τ ) τ Differentiation can be done using subtraction and time delay. Input a signal of: [ 2π f t ( t) ] K cos c θ ( t) The phase shift network (Time Delay) has a group delay of t 1 and a carrier delay t 0 designed so that : What comes out of the delay circuit is: 2π t0 = 2 π f c 2πf ( t t ) θ ( t t )] = K sin[2πf t θ ( t )] K cos[ c 0 1 c t1 Now LPF, the result is and sum in the original signal: If t1 is small and the change in θ(t) is small during t1 sec. Which results in our original message. y K 2 ( t) = sin[ θ ( t) θ ( t )] ( t) θ ( t) θ ( t ) d ( t) t θ ( t) 1 t 1 t 1 ( t) = t πf m( t) k m( t) 12 D d = f Standard Low Pass Filter (LPF). For a good read, check out Lathi page to see why we have BNC s 12,13,15, and 16. The Direct FM Multiplex Generator (941300) PreLab4_W2005 Page 3 of 5

4 Only using mono transmissions, use the LEFT input. Read Lathi page 243 for information about PREEMPHASIS. Following the signal, enable this section of the signal path. Set the level to CAL. All other switches should be in the out position, this will ground the inputs to the summing opamp reducing noise in the system. Direct FM generation is done here. The circuit here uses a tuned circuit to set the carrier frequency. A voltage applied to a varactor diode is used to vary the frequency of the tuned circuit and give us our FM signal. From the data sheet of the MV104 tuning diode we can see the capacitance changes as the voltage is varied. As the capacitance changes the frequency of our tuned circuit will change giving us an FM signal we can transmit. Leave the DEVIATION switch in for the standard 75 khz frequency deviation. From Summing Amplifier 12 V 12 V Carrier Frequency Adjust To RF Amp The RF amplifier before the antenna. PreLab4_W2005 Page 4 of 5

5 Matlab program to generate Bessel function plots. Bessel Functions used in Frequency Modulation (FM) Plotting the first 6 Bessel functions of the first kind. clear all, clf, clc ; Number of Bessel functions to plot n=0:5; Beta is our X axis range of values to plot Beta = 0:0.1:8; Matlab does all the hard work for us J = besselj(n,beta'); Setup plot to plot all n Bessel functions hold on grid for index = n 1 plot(beta,j(1:length(beta),index)) end label the plot xlabel('\bf\fontsize{25}\beta') ylabel('\bf\fontsize{25}j_{n}(\beta)') title('\fontsize{25}bessel Functions of the first kind') text(0.65,0.92,'\bfj_{0}') text(1.7,0.61, '\bfj_{1}') text(3.06,.53, '\bfj_{2}') text(4.42,.46, '\bfj_{3}') text(5.35,0.43,'\bfj_{4}') text(6.41,0.41,'\bfj_{5}') PreLab4_W2005 Page 5 of 5