T 7.2.1.3 Amplitude Modulation

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TPS <strong>7.2.1.3</strong> Double Sideband AM A C : V V 1 : f C : kHz b : Hz f r : kHz A M : V T : s f M : kHz V 2 Table 5.2.3-1: AM spectrum for square-wave modulation Signal parameter f KHz Measuerements Name S( n) V Analyzer settings S AM (n) V Theory S AM (n) V 5.3 AM demodulation (synchronous demodulation) 5.3.1 DSB Settings on the oscilloscope Input attenuator channel 1 Input attenuator channel 2 Time base Trigger / external 1 V/DIV 1 V/DIV 200 µs/DIV s M (t) Start with the settings from point 5.1. Set the phase controller on the CF transmitter to far left limit. Feed the DSB signal from the output of the modulator M2 directly into the demodulator D2 (do not use channel filter CH2!) Using a connecting lead feed the carrier signal (f C = 20 kHz) of the CF transmitter into the auxiliary carrier input of the demodulator D2. What have you achieved by this? Display the modulating signal s M (t) on the oscilloscope as well as the demodulated signal s D (t) at the output of the LP filter of the CF receiver. Sketch the curve of the modulating signal and the demodulated signal in Diagram 5.3.1-1. Tap the auxiliary carrier for the demodulator D2 in front of the phase shifter of the CF transmitter. Diagram 5.2.3-1: AM spectrum for square-wave modulation Diagram 5.3.1-1: Modulating and demodulated signal for the DSB, fixed phase relation (1): Modulating signal s M (t) (2): Demodulated signal s D (t) 34
TPS <strong>7.2.1.3</strong> Double Sideband AM Using the oscilloscope set the phase-shifts between the carrier of the transmitter and the carrier of the receiver as entered in Table 5.3.1-1. Measure the amplitude A D of the demodulated signal as a function of the phase φ. Complete Table 5.3.1-1. Plot the curve of A D /A Dmax in Diagram 5.3.1-1. Table 5.3.1-1: Phase response of the DSB φ degrees 0 18 36 54 72 90 108 s M (t): Sine A M = 2 V, f M = 2 kHz A D V A A D Dmax cos φ 5.3.2 Carrier recovery Carrier recovery is performed in the CF receiver using a PLL circuit. The PLL circuit is a control loop whose function is to match the frequency and phase of an oscillator to the reference oscillation. Fig. 5.3.2-1 illustrates the structure of a PLL circuit. Let's assume that the input signal s 1 (t) is supplied with the frequency f 1 to the phase detector. You can be fairly certain that the VCO is not going to be so friendly as to oscillate precisely at the same frequency. So its frequency f 2 will initially differ from f 1 . At the output of the phase detector an AC voltage is generated whose frequency is equal to the difference f 2 – f 1 . This AC voltage is now supplied to the input of the VCO via the loop filter. The VCO will respond to an AC voltage at its input with a corresponding change in frequency. In turn the VCO's changing frequency is detected by the phase detector. With a little luck the PLL locks into the frequency of the input signal. The PLL corrects the VCO until the input frequency and the VCO frequency coincide. A voltage U Φ arises behind the PD based on the phase shift. This is supplied to the VCO free of interfering AC components (U F ) through the loop filter. The following relationship prevails between the control voltage U F and the frequency f VCO of the VCO: f VCO = k F · U F (5.3.2-1) Diagram 5.3.1-1: Demodulated signal A D /A Dmax as a function of the phase φ Note: The synchronous demodulation is performed according to Equation (3-11), or (3-12) The control characteristic of the VCO (CF receiver) Remove the bridging plug between the loop filter and VCO input at the PLL. Feed a variable DC voltage U 1 from the function generator into the VCO input. Use this variable DC voltage to control the frequency of the VCO. Note down your measurement results in Table 5.3.2-1. Sketch the results in Diagram 5.3.2-1. Attention: 0 V< U F < 5 V W S 1 (t) X U Φ U F Fig. 5.3.2-1: Design of a PLL 1 Phase detector PD 2 Loop filter LF 3 VCO 35
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T 7.2.1.3 Amplitude Modulation

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