Signal Demodulation: Problems
Modulated waveform must be received and the message recovered. The message recovery in AM communication systems requires a circuit that is capable to extract the carrier envelope. A diode–capacitor based circuit known as envelope detector (or, peak detect
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Signal Demodulation: Problems
Modulated waveform must be received and the message recovered. The message recovery in AM communication systems requires a circuit that is capable to extract the carrier envelope. A diode–capacitor based circuit known as envelope detector (or, peak detector) is essential part of both AM and FM receivers, as well as many other analog signal processing systems. In this chapter we familiarize with the envelope detector operational principle and operation. Problems: 11.1. With reference to the simplified schematic Fig. 11.1 and if R = 2 k, estimate: 1. 2. 3. 4. 5.
Detector input impedance; Total signal power delivered to the detector; Voltages v0 (max), v0 (min), and V0 (DC); Average output current I0 (DC); and Appropriate capacitor value C to prevent diagonal clipping distortion for maximal modulation frequency f m (max) = 5 kHz and maximal modulation index m a = 0.9.
11.2. Assume that the input signal Vin of an AM diode detector, Fig. 11.2, is a 665 kHz IF carrier modulated with a 5 kHz tone. Component values are: C1 = 220 pF, C2 = 22 pF, R1 = 470 , R2 = 4.7 k, R L = 50 k. Diode I D versus VD characteristics is shown in the graph. 1. Sketch qualitatively the detector output tones along an ω axis showing relative amplitudes of the tones; 2. Sketch AM waveform shape at nodes 1–5; 3. Sketch equivalent circuit at 5 kHz. Calculate amplitude ratio of the input signal and signal at node 3; 4. Sketch equivalent circuit at 665 kHz. Calculate amplitude ratio of the input signal and signal at node 3; 5. Briefly comment on the above results.
R. Sobot, Wireless Communication Electronics by Example, 49 DOI: 10.1007/978-3-319-02871-2_11, © Springer International Publishing Switzerland 2014
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11 Signal Demodulation: Problems
Fig. 11.1 Problem 11.1: simplified schematic of peak detector
Fig. 11.2 Problem 11.2: simplified schematic of peak detector, and voltage current characteristics the diode
Fig. 11.3 Block diagram for Problem 11.3
11.3. Voltage signal received by a 50 antenna has amplitude of 10 µV. Gain contributions are noted next to each block of the system (see Fig. 11.3). Estimate: 1. input signal power in W and dBm units, 2. power delivered to the speaker in dBm and W. 11.4. A signal waveform used to modulate RF carrier has symmetrical triangular shape with zero DC = 1 V component and amplitude of Vb = 2 Vpp while the carrier wave has amplitude of VC = 2 Vp . Sketch these two waveforms before and after the mixer circuit. Estimate modulation index m by inspection of the plots. 11.5. For unmodulated signal, AM current in the antenna is I0 = 1 A, while sinusoidal modulation wave causes the antenna current to be Im = 1.1 A. Calculate the modulation index m if the antenna impedance is R.
11 Signal Demodulation: Problems
11.6. An FM signal S FM (t) is described as S FM (t) = 2,000 sin 2π × 108 + 2 × π × 104 cos (π × 104 t) t as it arrives at a 50 antenna. Determine: 1. 2. 3. 4. 5. 6.
the carrier frequency, the transmitted power, modulation index m f , the message signal
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