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EENG 3810 Chapter 4 Amplitude Modulation (AM) 1 Chapter 4 Homework 1. For an AM DSBFC modulator with a carrier frequency fc = 200KHz and a maximum modulating signal frequency fm(max) = 10 KHz, determine : a. Frequency limits for the upper and lower sidebands. b. Bandwidth. b. Upper and lower side frequencies produced when the modulating signal is a single-frequency 6 KHz tone. 2 Homework Continued 2. For the AM wave form above determine: 3 Homework Continued 3. 400 25 5 6 4 Homework Continued 4. Repeat steps (a) through (d) in Example 4 in these lecture slides for a modulation coefficient of 0.5. 5. For an AM DSBFC wave with a peak unmodulated carrier voltage Vc = 20 Vp, a load resistance RL = 20 , and a modulation coefficient m = 0.8, determine the power of the modulated wave 5 Homework Continued 6.Determine the noise improvement for a receiver with an RF bandwidth equal to 100 KHz and an IF bandwidth equal to 20 KHz. 6 Amplitude Modulation Transmission 7 AM Generation 8 Frequency Spectrum of An AM Double Sideband Full Carrier (DSBFC) Wave 9 Example 1 For an AM DSBFC modulator with a carrier frequency fc = 100KHz and a maximum modulating signal frequency fm(max) = 5 KHz, determine : a. Frequency limits for the upper and lower sidebands. b. Bandwidth. c. Upper and lower side frequencies produced when the modulating signal is a single-frequency 3 KHz tone. 10 Example 1 Solution a. b. c. 11 Example 1 d. The Output Spectrum For An AM DSBFC Wave 12 Phasor addition in an AM DSBFC envelope • For a single-frequency modulating signal, am AM envelop is produced from the vector addition of the carrier and upper and lower side frequencies. Phasors of the carrier, • The upper and lower frequencies combine and produce a resultant component that combines with the carrier component. • Phasors for the carrier, upper and lower frequencies all rotate in the counterclockwise direction. • The upper sideband frequency rotates faster than the carrier. (usf > c) • The lower sideband frequency rotes slower than the carrier. (usf < c) 13 Phasor addition in an AM DSBFC envelope 14 Modulation Coefficient 15 If the modulating signal is pure, single frequency sine wave and the modulation process is symmetrical, the % modulation can be derived as follows: 16 Peak Amplitudes of Upper and Lower Sidebands The peak change in amplitude of the output wave (Em) is equal to the sum of the voltages from the upper and lower sideband frequencies. Therefore, 17 Percent Modulation of An AM DSBFC Envelope ( a) modulating signal; (b) unmodulated carrier; (c) 50% modulated wave; (d) 100% modulated wave 18 Example 2 For the AM wave form above determine: 19 Example 2 20 Voltage Spectrum for an AM DSBFC Wave 21 Generation of an AM DSBFC Envelope Shown in The Time Domain –½ cos(230t) sin(225t) + ½ cos(220t) summation of (a), (b), and (c) 22 Voltage of an AM DSBFC Envelope In The Time Domain 23 Example 3 24 Example 3 Continued 25 Output Spectrum for Example 3 26 AM envelope for Example 3 27 Power for Upper and Lower Sideband 28 Total Power for an AM DSBFC Envelop 29 Power Spectrum for an AM DSBFC Wave with a Single-frequency Modulating Signal 30 Example 4 31 Power Spectrum for Example 4 32 Single Transistor, Emitter Modulator 33 Single Transistor, Emitter Modulator (output waveforms ) 34 Medium-power Transistor AM DSBFC Modulator 35 High-power AM DSBFC Transistor Modulator 36 Linear Integrated-circuit AM Modulator 37 Block Diagram of a Low-level AM DSBFC Transmitter 38 Block Diagram of a High-level AM DSBFC Transmitter 39 Single-Sideband 40 Conventional DSFC-AM 41 Single-side Band Full Carrier (SSBFC) The carrier is transmitted at full power and only one sideband is transmitted. 42 SSBFC waveform, 100% modulation 43 Single-Sideband Suppressed Carrier (SSBSC) The carrier is suppressed 100% and one sideband is removed. Only one sideband is transmitted. 44 SSBSC waveform 45 Single-Sideband Reduced Carrier (SSBRC) One sideband is removed and the carrier voltage is reduced to 10% of its un-modulated amplitude. 46 Independent Sideband (ISB) A single carrier is independently modulated by two different modulating signals. 47 ISB waveform 48 Vestigial Sideband (VSB) The carrier and one complete sideband are transmitted, but only part of the other sideband is transmitted. 49 50 Single-Sideband Generation 51 Balanced modulator waveforms 52 FET Balanced Modulator 53 AM DSBSC modulator using the LM1496/1596 linear integrated circuit 54 Amplitude Modulation Reception 55 Simplified Block Diagram of an AM Receiver 56 Simplified Block Diagram of an AM Receiver • Receiver front end = RF section – Detecting the signal – Band-limiting the signal – Amplifying the Band-limited signal • Mixer/converter – Down converts the RF signal to an IF signal • Intermediate frequency (IF) signal – Amplification – Selectivity • Ability of a receiver to accept assigned frequency • Ability of a receiver to reject other frequencies • AM detector demodulates the IF signal to the original signal • Audio section amplifies the recovered signal. 57 Noncoherent Tuned Radio Frequency Receiver Block Diagram 58 AM Superheterodyne Receiver Block Diagram 59 Bandwidth Improvement (BI) • • • • • Noise reduction ratio BI = BRF / BIF Noise figure improvement NFIMP = 10 log BI Determine the noise improvement for a receiver with an RF bandwidth equal to 200 KHz and an IF bandwidth equal to 10 KHz. – BI = 200 KHz / 10 KHZ = 20 – NFImp = 10 log 20 = 13 dB 60 Sensitivity • Sensitivity: minimum RF signal level that the receiver can detect at the RF input. • AM broadcast receivers – 10 dB signal to noise ratio – ½ watt (27 dBm) of power at the audio output – 50 uV Sensitivity • Microwave receivers – 40 dB signal to noise ratio – 5 mw (7 dBm) of power at the output • Aa 61 Dynamic Range • Dynamic Range – Difference in dB between the minimum input level and the level that will over drive the receiver (produce distortion). – Input power range that the receiver is useful. – 100 dB is about the highest posible. • Low Dynamic Range – Causes desensitizing of the RF amplifiers – Results in sever inter-modulation distortion of weaker signals 62 Fidelity • Ability to produce an exact replica of the original signal. • Forms of distortion – Amplitude • Results from non-uniform gain in amplifiers and filters. • Output signal differs from the original signal – Frequency: frequencies are in the output that were not in the orginal signal – Phase • Not important for voice transmission • Devastating for digital transmission 63 SSBRC Receiver 64 SSBFC Receiver 65