Amplitude Modulation (AM) 1

Tutorial 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. 4

Homework Continued Repeat steps (a) through (d) in Example 4 in these lecture slides for a modulation coefficient of 0.5. 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.

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(230t) sin(225t) + ½ cos(220t) 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

Conventional DSFC-AM

Single-side Band Full Carrier (SSBFC) The carrier is transmitted at full power and only one sideband is transmitted.

SSBFC waveform, 100% modulation

Single-Sideband Suppressed Carrier (SSBSC) The carrier is suppressed 100% and one sideband is removed. Only one sideband is transmitted.

SSBSC waveform

Single-Sideband Reduced Carrier (SSBRC) One sideband is removed and the carrier voltage is reduced to 10% of its un-modulated amplitude.

Independent Sideband (ISB) A single carrier is independently modulated by two different modulating signals.

ISB waveform

Vestigial Sideband (VSB) The carrier and one complete sideband are transmitted, but only part of the other sideband is transmitted.

Single-Sideband Generation

Balanced modulator waveforms

FET Balanced Modulator

AM DSBSC modulator using the LM1496/1596 linear integrated circuit

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 Low 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

SSBFC Receiver