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Lecture 2: AM Transmission

Prof. ParkELC 222

Essex County College

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Modulation• Modulation is the process of impressing a low-frequency

intelligence signal onto a higher-frequency carrier signal.• The low-frequency intelligence signal

– Intelligence– Modulating Signal– Information Signal– Audio Signal

• The high-frequency carrier signal– Radio-Frequency (RF) Signal

• Three Characteristics of Signal– Amplitude– Frequency– Phase

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AM Fundamentals

• Lower-Side Frequency (fc – fi)

• Carrier Frequency (fc)

• Upper-Side Frequency (fc + fi)

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Example 2-1

• A1.4 MHz carrier is modulated by a music signal that has frequency components from 20 Hz to 10 kHz. Determine the range of frequencies generated for the upper and lower sidebands.

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Example 2-2

• Determine the %m for the following conditions for an unmodulated carrier of 80V peak-to-peak (p-p).

Max. Vp-p Min. Vp-p

(a) 100 60(b) 125 35(c) 160 0(d) 180 0(e) 135 35

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Example 2-3

• Determine the maximum sideband power if the carrier output is 1 kW and calculate the total maximum transmitted power.

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Example 2-4

• A 500-W carrier is to be modulated to a 90 percent level. Determine the total transmitted power.

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Example 2-5

• An AM broadcast station operates at its maximum allowed total output of 50 kW and 95 percent modulation. How much of its transmitted power is intelligence?

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Example 2-6

• The antena current of an AM transmitter is 12A when unmodulated but increases to 13A when modulated. Calculate %m.

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Example 2-7

• An intelligence signal is amplified by a 70% efficient amplifier before being combined with a 10-kW carrier to generate the AM signal. If you want to operate at 100% modulation, what is the dc input power to the final intelligence amplifier?

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Example 2-8

• A transmitter with a 10-kW carrier transmits 11.2 kW when modulated with a single sine wave. Calculate the modulation index. If the carrier is simultaneously modulated with another sine wave at 50% modulation, calculate the total transmitted power.

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Mathematical Representation of Sine Wave

• v = Vp sin(t + )• Where v = instantaneous value• Vp = peak value = angular velocity = 2f = phase angle• AM: Amplitude Modulation• FM: Frequency Modulation• PM: Phase Modulation