Skip to document
Welcome to StudocuSign in to access the best study resources
Guest user
0followers
HomeMy LibraryAsk AI
  • You don't have any recent items yet.
My Library
  • You don't have any courses yet.
  • You don't have any books yet.
  • You don't have any Studylists yet.

Communication Principles - 6

The frequency of the modulating signal determines the frequency deviat...
Course

Electronics Engineering (CR 061)

95 Documents
Students shared 95 documents in this course
Academic year: 2022/2023
Uploaded by:
Anonymous Student
This document has been uploaded by a student, just like you, who decided to remain anonymous.
Samar State University

Comments

Please sign in or register to post comments.

Students also viewed

Related documents

Preview text

LESSON CONTENT

  • The reverse relationship can also be implemented. A decreasing modulating signal increases the carrier frequency above its center value, whereas an increasing modulating signal decreases the carrier frequency below its center value.
  • As the modulating signal amplitude varies, the carrier frequency varies above and below its normal center, or resting, frequency with no modulation.
  • Frequency Deviation 𝐀𝐀 = the amount of change in carrier frequency produced by the modulating signal.
  • The frequency of the modulating signal determines the frequency deviation rate, or how many times per second the carrier frequency deviates above and below its center frequency. If the modulating signal is a 500-Hz sine wave, the carrier frequency shifts above and below the center frequency 500 times per second.
  • An FM signal is illustrated in figure 5. Normally the carrier is a sine wave, but it is shown as a triangular wave here to simplify the illustration. With no modulating signal applied, the carrier frequency is a constant-amplitude sine wave at its normal resting frequency.
  • The modulating information signal is a low-frequency sine wave. As the sine wave goes positive, the frequency of the carrier increases proportionately. As the modulating signal amplitude decreases, the carrier frequency decreases. When the modulating signal is at zero amplitude, the carrier is at its center frequency point. Assume a carrier frequency of 150 MHz. If the peak amplitude of the modulating signal causes a maximum frequency shift of 30 kHz, the carrier frequency will deviate up to 150 MHz and down to 149 MHz. The total frequency deviation is 150 – 149 = 0 MHz = 60 kHz. In practice, however, the frequency deviation is expressed as the amount of frequency shift of the carrier above or below the center frequency. Thus, the frequency deviation for the 150-MHz carrier frequency is represented as ± 30 kHz.
  • Frequency Shift Keying (FSK) – when the modulating signal is a binary 0, the carrier frequency is the center frequency value. When the modulating signal is a binary 1, the carrier frequency abruptly changes to a higher frequency level. The amount of the shift depends on the amplitude of the binary signal. o Widely used in the transmission of binary data in Bluetooth headsets, wireless speakers and many forms of industrial wireless.
  1. Principles of Phase Modulation
    • When the amount of phase shift of a constant-frequency carrier is varied in accordance with a modulating signal, the resulting output is a phase modulation (PM) signal.
    • Imagine a modulator circuit whose basic function is to produce a phase shift o A time separation between two since waves of the same frequency.
    • Assume that a phase shifter can be built that will cause the amount of phase shit to vary with the amplitude of the modulating signal.
    • The greater the amplitude of the modulating signal, the greater the phase shift.
    • Assume further that positive alternations of the modulating signal produce a lagging phase shift and negative signals produce a leading phase shift.
    • If a constant-amplitude, constant-frequency carrier sine wave is applied to the phase shifter whose phase shift is varied by the intelligence signal, the output of the phase shifter is a PM wave.
    • As the modulating signal goes positive, the amount of phase lag, and thus the delay of the carrier output, increases with the amplitude of the modulating signal.
    • The result at the output is the same as if the constant-frequency carrier signal had been stretched out, or had its frequency lowered.
    • FM is produced only as long as the phase shift is varying.

This low pass filter called a frequency-correcting network, predistorter, or 1/f filter, causes the higher modulating frequencies to be attenuated.

Phase Shift Keying

  • PM is also used with binary signals, as Fig 5-6 shows.
  • When the binary modulating signal is 0 V, or binary 0, the PM signal is simply the carrier frequency.
  • When a binary 1 voltage level occurs, the modulator, which is a phase shifter, simply changes the phase of the carrier, not its frequency.
  • The process of phase-modulating a carrier with binary data is called phase-shift keying (PSK) or binary phase-shift keying (BPSK).
  • The PSK signal shown in fig 5-6 uses a 180 0 phase shift from a reference, but other phase shift values can be used.

Modulation Index and Sidebands - Any modulation process produces sidebands. - When a constant-frequency sine wave modulates a carrier, two side frequencies are produced. - The side frequencies are the sum and difference of the carrier and the modulating frequency. - In FM and PM, as in AM, sum and difference sideband frequencies are produced. - Fig 5-7 shows the frequency spectrum of a typical FM signal produced by modulating a carrier with a single-frequency sine wave.

Modulation Index - The ratio of frequency deviation to the modulating frequency is known as the modulation index mf

- 𝐀𝐀 = 𝐀𝐀𝐀𝐀

  • Where 𝐀𝐀 is the frequency deviation and 𝐀𝐀 is the modulating frequency.
  • Sometimes the lowercase Greek letter delta is used instead of 𝐀𝐀 to represent deviation.

Bessel Functions

  • Given the modulation index, the number and amplitudes of the significant sidebands can be determined by solving the basic equation of an FM signal.

  • The FM wave is expressed as a composite of sine waves of different frequencies and amplitudes that, when added, give an FM time-domain signal.

  • The first term is the carrier with an amplitude given by Jn coefficient, in this case Jo.

  • The next term represents a pair of upper and lower side frequencies equal to the sum and difference of the carrier and modulating signal frequency. The amplitude of these side frequencies is J 1.

  • The next term is another pair of side frequencies equal to the carrier ±2 times the modulating signal frequency. The other terms represent additional side frequencies spaced from one another by an amount equal to the modulating signal frequency.

Noise and Phase Shift - The maximum phase shift occurs when the noise and signal phasors are at a right angle to each other. This angle can be computed with the arcsine or inverse sine according to the formula

Preemphasis

  • Noise can interfere with an FM signal, and particularly with the high-frequency components of the modulating signal. Since noise is primarily sharp spikes of energy, it contains a lot of harmonics and other high-frequency components.
  • To overcome this problem, most FM systems use a technique known as preemphasis that helps offset high-frequency noise interference.

Frequency Modulation versus Amplitude Modulation - Advantages of FM o Noise Immunity o Capture Effect o Transmitter Efficiency - Disadvantages of FM o Excessive Spectrum Use o Circuit Complexity

Was this document helpful?

Communication Principles - 6

Course: Electronics Engineering (CR 061)

95 Documents
Students shared 95 documents in this course

University: Samar State University

Was this document helpful?
LESSON CONTENT
- The reverse relationship can also be implemented. A decreasing modulating signal increases the
carrier frequency above its center value, whereas an increasing modulating signal decreases the
carrier frequency below its center value.
- As the modulating signal amplitude varies, the carrier frequency varies above and below its
normal center, or resting, frequency with no modulation.
- Frequency Deviation �� = the amount of change in carrier frequency produced by the modulating
signal.
- The frequency of the modulating signal determines the frequency deviation rate, or how many
times per second the carrier frequency deviates above and below its center frequency. If the
modulating signal is a 500-Hz sine wave, the carrier frequency shifts above and below the center
frequency 500 times per second.
-

Students also viewed

Related documents