PDF Principles of Communication Modulation Handout 3 & 4

HANDOUT 3 PRINCIPLES OF COMMUNICATION Amplitude Modulation Modulation It is the process of converting raw message data into digital signals or waves over a modulator to optimize transmission. This process increases the strength of the signal to have maximum reach. Modulation is done to impro...

HANDOUT 3 PRINCIPLES OF COMMUNICATION Amplitude Modulation Modulation It is the process of converting raw message data into digital signals or waves over a modulator to optimize transmission. This process increases the strength of the signal to have maximum reach. Modulation is done to improve signals often unsuitable for the transmission device. Modulation Modulating Signal The message that must be transmitted Carrier Signal Steady waveform in terms of amplitude and frequency Modulated Signal The output signal after the modulation process Modulation The frequency people hear is in the audio spectrum only ranging from 20Hz to 20kHz. This makes sending audio signals wirelessly using radio frequency harder. One way to get a higher-frequency signal to carry a low-frequency signal is by amplitude modulation. Amplitude Modulation (AM) It is a modulation process focused on modulating or changing the signal’s amplitude. The instantaneous amplitude of a radio frequency carrier wave is varied in direct proportion to that of the modulating signal to obtain an amplitude-modulated signal. AM Characteristics 1 The carrier frequency stays constant during the modulation process, but its amplitude (height) changes based on the modulating signal. AM Characteristics 2 An increase in the amplitude of the modulating signal increases the amplitude of the carrier. AM Characteristics 3 Changes in the positive and negative peaks of the carrier wave depend on the modulating signal. AM Characteristics 4 An imaginary line connects the positive and negative peaks of the carrier waveform. This imaginary line is called an envelope. AM Characteristics 5 An increase/decrease in the amplitude of the modulating signal causes an increase/decrease in the positive and negative peaks of the carrier amplitude. Amplitude Modulation The figure shows the simplified waveform that uses equally spaced vertical lines to represent high-frequency carrier waves. The amplitude of these vertical lines also varies based on the modulating signal. Amplitude Modulation This shows the variation of carrier amplitude concerning time and are to be in time domain. Time domain signals are voltage or current variations that occur over time and are displayed on oscilloscope screen. Sidebands Also known as side frequencies, these are new signals which are generated at Amplitude different frequencies as part of the modulation process. Sidebands are found in the frequency spectrum above and below the carrier frequency. Frequency Sidebands Bandwidth fUSB = fc + fm BW = fUSB - fLSB fLSB = fc - fm BW = 2fm fUSB = upper sideband fLSB = lower sideband fc = carrier frequency fm = modulating frequency Sample Problems A radio station broadcasts at a carrier frequency of 105.0MHz. The 1 modulation frequency of the audio signal is 15kHz. Calculate the frequencies of the upper and lower sidebands, and its bandwidth. A radio station is transmitting at a carrier frequency of 1500kHz 2 with a modulation frequency of 20kHz. Identify the upper and lower sidebands, and its bandwidth. A radio station operates with a main frequency of 1060kHz and 3 modulates its signal using a frequency of 15kHz. What are the frequencies of the upper and lower sidebands, and bandwidth generated by this modulation? Modulation Index This is also referred to as the degree of Vm modulation or modulating m= factor/coefficient. Vc The amplitude of the modulating signal must always be less than the amplitude of m = modulation index the carrier or Vm < Vc. Otherwise, Vm = amplitude of modulating signal distortion will occur. Vc = amplitude of carrier signal Percentage of Modulation The percentage of modulation refers to how much of the carrier wave is being modulated by the signal in amplitude modulation (AM). %m = m x 100 It’s a way of expressing the modulation m = modulation index index in percentage form. Sample Problems Determine the modulation index and percentage of modulation if the 1 carrier voltage is 8V and the modulating signal is 6.85mV. A radio station is transmitting an AM signal with a carrier frequency 2 of 800kHz. The modulated signal voltage is measured to be 12V, while the carrier voltage is 15V. Calculate the modulation index. A radio station is transmitting an amplitude-modulated (AM) signal 3 at a carrier frequency of 1.2MHz. The carrier voltage is measured at 8MV, while the modulating signal voltage is 2.3kV. Calculate the modulation index. Modulation Index using Oscilloscope An oscilloscope can be used to derive the modulation index by measuring the values of the modulation and carrier voltages and Vmax – Vmin then calculating the ratio. m= Vmax + Vmin When the AM signal is seen on oscilloscope, the modulation index is computed from Vmax and Vmin. Modulation Index using Oscilloscope Modulation Index using Oscilloscope Vmax – Vmin Vmax + Vmin Vm = 2 Vc = 2 Vm = peak value of the modulating signal Vc = peak value of the carrier signal Sample Problems An AM signal is read with 4.9V divisions for Vmax and 1.4V 1 divisions for its Vmin on the oscilloscope. What is the percentage of modulation? Assuming an oscilloscope displays a peak of 8.37V and a trough of 2 3.21V. What is the modulation index? Find the peak value of the modulating signal and peak value of the 3 carrier signal if it has a Vmax of 13.13V and a Vmin of 1.23V. HANDOUT 4 PRINCIPLES OF COMMUNICATION Frequency & Phase Modulation Frequency Modulation (FM) A modulation process that puts the message in the carrier wave by changing the instantaneous frequency of the wave. Frequency Modulation (FM) Frequency Deviation (fd) It is the amount of change in the carrier frequency caused by the modulating signal. The maximum frequency deviation happens at the maximum amplitude of the modulating signal. Maximum Deviation = carrier frequency + maximum frequency Minimum Deviation = carrier frequency - maximum frequency Frequency Deviation = Maximum Deviation – Minimum Deviation Sample Problems A signal has a 150MHz carrier frequency. If the peak amplitude of 1 the modulating signal causes a maximum frequency shift of 40kHz. What are the frequency deviations? Find the total frequency deviation if a signal has a carrier frequency 2 of 103.5MHz with a maximum frequency of 7.3MHz. Compute the maximum, minimum, and frequency deviation if a 3 signal reached a maximum frequency of 40kHz and a carrier frequency of 1525kHz. Frequency-Shift Keying (FSK) In FM, a series of rectangular waves, such as serial binary data, can represent the modulating signal. This kind of modulating signal will only have two amplitudes and will also make the carrier frequency have two values (1 and 0). Frequency-Shift Keying (FSK) The modulating signal starts at binary 0, making the carrier frequency the center frequency value. When the modulating signal is at binary 1, the carrier frequency rises to a higher frequency level. The amount of shift depends on the amplitude of the binary signal. This modulation is called frequency-shift keying. Phase Modulation (PM) It is a modulation process wherein the phase of the carrier signal varies based on the amplitude variation of the message or modulating signal. PM is practically similar to FM, but in PM, the frequency of the carrier signal is not increased. Phase of a Signal Phase Modulation (PM) Lagging phase shifts are when the modulating signal lags or stretches. Leading phase shifts are when it starts to lead or compress. Phase-Shift Keying (PSK) When the binary 0 is 0V, the PM signal is simply the carrier frequency. When a binary 1 voltage level occurs (3V), the modulator or phase shifter changes the phase of the carrier, and not its frequency. Phase-Shift Keying (PSK) Each time the signal changes from 0 to 1 or 1 to 0, a 180° phase shift happens whether the signal is positive or negative. This process is called phase-shift keying or binary phase-shift keying. Sidebands In FM and PM, just like AM, it is the sum and difference of the carrier and the modulating frequency. The spectrum of an FM and a PM signal is wider than AM because of sideband pairs. Sidebands Sidebands fUSB1 = fc + fm fLSB1 = fc - fm fUSB2 = fc + 2fm fLSB2 = fc - 2fm fUSB3 = fc + 3fm fLSB3 = fc - 3fm and so on… Sample Problems A local radio station transmits its FM signal at a carrier frequency of 1 101.3MHz. The station uses an audio frequency of 5.5MHz to modulate its signal, which includes various local news updates. Calculate the frequencies of the first three pairs of sidebands generated by the FM signal. A satellite communication system operates by transmitting an FM 2 signal at a carrier frequency of 2.5GHz. To transmit data, the system modulates the signal using a data frequency of 205MHz. Determine the frequencies of the first two pairs of sidebands produced by the FM signal. Modulation Index (mf) fd mf = The amplitudes of the carrier and sidebands of the frequency spectrum of an FM signal still depend on the fm modulation index, still unitless. fd = frequency deviation fm = modulating frequency Sample Problems If the maximum modulating frequency is 3.5kHz and the maximum 1 frequency deviation of the carrier is ±14kHz, what is the modulation index? A radio station transmits an FM signal with a carrier frequency of 2 90MHz. The modulating signal is a sinusoidal audio tone with a frequency of 15kHz, and the peak frequency deviation of the FM signal is 75kHz.