02-Handout-1.pdf

IT1718 Types of Noise External Noise  Comes from sources which we have little or no control.  This noise can be classified as industrial noise, atmospheric noise, and extraterrestrial n...

IT1718 Types of Noise External Noise  Comes from sources which we have little or no control.  This noise can be classified as industrial noise, atmospheric noise, and extraterrestrial noise.  Industrial Noise is produced by manufactured equipment, such as automotive ignition systems, electric motors and generators.  Any electrical equipment that causes high voltages or currents to be switched produces transients that create noise. Example is the occurrence of large amplitude noise pulses when a motor device is turned on or off.  Atmospheric Noise is the electrical disturbances that occur in the earth’s atmosphere, often referred to as static which usually comes from lightning.  Extraterrestrial Noise is classified to as solar and cosmic which comes from the sources in space.  The sun radiates a wide range of signals in a broad noise spectrum, making it one of the primary sources of extraterrestrial noise. This is referred to as solar noise.  Stars also radiate noise in the same manner as the sun, but not as great of that produced by the sun, known as cosmic noise. Internal Noise  Produced by electronic components in a receiver circuit such as resistors, diodes, transistors, etc. Unlike external noise, there are some design to control over this type of noise.  Noise that are caused by random motion of free electrons in a conductor through heat is known as Thermal Noise, or is also referred to as white noise.  The amount of open-circuit noise voltage appearing across a resistor or the input impedance to a receiver can be calculated according to Johnson’s Formula: 𝑽𝒏 = √𝟒𝒌𝑻𝑩𝑹 Where: 𝑽𝒏 =rms noise voltage (Voltage) 𝐽𝑜𝑢𝑙𝑒 𝒌 =Boltzman’s Constant (1.38 x 10-23𝐾𝑒𝑙𝑣𝑖𝑛) 𝑻 = temperature, Kelvin (ᵒC + 273) 𝑩 = bandwidth, Hertz 𝑹 = resistance, Ω (ohms)  Thermal Noise can also be computed as a power level given as: 𝑷𝒏 = 𝒌𝑻𝑩 Where: 𝑷𝒏 = average noise power (watts) 𝐽𝑜𝑢𝑙𝑒 𝒌 =Boltzman’s Constant (1.38 x 10-23 ) 𝐾𝑒𝑙𝑣𝑖𝑛 𝑻 = temperature, Kelvin (ᵒC + 273) 𝑩 = bandwidth, Hertz Note: Three (3) temperature scales are in common use: Fahrenheit scale (ᵒF), Celsius scale (ᵒC), and Kelvin scale (K). When calculating noise values, you will frequently need to make conversions from one of these temperature scales to another. The most common conversion formulas are given as: 𝟓( 𝑻𝑭 −𝟑𝟐) (1) 𝑻𝒄 = 𝟗 or 𝑻𝒄 = 𝑻𝑲 − 𝟐𝟕𝟑 𝟗𝑻𝒄 (2) 𝑻𝑭 = + 𝟑𝟐 𝟓 (3) 𝑻𝑲 = 𝑻𝒄 + 𝟐𝟕𝟑 02 Handout 1 *Property of STI Page 1 of 1  IT1718 Formula (1) for Temperatures in Fahrenheit to Celsius, or Kelvin to Celsius, respectively. Formula (2) for Temperatures in Celsius to Fahrenheit. Formula (3) for Temperatures in Celsius to Kelvin.  Shot Noise is also white noise in that is contains all frequencies and amplitudes over a very wide range. The amount of shot noise is directly proportional to the amount of DC bias flowing in a device. The rms noise current in a device (𝑰𝒏 ) is calculated with the formula: 𝑰𝒏 = √𝟐𝒒𝑰𝑩 Where: 𝒒 = charge on an electron, 1.6 x 10-19 C 𝑰 = direct current, Amperes 𝑩 = bandwidth, Hertz  Transmit-time Noise refers to how long it takes for a current carrier such as a hole or electron to move from the input to the output.  Flicker Noise is the type of noise which is strongest at lower frequencies generally below a few kHz. The spectral density of this noise increases as the frequency decreases. Signal-to-Noise Ratio (SNR or S/N)  Indicates the relative strengths of the signal and the noise in a communication system.  Communication devices aims to produce the highest feasible SNR  SNR is computed by using either voltage or power values: 𝑽𝒔 𝑷𝒔 𝑺𝑵𝑹 = 𝑽𝒏 or 𝑺𝑵𝑹 = 𝑷𝒏 where: 𝑽𝒔 = 𝒔𝒊𝒈𝒏𝒂𝒍 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 𝑽𝒏 = 𝒏𝒐𝒊𝒔𝒆 𝒗𝒐𝒍𝒕𝒂𝒈𝒆 𝑷𝒔 = 𝒔𝒊𝒈𝒏𝒂𝒍 𝒑𝒐𝒘𝒆𝒓 𝑷𝒏 = 𝒏𝒐𝒊𝒔𝒆 𝒑𝒐𝒘𝒆𝒓 Noise Levels and Noise in Cascaded Stages  Noise Factor or Noise Ratio (NR) is the ratio of the Input SNR Power to the Output SNR Power.  NR is computed with the expression: 𝑺𝑵𝑹 𝒊𝒏𝒑𝒖𝒕 𝑵𝑹 = 𝑺𝑵𝑹 𝒐𝒖𝒕𝒑𝒖𝒕  Noise Figure (NF) is the noise factor expressed in decibels (dB), given as: 𝑵𝑭 = 𝟏𝟎𝒍𝒐𝒈(𝑵𝑹)  For cascaded stages the Overall Noise Performance (𝑵𝑹𝑻 ) can be obtained using Friis’ formula, given as: 𝑵𝑹𝟐 − 𝟏 𝑵𝑹𝟑 − 𝟏 𝑵𝑹𝒏 − 𝟏 𝑵𝑹𝑻 = 𝑵𝑹𝟏 + + +⋯+ 𝑨𝟏 𝑨𝟏 𝑨 𝟐 𝑨𝟏 𝑨𝟐 … 𝑨𝒏−𝟏 Where: 𝑵𝑹𝑻 = overall noise ratio 𝑵𝑹𝟏 = noise ratio of input or first amplifier to receive the signal. 𝑵𝑹𝟐 = noise ratio of second amplifier 02 Handout 1 *Property of STI Page 2 of 1  IT1718 𝑵𝑹𝟑 = noise ratio of third amplifier, and so on 𝑨𝟏 = power gain of first amplifier 𝑨𝟐 = power gain of second amplifier 𝑨𝟑 = power gain of third amplifier, and so on Concepts of Amplitude Modulation  Remember that modulation is an electronic method for transmitting information efficiently from a place to another.  The signal containing intelligence or information to be transmitted are called modulating signal. The sine wave modulating signal can be expressed with the expression: 𝑣𝑚 = V𝑚 sin(2𝝅𝑓𝑚 𝑡) Where: 𝑣𝑚 = instantaneous value of information signal V𝑚 = peak amplitude of information signal 𝑓𝑚 = frequency of the modulating signal 𝑡 = particular point in time during the cycle  Carrier frequency is greater than the modulating frequencies, and is expressed as: 𝑣𝑐 = V𝑐 sin(2𝝅𝑓𝑐 𝑡) Where: 𝑣𝑐 = instantaneous value of carrier sine wave voltage V𝑐 = peak amplitude of the constant unmodulated carrier sine wave (as measured between zero and the maximum amplitude of either the positive-going or the negative going alterations) 𝑓𝑐 = frequency of the carrier sine wave 𝑡 = particular point in time during the cycle  Amplitude Modulation (AM) is a modulation technique in which the modulating signal varies the amplitude of a sinusoidal carrier signal.  The resulting signal from the process of amplitude modulation is called AM wave.  In AM, amplitude of the modulating signal should be less than the amplitude of the carrier (V𝑚 < V𝑐 ). Distortion will occur otherwise. Through derivation, the instantaneous value of an AM wave (𝑣𝐴𝑀 ) can be computed by using the equation: 𝒗𝑨𝑴 = 𝐕𝒄 𝐬𝐢𝐧𝟐𝝅𝒇𝒄 𝒕 + (𝐕𝒎 𝐬𝐢𝐧𝟐𝝅𝒇𝒎 𝒕)(𝐬𝐢𝐧𝟐𝝅𝒇𝒄 𝒕) Where: 𝐕𝒄 𝐬𝐢𝐧𝟐𝝅𝒇𝒄 is the carrier waveform (𝐕𝒎 𝐬𝐢𝐧𝟐𝝅𝒇𝒎 𝒕)(𝐬𝐢𝐧𝟐𝝅𝒇𝒄 𝒕) is the carrier waveform multiplied by the modulating signal waveform 02 Handout 1 *Property of STI Page 3 of 1  IT1718 Figure 2.1 Amplitude modulator showing input and output signals Retrieved from: Frenzel L., Principles of Electronic Communication Systems, Page 95 Figure 2.2 sample of carrier signal, modulating signal and modulated signal waveforms Retrieved from: https://electronicspost.com/wp-content/uploads/2015/11/amplitude-modulation1  The dotted line in the modulated signal is called Envelope.  The circuit used for producing AM is called modulator. It computes the product of the carrier and modulating signals, as shown in figure 2.1.  Modulator changes a lower frequency modulating signal to a higher frequency signal. Modulation Index and Percentage of Modulation  Modulating signal voltage (V𝑚 ) must be less than the carrier voltage (V𝑐 )  The relationship between modulating signal voltage, and carrier voltage is known as modulation index (m) given as: 𝐕𝒎 𝒎= 𝐕𝒄  Multiplying the modulation index (m) by 100 gives the percentage of modulation.  The value of modulation Index (𝒎) should be a number between zero (0) and one (1).  Distortion will occur if 𝒎 exceeds one (1). Which results to garbled or harsh unnatural sounds in the speaker for voice, while scrambled and inaccurate picture on a TV screen for video.  The ideal condition for AM is 𝑚 = 1, that is when V𝑚 = V𝑐 , which gives 100 percent modulation. This results in the greatest output power at the transmitter and greatest output voltage at the receiver, without distortion. 02 Handout 1 *Property of STI Page 4 of 1  IT1718  Modulation Index can be computed from Vmax and Vmin when the AM signal is displayed on an oscilloscope shown in figure 2.3. Formula for the peak value of the modulating signal (Vm) is given as: 𝐕𝒎 𝐕𝒎𝒂𝒙 − 𝐕𝒎𝒊𝒏 𝒎= = 𝐕𝒄 𝐕𝒎𝒂𝒙 + 𝐕𝒎𝒊𝒏 Figure 2.3 Sample of an AM Wave Retrieved from: Frenzel L., Principles of Electronic Communication Systems, Page 97 Sidebands and the Frequency Domain  Sidebands are new signals at different frequencies generated whenever a carrier is modulated by an information signal.  Sidebands occur in the frequency spectrum directly above and directly below the carrier frequency.  Through modulation, signals that are originally had the same spectrum can be translated to new frequencies and thus not interfere with each other.  Sidebands occur at frequencies that are the sum and difference of the carrier and modulating frequencies, computed as: 𝒇𝑼𝑺𝑩 = 𝒇𝒄 + 𝒇𝒎 𝒇𝑳𝑺𝑩 = 𝒇𝒄 − 𝒇𝒎 Where: 𝒇𝑼𝑺𝑩 = upper sideband 𝒇𝑳𝑺𝑩 = lower sideband 𝒇𝒄 = carrier frequency 𝒇𝒎 = modulating frequency  Sideband signals are usually illustrated in a frequency domain, where the horizontal axis represents frequency, and the vertical axis represents the magnitude of the signals (may be in voltage, current, or power amplitude) as shown in figure 2.4. 02 Handout 1 *Property of STI Page 5 of 1  IT1718 Figure 2.4 Frequency-domain display of an AM signal Reference: Frenzel L., Principles of Electronic Communication Systems, Page 100 Amplitude Modulation Power  In radio transmission, the AM signal is amplified by a power amplifier and is fed to the antenna.  AM signal is really a composite of several signal, namely the carrier and the two sidebands, with each producing power in the antenna.  The total transmitted power (PT) is simply the sum of the carrier power (Pc) and the two power in the two sidebands PUSB and PLSB: 𝑷𝑻 = 𝑷𝒄 + 𝑷𝑳𝑺𝑩 + 𝑷𝑼𝑺𝑩 Where: 𝑷𝑻 =Total transmitted power 𝑷𝑪 = Carrier power 𝑷𝑳𝑺𝑩 = Lower sideband power 𝑷𝑼𝑺𝑩 =Upper sideband power  Total transmitter power (PT) can also be computed when the carrier power (PC) and the percentage of modulation are known, given as: 𝑚2 𝑷𝑻 = 𝑷𝑪 (1 + ) 2 Where: 𝑷𝑻 =Total transmitted power 𝑷𝑪 = Carrier power 𝒎 = Modulation index References: Beasley, J. & Miller, G. (2014). Modern Electronic Communication, Ninth Edition. London: Pearson Education Limited Frenzel, L. (2016). Principles of Communication Systems. New York: McGraw-Hill Education Madhow, U. (2014). Introduction to Communication Systems. California: University of California, Santa Barbara 02 Handout 1 *Property of STI Page 6 of 1