Nothing Special   »   [go: up one dir, main page]
Scribd Logo
Upload

Welcome to Scribd!

  • 68 views

    Noise

    Uploaded by

    nyan nyan nyan
    1) Noise is any unwanted signal present in a system. The signal-to-noise ratio is important for communication systems as it determines intelligibility. SNR is calculated as the ratio of signal power to noise power. 2) Noise sources can be external such as atmospheric, solar, or man-made interference, or internal such as white noise, pink noise, or shot noise generated within electronic components. 3) White noise power is directly proportional to temperature and bandwidth according to Johnson's formula. Shot noise power increases with bias current in transistors.

    Copyright:

    © All Rights Reserved
    Download as DOC, PDF, TXT or read online from Scribd

    Noise

    Uploaded by

    nyan nyan nyan
    68 views6 pages
    1) Noise is any unwanted signal present in a system. The signal-to-noise ratio is important for communication systems as it determines intelligibility. SNR is calculated as the ratio of signal power to noise power. 2) Noise sources can be external such as atmospheric, solar, or man-made interference, or internal such as white noise, pink noise, or shot noise generated within electronic components. 3) White noise power is directly proportional to temperature and bandwidth according to Johnson's formula. Shot noise power increases with bias current in transistors.

    Original Description:

    est

    Copyright

    © © All Rights Reserved

    Available Formats

    DOC, PDF, TXT or read online from Scribd

    Did you find this document useful?

    Is this content inappropriate?

    1) Noise is any unwanted signal present in a system. The signal-to-noise ratio is important for communication systems as it determines intelligibility. SNR is calculated as the ratio of signal power to noise power. 2) Noise sources can be external such as atmospheric, solar, or man-made interference, or internal such as white noise, pink noise, or shot noise generated within electronic components. 3) White noise power is directly proportional to temperature and bandwidth according to Johnson's formula. Shot noise power increases with bias current in transistors.

    Copyright:

    © All Rights Reserved
    Download as DOC, PDF, TXT or read online from Scribd
    Download as doc, pdf, or txt
    68 views6 pages

    Noise

    Uploaded by

    nyan nyan nyan
    1) Noise is any unwanted signal present in a system. The signal-to-noise ratio is important for communication systems as it determines intelligibility. SNR is calculated as the ratio of signal power to noise power. 2) Noise sources can be external such as atmospheric, solar, or man-made interference, or internal such as white noise, pink noise, or shot noise generated within electronic components. 3) White noise power is directly proportional to temperature and bandwidth according to Johnson's formula. Shot noise power increases with bias current in transistors.

    Copyright:

    © All Rights Reserved
    Download as DOC, PDF, TXT or read online from Scribd
    Download as doc, pdf, or txt
    of 6

    Noise Notes

    Noise - Noise is any signal present in your system that you do not want to be in the
    system.

    Signal-to-Noise Ratio
    The signal to noise ratio is the most important statistic in any communication system.
    Intelligibility is dependent upon having a good signal to noise ratio.

    SNR = 10 x log signal power = 20 x log signal voltage


    10 noise power 10 noise voltage

    Noise Sources
    External:
    Natural:
    Atmospheric ( mostly lightning ) - 1/f
    Solar
    Cosmic
    Man-Made:
    Welders
    Ignition
    60 Hz and harmonics
    Radio
    TV
    etc.
    Internal Noise Sources
    White noise - also known as Johnson or thermal noise
    Discovered by J.B. Johnson in 1928
    Caused by thermal interaction of free electrons and vibrating ions
    Random in nature
    Varies in amplitude
    Equal power throughout the spectrum
    The equation for white noise in a metal resistor is given by:
    P = kTf
    n
    e = 4kTfR
    n

    Where:
    P = noise power in watts
    n
    e = noise voltage in volts rms
    n
    -23
    k = 1.38 x 10 J/ K
    T = temperature in K
    f = bandwidth of circuit/system in Hz
    Pink noise - also known as 1/f noise
    Amplitude is 1/f
    Of little consequence except at low frequencies
    Should be infinite @ an infinite period - "big bang"
    Shot noise - also known as transistor noise
    Shot noise is caused by random movement of electrons within semiconductor devices.
    The equation for shot noise in a diode is:
    i = 2qI dc f
    n
    Where:
    -19
    q = 1.6 x 10 coulombs (C)
    I = dc current in amps
    dc
    f = bandwidth of circuit/system in Hz
    There is no exact formula for shot noise in a transistor. Shot noise generally increases
    with an increase in bias current except in MOSFET's where it is relatively independent.
    Noise Figure
    The noise figure of an amplifier is a relative indication of the ability of an amplifier to
    amplify signals without amplifying noise. It is given by the following:
    NF = 10 x log = 10 x log NR
    10 10
    NR =

    Equivalent Noise Bandwidth

    f = f
    eq 2

    Real filters do not have square bandpasses. Rather bandwidth is defined by the 3 db
    points. Noise beyond the 3 db points contributes to noise but at a reduced rate. These
    effects may be accounted for by using an equivalent bandwidth called the noise equivalent
    bandwidth. It is given by:

    Friss' Formula
    NR = NR + + ...+
    1

    Low Noise Amplifiers


    1. The formula for white noise is only true for metal resistors; other materials are
    worse.
    2. Noise effects may be minimized by cooling.
    3. Noise effects may be minimized by limiting the bandwidth of the circuit/system.
    4. Noise amplification may be minimized by impedance matching to transfer the
    maximum noise possible out of the circuit/system.
    5. Noise minimization is most important in the first stage of amplification.
    Coherent Noise
    Definition: - Coherent noise is noise that is in phase with the signal. If there is no signal,
    there is no noise.
    Amplitude Distortion: - Distortions in a signal are referred to as amplitude distortion if
    we are working in the time domain.
    Harmonic Distortion: - Distortions in a signal are referred to as harmonic distortion if
    we are working in the frequency domain.
    harmonic distortion = x 100%

    total harmonic distortion = x 100%


    Example Problem
    A three stage amplifier has a 3-db system bandwith of 200 kHz determined by an LC tuned
    circuit at it's input, and operates at 22 C. The first stage has a power gain of 14 db and a
    NF of 3 db. The second and third stages are identical, with power gains of 20 db and NF
    = 8 db. The output is 300 . The input noise is generated by a 10 k resistor. Calculate:
    a. The noise and power at the input and output of the system assuming
    noiseless amplifiers
    b. The overall noise figure for the system
    c. The actual noise voltage and power.
    (a)

    f = f = 200 kHz = 3.14 x 10 Hz


    5
    eq 2 2

    -23 5 -15
    P = kTf = 1.38 x 10 x (273 +22) x 3.14 x 10 = 1.28 x 10 W
    n
    e = 4kTfR = = 7.15 V
    n
    5
    Total power gain = 14 db + 20 db + 20db = 54 db = 2.51 x 10
    For noisless amplifiers:
    5 -15 -10
    P = 2.51 x 10 x 1.28 x 10 = 3.22 x 10 W
    noise out
    10
    3.22 x 10 x 10 W=
    e = 0.311 mV
    noise out
    (b)
    With noisy amplifiers:
    Pg = 14 db = 25.1
    1

    Pg = Pg = 20 db = 100
    2 3
    NF = 3 db NR = 2
    1 1
    NF = NF = 8 db NR = NR = 6.31
    2 3 2 3

    NR = NR + + ... +
    1
    NR = 2 + + = 2.212
    NF = 10 x log NR = 10 x log (2.212) = 3.45 db
    10 10
    (c)
    NR = = 2.212
    5
    P = = 2.51 x 10
    g
    2.212 = =
    2.212 =
    -10
    N = 7.11 x 10 W
    o
    -10
    7.11 x 10 W=
    e = 0.462 mV
    noise out

    You might also like