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    Al-Balqa' Applied University Faculty of Engineering Technology

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    M Odeh

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    Al-Balqa' Applied University Faculty of Engineering Technology

    Uploaded by

    M Odeh
    3 views6 pages

    Original Title

    PCM (1)

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    3 views6 pages

    Al-Balqa' Applied University Faculty of Engineering Technology

    Uploaded by

    M Odeh

    Copyright:

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

    Al-Balqa' Applied University

    Faculty of Engineering Technology

    Laboratory Communication Lab


    Experiement PCM
    Name Ahmad Imad Hasan Issa
    Reg Number 31815020024
    Instructor Dr.Aws Al-Qaisy
    Lab Time Wed 2-5
    Date 25/5/2021

    Al-Balqa' Applied University


    ** Objectives :
    To examine the operation of the linear 8-bit PCM coder
    and decoder.
    To plot the quantization curve and check the coding law.
    To examine the eye diagram.
    To check the voice transmission quality at variation of the
    channel characteristics.
    ** Material :
    Power unit PSU or PS1; Module holder base.
    Individual Control Unit SIS1, SIS2 or SIS 3 (or switches S).
    Experiment module MCM30.
    Oscilloscope.
    ** Theoretical Notions :
    Coder
    The block diagram of the linear PCM communication system
    is shown in fig.993.1.
    The input analog signal (fig.993.2a) crosses a 3.4kHz low pass
    filter (anti-aliasing filter) and reaches the sampler (Sample & Hold).
    The sampling frequency (TX Frame Sync) is 8 kHz. The sampled
    signal (fig.993.2c) is applied to an A/D converter, carrying out an 8-
    coding. The parallel output of the A/D is transformed into serial by
    the next Parallel-to-Serial converter. Considering the sampling
    frequency fs and the number of N bit per sample, the transmission
    rate of the bits of the PCM is equal to:
    v = N_fs
    With sampling frequency of 8 kHz and 8 bit/sample there is a
    rate of 64 kbit/s (fig.993.2d).
    Transmission channel and decoder
    The PCM signal from the Parallel-to-Serial converter is
    filtered by TX FILTER and then transmitted. It crosses the artificial
    line and reaches the receiving section, where:
    it is filtered by the RX FILTER.
    it is sampled at the maximum amplitude point of each
    pulse. To point out the effect of this circuit, the reception
    clock phase can changed with the Phase Adj.

    The samples are applied to a threshold circuit (decision


    value), supplying the output with the PCM NRZ signal
    reconstructed in reception.
    The unit (" TX Filter " + " Channel " + " RX Filter ")
    constitutes the transmission channel, and the quality of the received
    pulses depends on the frequency response (inter symbol interference
    and eye diagram).
    ** Quantization and serial PCM flow:
    Power the module
    Set the circuit to linear PCM mode and connect a 40-kHz line
    (SW5=Lin, J1=40, J2=d, fig.993.3).
    Connect TP28 (DC OUT) a TP3O. Connect the oscilloscope in
    DC (or a voltmeter) to TP3O
    t
    t
    t
    t
    t
    t
    Analog signal
    Sampling Pulses
    Sampled Signal
    Sampled Signal
    Serial PCM Signal
    D/A receiver signal

    Change the DC OUT potentiometer an see how the lighting of


    the led set on the parallel output of the A/D converter changes.
    Consider the input voltage jump necessary to change of 1 bit
    the converter output (the Led 1 is the less significant bit). The
    measurement is difficult to carry out as the difference between
    the adjacent quantization levels is very low
    Q1 : What is the amplitude of the quantization levels?
    Ans. about 8 mV.
    Synchronize the oscilloscope to the frame synchronism (TX
    Frame Sync, TP35), and examine the serial PCM signal in
    TP37.
    Vary the potentiometer DC OUT and see how the wave-form
    of the serial PCM signal changes.
    See that each bit is represented in NRZ format, i.e. with a
    positive (bit 1) or null (bit 0) voltage value with duration equal
    to the bit clock period (TP36).
    See that each sample is converted into a set of serial bits,
    which are allocated between two next frame synchronism
    pulses.
    Q2 : Which is the duration of the T frame? Which is the duration of
    the bit (bit interval) TBIT ? How many bits are included within next
    frame synchronisms?
    Ans. T = l25μs ; TBIT = 15.625μs ; 8bit .
    Q3 : What is the rate (signaling rate) of the PCM signal examined in
    TP37 ?
    Ans. 64 kbit/s (equal to the inverse of the bit interval). This is the
    typical rate of single PCM telephone channel, with sampling at 8
    kHz and 8-bit conversion.
    ** Wave-forms of the coder :
    Keep the last setting (fig.993.3)
    Connect l-kHz l-Vp-p to the analog input of the modulator
    (connet TP24 to TP30 and adjust the signal level to lVp-p).
    Synchronize the oscilloscope to the input analog signal
    (TP30), and examine :
    - TP33: pulses for the sampling of the analog signal.
    - TP34: step signal supplied by the Sample & Hold.
    Synchronize the oscilloscope to the frame synchronism pulse
    (TP35), and examine:
    - TP37: serial PCM signal, in NRZ format (bit1 = 5V,
    bit0 = 0 V).
    - TP36: bit clock, which period determines the bit
    duration of the serial PCM signal.
    - see that between two next synchronism pulses there are
    8 bits changing in continuous.
    ** Line circuits and decoder :
    Line circuits
    Keep the last setting (fig.993.3). Set Attenuation and Noise to
    the minimum. Connect TP44 to EXT IN.
    Connect l-kHz l-Vp-p to the analog input of the modulator
    (connect TP24 to TP30 and adjust the signal level to lVp-p).
    Synchronize the oscilloscope to the frame synchronism pulses
    (TP35), and examine the wave-forms of the PCM signal
    through the communication channel:
    - TP37: serial PCM signal, in NRZ format (bit1 = 5V,
    bit0 = 0V).
    - TP38: output of the transmission filter. The PCM signal
    is distorted, by effect of the filter
    - TP39: line output. The PCM signal is attenuated and
    further distorted.
    - TP40: output of the reception filter. The PCM pulses are
    shaped to get a proper eye diagram.
    Eye diagram
    to examine the eye diagram, synchronize the oscilloscope on
    the bit clock (TP36), set the time base on 5μs/div and examine
    the wave form in TP40. You get the eye diagram, similar to
    the one of fig.993.4.
    Decision element
    The PCM pulses (after the shaping) are sampled at their
    maximum value (at the eye center), and a next threshold
    circuit gives the sampled value a high (bit 1) or low value (bit
    0).

    Synchronize the oscilloscope on the reception bit synchronism


    (TP41), and examine the signals related to the reception
    sampling and to the decision element:
    - TP40: shaped PCM signal.
    - TP42: output of the reception sampler.
    - TP43: output of the decision element. It takes two levels:
    high (about +4V) if the sampler output is higher than
    0V, low (0V) if the output of the sampler is inferior to
    0V.
    Q4 : Which is the effect of the Phase Adjust potentiometer?
    Ans. to optimize the phase equalization of the received PCM signal.
    In TP44 there is a step wave-form (supplied by the D/A
    converter) which approximates the starting analog signal
    (TP30). Act on Phase Adjust to obtain the best wave-form.
    Examine the wave-form of the signal at the reception filter
    output (TP21), and see the correspondence with the
    transmitted analog signal (TP1). Adjust LEVEL F1 to get equal
    amplitudes.
    Q5 : The received signal (TP21) is absent much distorted. What is
    the reason ?
    Ans. the sampling and reception pulses (TP41) are not in phase with
    the center of the received PCM pulses (TP40).
    Keep the last setting (fig.993.3). Set Attenuation and Noise to
    the minimum Connect TP44 to EXT IN.
    Connect 1kHz 1-Vp-p to the analog input of the modulator
    (connect TP24 to TP30 and adjust the signal level to l Vp-p).
    Examine the eye diagram in TP40 (synchronize the
    oscilloscope or the bit clock TP36, and set the time base on
    5μs/div).
    Increase the noise and the line attenuation. Observe the
    gradual closing of the eye (fig.993.5). Note that the received
    signal (TP21) gets worse. See that, with the same noise and
    line attenuation, the received signal quality will be better if the
    PCM pulses are sampled at the eye center (change Phase
    Adjust) lower the band pass of the line (connect J1=20 kHz)
    and see the eye diagram gets much worse (fig.993.6).

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