Page 1 of 1 MTI 7601 PAM Modulation and Demodulation Contents Aims of the Exercise Learning about the functioning principle of the pulse-amplitude modulation (sampling, time division multiplex operation) Overview of exercises Pulse-Amplitude Modulation Exercise 1: PAM modulation (sampling) The principle of the pulse-amplitude modulation for single-channel operation Display of the signals for sinusoidal and direct voltage as output signal Exercise 2: PAM demodulation The principle of the pulse-amplitude modulation for single-channel operation Display of the signal within the PAM demodulator for transmission of sinusoidal and direct voltage Exercise 3: Time division multiplex Principle of the time division multiplex operations pulse amplitude-modulated signals
Page 1 of 3 MTI 7601 PAM Modulation and Demodulation Introduction Pulse Amplitude Modulation (PAM) The pulse amplitude modulation (PAM) stage, used as a pre-selector stage to pulse code modulation (PCM), belongs to the family of pulse modulation methods. Other methods in this group, include: Pulse frequency modulation (PFM) Pulse phase modulation (PPM) Pulse duration modulation (PDM) A common factor in all methods, is that a pulsed carrier signal is modulated by an analog information signal. In the case of pulse amplitude modulation, the following block schematic diagram shows the construction of a PAM modulator: Fig. 1: PAM modulator, block schematic diagram After passing through an anti-aliasing filter, the information signal is sampled by a sequence of digital pulses (sampling), where the frequency of the sampling signal must be at least twice the value of the highest frequency information signal that may be present. In this exercise, the sampling frequency is: f Sample = 8 khz After modulation, a train of pulses is produced, the amplitudes of which correspond exactly, to the values of the input signal at the sample points.
Page 2 of 3 Fig. 2: Timing chart, PAM Demodulation of a PAM signal is achieved by a steep-edged lowpass filter. Time Division Multiplex (TDM) The techniques of time division multiplex (TDM) in communications, allow multiple use of a single transmission line (or path). There are large gaps (relatively speaking) between the modulated pulses of a PAM signal and in a TDM system these gaps are used for inserting modulated signals of other information. Synchronous multiplexing in modulator and demodulator, as well as a temporal separation between the sampling pulses of individual channels, ensure that the various channels do not interfere with one another and that the demodulation process of the signals is clean and the channels are clearly separated. In this exercise, two channels are transferred simultaneously, using time division multiplex.
Page 3 of 3 Fig. 3: Principle of TDM The standardised international system PCM 30, is used for the simultaneous transmission of 30 telephone channels.
Page 1 of 4 MTI 7601 PAM Modulation and Demodulation Exercise Assembly Insert the PAM / PCM modulator in the left hand side of the Experimenter and the PAM / PCM demodulator in right hand side. Using 2mm connection cables, connect the signal outputs of the PAM modulator to the inputs of the PAM demodulator: CLCK - CLCK, SYNC. - SYNC., PAMout - PAMin. Before switching on the power supply, set all the following switches and potentiometers in their initial position: Experimenter card Element Position SO4201-7R Gain potentiometer, channel 1 Fully CCW SO4201-7R Gain potentiometer, channel 2 Fully CCW SO4201-7R Compression mode selector A-Law linear SO4201-7R Channel selector Channel 1 SO4201-7T Expansion mode selector A-Law linear SO4201-7T Channel selector Channel 1 When the power supply of the UniTr@in-I Interface is switched on, all necessary voltage supplies are available and the exercise can commence.
Page 1 of 4 MTI 7601 PAM Modulation and Demodulation Principle of Pulse Amplitude Modulation in Single-channel Operation Exercise 1: Displaying the signal waveforms, with a sinusoidal input signal On the PAM-/PCM modulator Experimenter card, connect the "LF1" input to the "SIN 1kHz" output. Set the gain for channel 1 to the value 1 (fully counter-clockwise). Connect input "LF2" with a short 2mm connection cable to the "AGND" socket located above this input socket. On channel A of the oscilloscope, measure the output voltage at the output of the 4kHz lowpass filter "U1.1" and on channel B, measure the signal at the outputs of the Sample and Hold elements "U1.2" and "PAM-out". Synchronise on channel A. Using the gain control for channel 1 of the PCM transmission path, adjust the amplitude of the sinewave signal at the output of the lowpass filter, to approximately 3-4 Vpp. For evaluation purposes, display the signals on the oscilloscope on top of each other. How is a pulse amplitude modulated signal generated from an analog signal What Sample frequency is used in the exercise here Comment on the results obtained. Note: For determining the Sample frequency, use the time marker of the oscilloscope! In the lower part of the operating bar of the oscilloscope, the button will be seen for the cursor function. Set this for channel A. Also, two amplitude markers are available for measuring voltages and two time markers for measuring time or frequency. The marker can be moved with the mouse to the position required. The values detected are shown at the top right. Results: Chan. A= 1 V/DIV DC Chan. B= 1 V/DIV DC
Page 2 of 4 Fig.: 1: Signal at the output of the lowpass filter and at the output of the Sample and Hold element Chan. A= 1 V/DIV DC Chan. B= 1 V/DIV DC Fig.: 2: Signal at the output of the lowpass filter and at the output PAMout
Page 3 of 4 Exercise 2: Displaying the signal waveforms, with a DC voltage as input signal NOTE: The possibility of transmitting DC voltages is purely a teaching facility incorporated in the training system used here. In practice, CODEC's incorporate a DC suppression system (f > 200 Hz). On the PAM-/PCM modulator Experimenter card, connect the "LF1" input to the output of the internal DC source, "DC +5V/-5V". Adjust the DC source voltage potentiometer for an output of +3.0V (check with a multimeter). Set the gain for channel 1 to the value 1 (fully counter-clockwise). On channel A of the oscilloscope, measure the output voltages at the output of the 4kHz lowpass filter, at the output of the Sample and Hold element and at the PAM-out socket. On channel B of the oscilloscope, display the synchronisation pulse at the "SYNC" output. Trigger on B. For more stable triggering, set the trigger point to 50% of the amplitude of the pulse. Comment on the results obtained. Results: Chan. A= 2 V/DIV DC Chan. B= 2 V/DIV DC Fig. 3: Signal at the output of the lowpass filter Chan. A= 2 V/DIV DC Chan. B= 2 V/DIV DC
Page 4 of 4 Fig. 4: Signal at the output of the Sample and Hold element Chan. A= 2 V/DIV DC Chan. B= 2 V/DIV DC Fig. 5: Signal at the output PAMout
Page 1 of 4 MTI 7601 PAM Modulation and Demodulation Principle of PAM Demodulation in Single-channel Operation Exercise 1 Displaying the signal waveforms within the PAM demodulator with a sinusoidal signal (applied to the input of the PAM modulator) Ensure that all initial settings as given in the exercise assembly notes, have been completed. On the PAM-/PCM modulator Experimenter card, connect the "LF1" input to the "SIN 1kHz" output. Set the gain for channel 1 to the value 1 (fully counter-clockwise). Connect input "LF2" with a short 2mm connection cable to the "AGND" socket located above this input socket. On channel A of the oscilloscope, measure the output voltage at the output of the 4kHz lowpass filter "U1.1" on the PAM / PCM modulator, and on channel B, measure the signals at the outputs "PAM-out", the Sample and Hold element "Hold1" and "LF1"of the PAM / PCM demodulator. Synchronise on channel A. Using the gain control for channel 1 of the PCM transmission path, adjust the amplitude of the sinewave signal at the output of the lowpass filter, to approximately 3-4 Vpp. Comment on the results obtained. Results: Chan. A= 1 V/DIV DC Chan. B= 1 V/DIV DC Fig. 1: PAM input signal at the demodulator
Page 2 of 4 Chan. A= 1 V/DIV DC Chan. B= 1 V/DIV DC Fig. 2: Demultiplexed and Hold signal before filtering Chan. A= 1 V/DIV DC Chan. B= 1 V/DIV DC Fig. 3: Filtered output signal of channel 1
Page 3 of 4 Exercise 2: Displaying the signal waveforms within the PAM demodulator with a DC voltage (applied to the input of the PAM modulator) On the PAM-/PCM modulator Experimenter card, connect the "LF1" input to the output of the internal DC source, "DC +5V/-5V". Adjust the DC source voltage potentiometer for an output of +3.0V (check with a multimeter). Set the gain for channel 1 to the value 1 (fully counter-clockwise). On channel A of the oscilloscope, measure the signals at the outputs "PAM-out", the Sample and Hold element "Hold1" and "LF1" of the PAM / PCM demodulator. On channel B of the oscilloscope, display the synchronisation pulse at the "SYNC" output. Trigger on B. For more stable triggering, set the trigger point to 50% of the amplitude of the pulse. Comment on the results obtained. Results: Chan. A= 2 V/DIV DC Chan. B= 2 V/DIV DC Fig. 4: PAM input signal at the demodulator Chan. A= 2 V/DIV DC Chan. B= 2 V/DIV DC
Page 4 of 4 Fig. 5: Demultiplexed and Hold signal before the filtering Chan. A= 2 V/DIV DC Chan. B= 2 V/DIV DC Fig. 6: Filtered output signal of channel 1
Page 1 of 4 MTI 7601 PAM Modulation and Demodulation Principle of Time Division Multiplex of PAM Signals Exercise: Displaying the signal waveforms of PAM modulated signals in time division multiplex (TDM) On the PAM-/PCM modulator Experimenter card, connect the "LF1" input to the "SIN 1kHz" output and the input "NF2" to the "SIN 500Hz" output. Set the gain for channels 1 and 2 to the value 1. On channel A of the oscilloscope, measure the output voltages at the outputs "U1.1" and "U1.2", and on channel B, measure the signals at the outputs "U2.1" and "U2.2".. Synchronise on channel B. Using the gain controls for channels 1 and 2 of the PCM transmission path, adjust the amplitude of the sinewave signal at the output of the lowpass filter, to approximately 3-4 Vpp. Now, measure the multiplexed signal at the PAM output on channel A of the oscilloscope. Channel B remains connected to the output of the lowpass filter, "U2.2". How is the sampling made on the two channels Describe the multiplexed PAM output signal! On the PAM / PCM demodulator (SO4201-7T), using the oscilloscope, measure the demultiplexed and Hold signal before the filter stages at test points "Hold 1" and "Hold 2", and the filtered output signals of channel 1 at LF1out and channel 2 at LF2out. Important: Remember that the channel selector for the LED on the PAM / PCM demodulator SO4201-7T, must be set for both channels! In addition to the function of indicating the PCM code on the LED, this selector also switches both channels for receiving a PCM transmission. How are the channels separated Vary the amplification of the input signals. Does this introduce any interference between the two channels (e.g. cross-coupling of the signal in each channel) Comment on the results. Results: X = 200 µs/div X/T (B) Chan. A= 1 V/DIV DC Chan. B= 1 V/DIV DC