CH-4: PULSE COMMUNICATION

CH-4: PULSE COMMUNICATION

You have undoubtedly drawn graphs of continuous curves many times during your education. To do that, you took data at some finite number of discrete points, plotted each point, and then drew the curve. Drawing the curve may have resulted in a very accurate replica of the desired function even though you did not look at every possible point. in effect, you took samples and guessed where the curve went in between the samples. If the samples had sufficiently close spacing, the result is adequately described. It is possible to app0ly this line of thought to the transmission of an electrical signal, that is, to transmit only the samples and let the receiver reconstruct the total signal with a high degree of accuracy. This is termed pulse modulation.

It will be shown later that a signal sampled at twice the rate of its highest significant frequency component can be fully reconstructed at the receiver to a high degree of accuracy. Stated inversely, a given bandwidth can carry pulse signals of half its high frequency cut-off. This is known as the Nyquist rate. In the case of voice transmission the standard sampling rate is 8 KHz, it being just slightly more than twice the highest significant frequency component. This implies a pulse rate of 8 K Hz or 125 micro second period. Since a pulse duration of 1 microsec. may be adequate, it is easy to see that a number of different message could be multiplexed (TDM) on the channel, or alternatively it would allow a high peak transmitted power with a much lower (1/125) average power. The high peak power can provide a very high signal to noise ratio or a greater transmission range.

It should be careful to realize that a price must be paid for system gains obtained by pulse modulation schemes. More important than the greater equipment complexity is the requirement for greater channel (bandwidth) size. If a maximum 3 KHz signal directly amplitude modulates a carrier, a 6 KHz bandwidth is required. If a 1 microsecond pulse does the modulating, just allowing its fundamental component of 1/1 micro second or 1 MHz to do the modulating means a 2 M Hz bandwidth is required in AM. In spite of the large bandwidth required, TDM is still preferable (if not the only possible way) to using 100 different transmitters, antennas or transmission lines, and receivers in cases where large numbers of messages must be conveyed simultaneously.

In its strictest sense, pulse modulation is not modulation but rather a message processing technique. The message to be transmitted is sampled by the pulse, and the pulse is subsequently used to either amplitude or frequency modulate the carrier. The three basic forms of pulse modulation are illustrated in Fig. 1. The three types we shall consider here are usually termed pulse-amplitude modulation (PAM), pulse width modulation(PWM), and pulse position modulation(PPM). For the sake of clarity, the illustration of these modulation schemes has greatly exaggerated the pulse widths. Since a major application of pulse modulation occurs when TDM is to be used, shorter pulse durations, leaving room for more multiplexed signals, are obviously desirable. As shown in Fig. 1, the pulse parameter that is varied in step with the analog signal is varied in direct step with the signal's value at each sampling interval. notice that the pulse amplitude in PAM and pulse width in PWM are not zero when the signal is minimum. This is done to allow a constant pulse rate and is important in maintaining synchronization in TDM systems.

Fig. 1: Types of pulse modulation

Pulse amplitude Modulation:In pulse-amplitude modulation, the pulse amplitude is made proportional to the modulating signal's amplitude. This is the simplest pulse modulation to create in that a simple sampling of the modulating signal at a periodic rate can be used to generate the pulses, which are subsequently used to modulate a high-frequency carrier. An eight-channel TDM PAM system is illustrated in Fig. 2

Fig. 2: Eight channel TDM PAM system.

At the transmitter, the eight signals to be transmitted are periodically sampled. The sampler illustrated is a rotating machine making periodic brush contact with each signal. A similar rotating machine at the receiver is used to distribute the eight separate signals, and it must be synchronized to the transmitter. A mechanical sampling system such as this may be suitable for low sampling rates such as encountered in some telemetry systems but would not be adequate for the 8 KHz rate required for voice transissions. In that case an electronic switching system would be incorporated.

At the transmitter, the variable amplitude pulses are used to frequency modulate a carrier. A rather standard FM receiver recreates the pulses, which are then applied to the electromechanical distributor going to the eight individual channels. The pulses applied to each line go into an enveloped detector that serves to recreate the original signal. This can be a simple low-pass RC filter such as is used following the detection diode in a standard AM receiver.

While PAM finds some use due to its simplicity, PWM and PPM use constant amplitude pulses and provide superior noise performance. The PWM and PPM systems fall into a general category termed pulse time modulation (PTM) since their timing, and not amplitude , is the varied parameter.

There are two basic sampling techniques used to create a PAM signal. The first is called natural sampling. Natural sampling is when the tops of the sampled waveform (the sampled analog input signal) retain their natural shape. An example of natural sampling is shown in Fig. 3. (a). Notice that one side of the analog switch is connected to ground. When the gate is asserted the JFET will short the signal to ground, but it will pass the unaltered signal to the output when the gate is not asserted. Note too that there is ano hold capacitor present in the circuit.

Fig. 3: Natural sampling; (b) flat-top sampling

Probably the most popular type of sampling used in PAM system is called flat-top sampling. In flat-top sampling the sample signal voltage is held constant between samples. The method of sampling creates a staircase that tracks the changing input signal. This method is popular because it provides a constant voltage during a window of time for the binary conversion of the input signal to be completed. An example of flat-top sampling is shown in Fig. 9-4(b). With flat-top sampling the analog switch connects the input signal to the hold capacitor.

The Sample Frequency:

One of the most critical specifications in a pulse modulated or digital modulated (PCM) system is the selection of the sample frequency. The sample frequency is governed by the Nyquist rate. The Nyquist rate states that the sample frequency (fs) must be at least twice the highest input frequency (fa)

If the sampling criterion is not met, the original analog signal frequency is lost and an alias frequency is produced instead. The frequency of the alias signal is given by

This is shown in Fig. 4, where the sampling rate is 2/3 times the 1 KHz signal rate. The output signal in fig. 3 is at 1000 - 667 Hz , or 333 Hz, and will bear no resemblance to the original signal.

Fig. 4: Generation of alias frequency.

A clock signal (at the sampling rate) is necessary so that the digital-to-analog converter (DAC) knows when all the input bits are valid as a group. Erroneous outputs would otherwise occur since small in the physical length or momentary shift (jitter) caused by circuitry action cause bits to arrive at different times.

Pulse width modulation:

Pule-width modulation (PWM), a form of PTM, is also known as pulse-duration modulation (PDM) and pulse-length modulation (PLM). A simple means of PWM generation is provided in Fig.5

Fig. 4: PWM generation waveforms

Pulse-Position Modulation

PWM and pulse - position modulation (PPM) are very similar, a fact that is underscored in Fig. 5, which shows PPM being generated from PWM .

Fig. 6: PPM generation

Since PPM has superior noise characteristics, it turns out that the major use for PWM is to generate PPM. By inverting the PWM pulses in fig.6 and then differentiating them, the positive and negative spikes shown are created. By applying them to a Schmitt trigger sensitive to only positive levels, a constant amplitude and constant pulse width signal if formed. however, the position of these pulses is variable and now proportional to the original modulating signal, and the desired PPM signal has been generated. The information content is not contained in either the pulse amplitude or width as in PAM and PWM, which means the signal now has a greater resistance to any error caused by noise. In addition, when PPM modulation is used to amplitude-modulate a carrier, a power savings results since the pulse width can be made very small.

At the receiver, the detected PPM pulses are usually converted to PWM first and then converted to the original analog signal by integrating as previously described. Conversion from PPM to PWM can be accomplished by feeding the PPM signal into the base of one transistor in a flip-flop. The other base is fed from synchronizing pulses at the original (transmitter) sampling rate. The period of time that the PPM-fed transistor's collector is low depends on the difference in the two inputs, and it is therefore the desired PWM signal.

Pulse-code Modulation:

Pulse-code modulation (PCM) is the most common technique used today in digital communications for representing an analog signal by a digital word. PCM is used in many applications, such as your telephone system, digital audio recording (DAT or digital audio tape), CD laser disks, digitized video special effects, voice mail, and many other applications. PCM techniques and applications are a primary building block for many of today's advanced communications systems.

Our discussion of PCM systems begins with a look at the analog-to-digital conversion process- ADC or A/D converter. A simple example of the A/D process is to think in terms of our voice and the mechanism required to convert it into a digital data format suitable for inputting to a computer. This requires that the analog signal, which is a continuous-time signal, be converted into a series of quantized values that then represent the original analog signal in digital form. This digitized signal can then be digitally processed by the interface circuitry to the computer or by the computer itself. the data can then be held in the computer until accessed by the DAC, the ditital to analog converter.

Pulse-code modulation is a technique for converting the analog signals into a digital representation. The PCM architecture consists of a sample-and hold (S/H) circuit and a system for converting the sampled signal into a representative binary format. First, the analog signal is input into a sample and hold circuit. At fixed time intervals, the analog signal is sampled and held at a fixed voltage level until the circuitry inside the A/D converter has time to complete the conversion process of generating a binary value. A block diagram of the process is shown in fig. 7

Fig. 7: Block diagram of PCM process

As we have seen in the block diagram of the PCM circuit (Fig.7), the analog to digital converter (ADC) is used to convert the information signal to digital format. This process is known as dgitizing. We will see later how ADC and digital to analog converters (DAC) are used in coding and decoding. A block diagram of a PCM system (transmitter and receiver) is shown in fig. 8 The ADC is shown in the transmitting section and the DAC in the receiver section.

Fig. 8: PCM communication system

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