## Pulse Amplitude Modulation Circuit Diagram

Other larger groups can be formed, for example 6 mastergroups may be combined to form a jumbogroup with 3600 voice channels.

To recover the original baseband signals from the various groups, the appropriate number of filtering/demodulation processes will have to be carried out. At each stage of the demodulation process, the correct carrier will have to be reinstated for this to be possible.

9.3 TIME-DIVISION MULTIPLEX (TDM)

In FDM, voice signals were ''stacked'' in the frequency spectrum so that many such signals could be transmitted over the same channel without interference. In timedivision multiplex (TDM), each voice signal is assigned the use of the complete channel for a very short time on a periodic basis. The theoretical basis of this technique is the Sampling Theorem. An informal statement of the sampling theorem is:

If the highest frequency in a signal is B Hz, then the signal can be reconstructed from samples taken at a minimum rate of 2B samples per second (Nyquist sampling rate or frequency).

The proof of this theorem is beyond the scope of this book. However, there are a number of practical problems which arise in the application of the theorem:

(1) The theorem assumes that the samples have infinitesimally narrow pulse widths. This is clearly not so in a practical circuit. The sampling rate is usually chosen to be higher than the Nyquist frequency since it is the minimum; it is discrete to avoid extreme conditions when dealing with an imperfect situation.

(2) The theorem assumes that an ideal low-pass filter is used to remove all frequencies above B Hz ahead of the sampler. When using a practical filter, it is necessary to sample the signal at a higher rate (oversampling) to avoid distortion due to aliasing.

A TDM system with two input signals is illustrated in Figure 9.5. The samplers or commutators are shown here as switches which are driven in synchronism.

The TDM system shown in Figure 9.5 is an example of pulse amplitude modulation (PAM). Practical TDM systems based on PAM have been built and used in the telephone system (No. 101 ESS-PBX) [8].

### 9.3.1 Pseudodigital Modulation

To code an analog signal in pulse form one can use the height of the pulse, the width (or duration), or the position of the pulse relative to standard position. When the height is used, it is called pulse-amplitude modulation (PAM). When the coding is in terms of the width it is called pulse-width modulation (PWM) and when the position is used it is called pulse-position modulation (PPM). Pulse height, width, and position are analog quantities which in turn can be quantized and represented by a binary code where the digits are present, 1, or absent, 0. When this has been done the modulation scheme is called pulse-code modulation (PCM). Although PCM is qualitatively different from the other modulating schemes, they are compared in Figure 9.6.

These schemes would work equally well in a noiseless environment. When noise is present, and it always is, PCM has a clear advantage over the others. In the case of PAM, PWM and PPM the receiver has to determine what the original amplitude, width, and position were respectively in order to reconstruct them. In PCM, the decision is simplified to whether the digit sent was a 1 or a 0. In all cases, it is necessary to transmit timing information with the signal so that the receiver knows where the bit stream starts and stops.

### 9.3.2 Pulse-Amplitude Modulation Encoder

To illustrate the design principle of a PAM communication channel, a four-channel PAM system has been chosen. The coder or commutator is shown in Figure 9.7.

The master clock drives the four-phase ring counter. The ring counter drives four sampling gates on and off in the correct sequence. When one of the four outputs is on, 1, all the others are off, 0, so only the sampling gate with the 1 is connected to the adder. Note that the second input to the fourth sampling gate are connected to the master clock. This means that channel 4 will always produce a positive pulse. The amplitude of this pulse is adjusted to be higher than the most positive value of the analog input voltage. This is called the synchronization pulse or sync pulse for short. It is used to identify and time the other channels. The design of the component circuits now follows.

9.3.2.1 Four-Phase Ring Counter. The four-phase ring counter and its timing diagram are shown in Figure 9.8.

Commutators

Figure 9.5. A mechanical illustration of time-division multiplex (TDM) with pulse amplitude modulation (PAM). Reprinted with permission from B. P. Lathi, Modern Digital and Analog Communication Systems, CBS College Pub., New York, 1983.

Figure 9.6. A comparison of PAM, PWM, PPM and PCM. Note that PAM, PWM and PPM are not truly digital since they convey information by the variation of analog quantities, that is, amplitude, duration and position in time. Reprinted with permission from B. P. Lathi, Modern Digital and Analog Communication Systems, CBS College Pub., New York, 1983.

Figure 9.6. A comparison of PAM, PWM, PPM and PCM. Note that PAM, PWM and PPM are not truly digital since they convey information by the variation of analog quantities, that is, amplitude, duration and position in time. Reprinted with permission from B. P. Lathi, Modern Digital and Analog Communication Systems, CBS College Pub., New York, 1983.

Figure 9.7. A block diagram for a four-channel PAM system. Note that channel 4 is used for timing purposes.

Sync.

Channels

Figure 9.7. A block diagram for a four-channel PAM system. Note that channel 4 is used for timing purposes.

Figure 9.8. A block diagram of a four-phase ring counter with its timing diagram.

It can be seen from the diagram that, in the time taken by one frame, the output pulses go through one cycle. The outputs are used to drive the sampling gates.

9.3.2.2 Series Sampling Gate. The configuration of the series sampling gate is shown in Figure 9.9.

Figure 9.9. The series sampling gate using a FET.

The transistor is an open-circuit when the control signal is a 0 and a short-circuit when it is a 1. The output is as shown.

9.3.2.3 Shunt Sampling Gate. The shunt sampling gate is shown in Figure 9.10.

The transistor acts as a switch and short-circuits the output when the gate voltage is a 1. When the gate voltage is a 0, it is an open-circuit and a path exists between the input and the output.

9.3.2.4 Series-Shunt Sampling Gate. The two circuits shown above have an inherent deficiency because, when the transistor is on, its source-to-drain impedance is low but not equal to zero. To improve the performance, the action of the two gates can be combined as shown in Figure 9.11.

Figure 9.10. The shunt sampling gate.

Input

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### Responses

• Eden Kiros
How is PCM different from PAM, PWM, and PPM?
6 years ago
• yorda
Why transistor acts as a sampler in PAM circuit?
6 years ago
• lily
What is pam in practical circuits?
6 years ago
• courtney
Which gate or filter is used for sampling in tdm?
3 years ago
• Zachary MacDonald
How transistors work in a PAM circuit?
3 years ago
• elizabeth
What is pam with diagram in analog communication?
3 years ago
• AARON
Who make a create pulse amplitude modulation circuit?
2 years ago
• Tuukka
What is the role of transistor in a pulse amplitude modulation?
2 years ago