JNTU B.Tech IV TELEMETRY AND TELECONTROL Previous Question Paper (July/August - 2022 - R18)

A voltage telemetry system transmits information by varying the magnitude of a DC or AC voltage signal proportional to the measured physical parameter. The system consists of a sensor, a signal conditioning unit (amplifier/attenuator), a transmission medium (wires), and a receiving indicator or recorder.

The sensor converts the physical quantity into a voltage. The signal conditioner ensures the voltage level is compatible with the transmission line. The receiver detects the voltage level and displays the corresponding physical value.

Advantages:

• Simple and inexpensive to implement.
• Easy to calibrate and troubleshoot using standard voltmeters.

Disadvantages:

• Highly susceptible to noise and electromagnetic interference (EMI).
• Voltage drops occur over long distances due to line resistance, leading to signal attenuation and inaccuracy.

Line and channel coding are essential for reliable data transmission in telemetry systems to ensure synchronization and error detection.

Line Coding: This refers to the process of converting binary data into digital signals (voltage levels) for transmission over a physical medium. Common codes include:

• Unipolar NRZ: Uses a positive voltage for logic 1 and zero for logic 0.
• Polar NRZ: Uses positive voltage for 1 and negative for 0, providing better noise immunity.
• Manchester Coding: Represents each bit by a transition in the middle of the bit period, ensuring self-clocking and eliminating DC bias.

Channel Coding: This involves adding redundant bits to the data stream to detect or correct errors caused by channel noise. Key techniques include:

• Parity Bits: A simple method where an extra bit is added to make the total number of 1s even or odd, allowing for single-bit error detection.
• Hamming Code: A block code that adds multiple parity bits to detect and correct single-bit errors.
• Cyclic Redundancy Check (CRC): Uses polynomial division to generate a checksum, highly effective for detecting burst errors in telemetry channels.