JNTU B.Tech III ELECTRONIC INSTRUMENTATION Previous Question Paper (August - 2022 - R13)

Gross errors are primarily caused by human mistakes in reading, recording, or calculating data. To minimize these errors during steady-state measurements, the following precautions should be taken:

• Proper Instrument Selection: Ensure the instrument is appropriate for the range and type of measurement to avoid parallax errors or improper scaling.
• Parallax Error Prevention: Always view the pointer or digital display directly from the front. Many high-precision analog meters include a mirror strip to align the eye correctly.
• Systematic Recording: Maintain a logbook to record data immediately after observation to prevent memory-related errors. Cross-check readings with a second observer if possible.
• Calibration Checks: Periodically calibrate instruments against a known standard to ensure the steady-state value is not drifting due to aging components.
• Environmental Control: Ensure the measurement environment is stable, as temperature fluctuations or electromagnetic interference can introduce transient errors that appear as steady-state inaccuracies.
• Proper Connection: Verify that all leads and terminals are secure and correctly connected to prevent contact resistance or ground loops.

Loop gain is a critical parameter in feedback systems, representing the product of the forward path gain and the feedback path gain. It is measured to determine system stability and performance.

Case 1: Measurement in a Closed-Loop Feedback Amplifier

To measure the loop gain of an operational amplifier or feedback circuit, the feedback loop is physically opened at a point where the impedance looking into the feedback network is high and the impedance looking into the forward amplifier is low. A test signal is injected at the input of the forward path, and the return signal is measured at the output of the feedback path. The loop gain is defined as:

Case 2: Measurement using Bode Plot Analysis (Stability Margin)

In practical control systems, the loop gain is often determined by measuring the frequency response. By sweeping a sinusoidal signal across the system's bandwidth, the magnitude and phase shift of the open-loop transfer function are recorded. The loop gain is identified at the crossover frequency where the magnitude is 0 dB (unity gain). The phase margin is then calculated as the difference between -180° and the phase at this crossover frequency, providing a direct measure of the system's stability margin.