Why is a delay line used in an oscilloscope?

The correct answer is: 4) To synchronise the input signal with the electron beam

Explanation:

A delay line in an oscilloscope temporarily holds the input signal to ensure it reaches the vertical deflection plates at the same time as the sweep generator (horizontal beam) triggers. This synchronization allows the waveform to be displayed stably and accurately from the start of the sweep.

The calibration signal usually available from a Cathode Ray Oscilloscope (CRO) is typically a square wave voltage.

This signal helps in adjusting and verifying the time base and amplitude settings of the oscilloscope.

The calibration signal available from CRO is a saw-tooth waveform. It is applied at vertical plates whereas input is applied at horizontal plates.

Option Analysis

1. Reduce noise → Delay lines don’t filter noise; they introduce a fixed time delay.

2. Enhance resolution → Resolution depends on ADC and display, not the delay line.

3. Increase amplitude → Delay lines don’t amplify signals; they preserve their shape.

Block diagram of CRO

1. The Vertical Amplifier: It block amplifies the weak signals so that they produce measurable deflection on the screen; this amplifier decides the sensitivity and bandwidth of the CRO. Its output is given as input to the delay line.
2. Horizontal amplifier: The last block is the strength of a sawtooth signal available at the output of a time-based generator is not sufficient thus before applying it to the horizontal plates the signal is amplified using a horizontal amplifier.
3. Trigger Circuit: The third block is a trigger circuit. This circuit generates trigger pulses that keep the synchronization between the input signal and the horizontal deflection circuit.
4. The time-based generator generates sawtooth waveforms and applies them between the horizontal deflection plates as the Sawtooth wave varies linearly with time and takes place at a constant velocity hence x-axis of the CRO can be calibrated in terms of time and input can be displayed with respect to time.
5. Delay line: The input signal is applied to the horizontal as well as the vertical deflection plates but before reaching the horizontal deflection plates. The signal passes through different blocks such as trigger circuit time-based generator and the horizontal amplifier thus a small delay occurs and the signal reaches the vertical deflection plates before the horizontal plates but this causes the distortion of a signal on the screen to avoid this situation a small amount of delay is added using the delay line block after the vertical amplifier.