AC MEASUREMENTS AND OSCILOSCOPES

Question 1

What is Auto (trigger) mode and how does it work?

Answer 1

sweep signal is generated even when there is no triggering signal or trigger condition is not met. In this way, the screen will display a trace (when there is no signal) or the signal (but may not be synchronised).

Question 2

Flow diagram of how a digital scope works:

Answer 2

Input -> Amplify -> Digitise -> Store/Process -> Display

Question 3

Advantage of digital scope over analogue ones

Answer 3

• Digital scope has made capturing non-periodic signals much easier as it has separate signal acquisition and display units. Captured signals can be repeated sent to display unit for a stable display.
• Another advantage of digital scopes is that they offer automatic measurements of signal parameters such as peak voltage, peak-to-peak voltage, rms voltage, frequency, delay or pahse difference (of two waves), etc, which can be shown on the screen at press of a button. This is referred to as the Quick Measurement or Auto Measurement function.

Question 4

What are the 4 basic types of ac (analogue) meter movement?

Answer 4

(a) Thermocouple - used mainly for measurements in the radio frequency range : A heating element is heated by the current; a thermocouple is attached to it, and produces an output voltage which can be calibrated to the ac parameters.
(b) Hot-wire - a tensioned resistance wire responds to the current by expanding as it heats, moving an attached indicating needle.
(c) Electrodynamometer - the current is used to generate a magnetic field via two coils which carry out the function of the Permanent Magnet in a PMMC, and a third coil on a pivot causes a needle to rotate, just as in the PMMC. This system gives a reading relating to the average current, Iav .
(d) Iron-vane - see Carr, Chapter 3.

Question 5

Rectification of ac Signals:

Answer 5

When the diode is forward-biased current flows. No current flows if it is reverse-biased.