Electronic Measurement and Test Instruments
Electronic Measurement and Test Instruments is examined through instrument selection, connection, reading interpretation and measurement safety.
What the exam tests
Measurement questions test the digital multimeter, analog multimeter, CRO/DSO, function generator, LCR meter and DC power supply. You must distinguish accuracy from precision, read a trace using volts/div and time/div, and allow for probe attenuation.
Core rules
- Voltmeter in parallel, high input impedance; ammeter in series, low resistance; ohmmeter only on an isolated, unpowered component.
- Digital meters give resolution (least count); accuracy is a tolerance, e.g. ±(0.5% + 2 digits). True RMS meters are needed for distorted waveforms; average-responding meters misread non-sine AC.
- CRO/DSO: vertical reading = divisions × volts/div × probe factor; time reading = divisions × time/div; frequency = 1/T. Trigger stabilises the display.
- AC mains 230 V is RMS; peak = 230 × √2 ≈ 325 V, peak-to-peak ≈ 650 V. Do not confuse these.
- Diode/continuity mode is for open/short checks, not accurate resistance.
Worked example
A sine wave on a CRO occupies 4 divisions peak-to-peak with 2 V/div and a 10× probe. Actual peak-to-peak = 4 × 2 × 10 = 80 V. Peak = 40 V. RMS = 40/√2 ≈ 28.3 V. One cycle spans 5 divisions at 1 ms/div, so T = 5 ms and f = 1/0.005 = 200 Hz. The reasoning matters: apply the probe factor once, then convert peak-to-peak to peak before RMS.
Traps that cost marks
Forgetting the 10× probe; treating peak-to-peak as peak; putting an ammeter across a supply; measuring resistance in a live circuit; reading a DSO cursor value without checking zero reference; assuming a meter's display digits equal its accuracy.