Electronic Measurements and Test Instruments
The Electronic Measurements and Test Instruments domain checks whether you can select the right instrument, connect it correctly and interpret its reading. Expect multimeter, CRO/DSO, function generator and LCR meter questions, including accuracy, loading and probe attenuation.
Core rules and concepts
- A voltmeter connects in parallel and should have high input resistance; an ammeter connects in series and should have low resistance. An ohmmeter is used only on an isolated, unpowered component.
- A digital multimeter’s accuracy is often given as ±(% of reading + number of counts). Resolution is the smallest display step, not the accuracy.
- On a CRO/DSO, voltage = vertical divisions × volts/div × probe factor; time = horizontal divisions × time/div. A 10× probe multiplies the displayed voltage by 10.
- For a sine wave: V_peak = √2 × V_rms; V_pp = 2 × V_peak. Use DC coupling for offset measurements and AC coupling to remove DC.
- Match instrument impedance and bandwidth to the signal; a meter with low bandwidth will under-read high-frequency AC.
Worked example
A sine wave on a CRO fills 4 vertical divisions peak-to-peak at 2 V/div, using a 10× probe. V_pp = 4 × 2 × 10 = 80 V. V_peak = 40 V. V_rms = 40/√2 = 28.3 V.
Traps that cost marks
Forgetting the 10× probe factor; reading peak-to-peak as RMS; connecting an ammeter in parallel and blowing its fuse; measuring resistance in a live circuit; treating display counts as accuracy; using a meter beyond its CAT safety rating.