Electronic Measuring Instruments and Testing
What the exam tests
Electronic Measuring Instruments and Testing covers multimeters, cathode-ray oscilloscopes (CRO) and digital storage oscilloscopes, signal generators, frequency counters and component testers. Questions ask you to interpret a display, calculate a quantity from settings, and judge whether a measurement loads the circuit.
Core rules
- A voltmeter is connected in parallel and should have high resistance; an ammeter is in series and should have low resistance. Loading error appears when meter resistance is comparable with circuit resistance.
- CRO vertical scale is volts/division; horizontal scale is time/division. For a repetitive waveform, period T=(horizontal divisions per cycle)×(time/div), f=1/T.
- Peak-to-peak voltage Vpp=(vertical divisions)×(volts/div). For a sine, Vrms=Vp/√2=Vpp/(2√2).
- A ×10 probe multiplies the displayed voltage by 10 and raises input impedance; compensate it before use.
- Accuracy is not the same as resolution. A 3½-digit DMM may display 0.01 V but still have ±(0.5%+2 counts) error.
Worked example
A sine wave on a CRO occupies 5 horizontal divisions at 2 ms/div, so T=10 ms and f=100 Hz. It spans 6 vertical divisions peak-to-peak at 0.5 V/div, so Vpp=3.0 V, Vp=1.5 V, and Vrms=1.5/1.414=1.06 V. If a ×10 probe is fitted and the scope is set to 0.5 V/div, the actual Vpp is 30 V, not 3 V.
Traps
Quoting peak-to-peak as amplitude; forgetting the probe factor; using AC coupling when measuring DC level; and assuming a meter reads true rms on a distorted waveform. Most cheap DMMs are average-responding and calibrated for sine waves.