Electronic Measurements and Instrumentation
Electronic Measurements and Instrumentation tests units, errors, meter loading, oscilloscope reading, bridges, transducers, and calibration. Expect practical interpretation of displays and instrument specifications.
Core rules
- Accuracy is closeness to true value; precision is repeatability. Resolution is the smallest discernible change.
- A voltmeter should have high input impedance; an ammeter should have low resistance. Meter loading changes the circuit being measured.
- A multiplier resistor extends a voltmeter range; a shunt extends an ammeter range.
- CRO/DSO voltage = vertical divisions × volts/div × probe attenuation. Period = horizontal divisions × time/div; f = 1/T.
- A 10× probe multiplies the displayed voltage by 10 and raises input impedance.
- True-RMS meters are needed for distorted waveforms; average-responding meters are calibrated for sine waves.
- A Wheatstone bridge balances when R1/R2 = R3/R4.
Worked example
A trace spans 4 vertical divisions at 0.5 V/div with a 10× probe. Peak-to-peak voltage = 4 × 0.5 × 10 = 20 Vpp. One cycle spans 2 horizontal divisions at 1 ms/div, so T = 2 ms and f = 1/0.002 = 500 Hz. If the waveform is a sine, RMS voltage is 20/(2√2) ≈ 7.07 V.
Traps
Reporting peak-to-peak as RMS, or forgetting probe attenuation, gives large errors. Assuming a meter’s reading is exact when its input impedance loads a high-resistance divider is common. Reading oscilloscope divisions from the graticule centre without checking zero reference causes DC offset errors. Instrument bandwidth must exceed the signal’s highest significant harmonic.