Electronic Measuring Instruments and Testing
What this domain covers
Electronic Measuring Instruments and Testing tests your ability to select an instrument, read a scale and calculate a measurement from a CRO or DSO display, plus fault-finding with a multimeter.
Core rules
- A multimeter measures voltage, current and resistance; connect it in parallel for voltage and in series for current. Select a range above the expected value.
- A CRO displays voltage against time. Amplitude = vertical divisions × volts/division × probe attenuation; period = horizontal divisions × time/division; frequency = 1/period.
- Instrument loading: a voltmeter of finite resistance draws current and lowers the reading across high-resistance circuits; a CRO with a ×10 probe presents about 10 MΩ.
- Accuracy is closeness to the true value; precision is repeatability. Sensitivity is the smallest change an instrument can detect.
- A continuity/diode test identifies open circuits and checks diode polarity.
Worked example
A signal occupies 4 vertical divisions peak-to-peak on a 2 V/div setting, and one cycle spans 5 horizontal divisions at 1 ms/div. Peak-to-peak voltage = 4 × 2 = 8 V, so peak = 4 V and rms (sine) = 4/√2 ≈ 2.83 V. Period = 5 × 1 ms = 5 ms; frequency = 1/0.005 = 200 Hz.
Traps that cost marks
Reporting peak-to-peak as rms; forgetting the ×10 probe factor; reading the whole screen instead of one cycle; connecting an ammeter in parallel; and ignoring meter loading on a high-resistance node.