Avionic General Test Equipment
Operation and use of the multimeter, oscilloscope, signal generator and insulation tester, and what each one actually measures. Examined for Category B2 only.
Avionic test equipment — summary notes
Free- Covers the general-purpose instruments used on avionic systems — the multimeter, oscilloscope, signal generator, frequency counter and insulation tester — and what each one actually measures.
- The multimeter measures voltage, current and resistance. Voltage is measured in parallel with the load, current in series with it, and resistance only on a de-energised circuit — measuring resistance on a live circuit damages the meter and gives a meaningless result.
- The oscilloscope displays voltage against time, so it shows waveform SHAPE: amplitude, period and frequency, rise time, distortion and noise that a multimeter would average away.
- The signal generator injects a known waveform of chosen frequency and amplitude so a system's response can be measured — the standard way to trace a fault stage by stage through an amplifier or receiver.
- The insulation tester (megohmmeter) applies a high DC voltage to measure insulation resistance in megohms. It must not be used on solid-state equipment, which its test voltage would destroy.
- All test equipment must be in calibration, correctly ranged before connection, and connected with the aircraft in the state the maintenance data specifies.
- ⚠ Exam trap: an oscilloscope shows the waveform against TIME; a multimeter gives a single averaged or RMS value and cannot reveal shape or distortion.
- Meter connection
- voltmeter in parallel · ammeter in series · ohmmeter on a dead circuit
- Oscilloscope axes
- vertical = voltage · horizontal = time
- Frequency from the trace
- f = 1 ÷ T, T read from the timebase
A signal is suspected of being distorted rather than merely low. Which instrument settles it, and why?
The oscilloscope. It plots voltage against time, so clipping, ringing or noise are visible directly. A multimeter would return only an averaged value, which can look perfectly normal while the waveform is badly distorted.
Why must an insulation tester never be used on a circuit containing solid-state devices?
It applies a high DC test voltage — hundreds of volts — to force a measurable leakage current through the insulation. Semiconductor junctions break down far below that, so the test itself would destroy the components it was applied to.
Avionic test equipment concept map
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