Test Equipment, Measurement, and Troubleshooting
Core concepts
The multimeter measures voltage, current, resistance, continuity and diode drop. A voltmeter goes in parallel and should have high impedance; an ammeter goes in series and has low resistance. Never measure resistance or continuity on a powered circuit. True-RMS meters matter for distorted waveforms; average-responding meters misread them.
An oscilloscope shows amplitude versus time; set volts/div, time/div and trigger, and use a 10x probe with compensation. A megohmmeter checks insulation resistance, a bond meter measures very low resistance in milliohms, and a time-domain reflectometer locates opens, shorts and mismatches in coax. Signal generators, frequency counters, logic analyzers and spectrum analyzers extend the toolkit.
Troubleshoot logically: confirm power and grounds, inspect connectors, half-split the circuit, substitute known-good LRUs, and compare with a known-good system. Read built-in test codes before replacing parts.
Traps
Typical traps are using an ohmmeter on a live circuit, connecting an ammeter in parallel, confusing peak and RMS, and forgetting the round trip in TDR measurements.
Worked example
A TDR pulse reflects after 1.0 microsecond in coax with velocity factor 0.7. Distance = (0.7 x 3 x 10^8 m/s x 1.0 x 10^-6 s) / 2 = 105 m. The divide-by-two accounts for the trip out and back.