Measurements and Test Equipment
Instruments and what they measure
A multimeter measures DC and AC voltage, current and resistance; true-RMS meters correctly read non-sinusoidal waveforms, while average-reading meters are calibrated for sine waves and misread others. An oscilloscope shows waveform shape, amplitude and timing, but probe capacitance and 10× attenuation load the circuit. A signal generator provides known test levels; a spectrum analyser displays amplitude versus frequency and is used for harmonics, spurious outputs and occupied bandwidth. A directional power or SWR meter and a dummy load allow transmitter testing without radiating.
Accuracy, resolution and units
Accuracy is closeness to the true value; resolution is the smallest displayed change. Calibration and lead resistance matter at low values. RF power is often expressed in dBm: 0 dBm = 1 mW, 30 dBm = 1 W, 40 dBm = 10 W, 50 dBm = 100 W. Average power and peak envelope power differ for SSB and pulsed modes.
Traps and rule of thumb
Do not test a transmitter into an open antenna when a dummy load is appropriate, and do not confuse peak with RMS on a scope. A meter's rated bandwidth matters: a low-frequency multimeter will not measure RF accurately.
Worked example: 100 W in dBm = 10 log10(100/0.001) = 50 dBm. Conversely, 37 dBm ≈ 5 W.