Analog dBm Meters
Analog dBm meters are instruments that measure signal power. They display values on a dial, usually with a needle or moving pointer. Signal power, which can be expressed in decibels (dB) when the unit of reference is known, is measured in dBm to represent the decibels referenced to a power level of one milliwatt (mW). Expressed as a logarithm, dBm = 10 log (P / P Ref), where P is the measured power and P Ref is the reference power of 1 mW. Analog dBm meters that convert dBm into voltage (V) measure impedance in ohms (W) so that any voltage across any resistance (R) that results in 1 mW equals 0 dBm. Expressed as an equation, 0 dBm = V^2/R = 1 mW, where V is the voltage and R is the circuit resistance. Consequently, 0.775 V across 600 W is 0 dBm, 1 V across 1000 is W 0 dBm, and both voltages result in 1 mW.
Analog dBm meters are available in handheld and benchtop models. Many devices are portable, low cost, and designed for test environments. Battery powered units can be operated without plug-in power and include battery test methods that alert users to power supply problems. Some analog dBm meters include fuses for overload protection and temperature-compensated programming or electrical devices designed to counteract known errors caused by temperature changes. Other analog dBm meters include a mirrored scale that makes it easier to read measurements to a given accuracy by enabling users to avoid parallax errors. Range switches permit the selection of a range of units to measure. Analog dBm meters with diode test methods are also available.