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.
Analog dBm meters are often used to measure the sound levels of devices such as sirens, horns, motors, and buzzers. They are also used to measure power loss or line loss on voice frequency (VF) communication lines such as two and four wire telephone lines. & Power loss refers to the measured attenuation of a test 820 Hz tone transmitted at 0 dBm. When VF transmission lines are installed, active line components are adjusted to yield a 16 dB 1 dBm attenuation of the 0 dBm, 820 Hz test tone. Subsequent testing of the VF lines typically yields loss measurements of 16 dBm 4 dBm. The Gain/Slope frequency response test, another common VF line measurement that uses analog dBm meters, measures the attenuation at four specific frequencies: 300 Hz, 820 Hz, 2000 Hz, and 3000 Hz. Calculating the difference in attenuation between the lowest readings and the other three readings provides a good indication of the frequency response of the transmission line.
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