Before you start evaluating radio frequency devices, you need to understand some basic concepts and terms that are used in radio frequency engineering. These include frequency, wavelength, amplitude, phase, modulation, bandwidth, impedance, power, and noise. Frequency is the number of cycles per second of a periodic signal, measured in hertz (Hz). Wavelength is the distance between two consecutive peaks or troughs of a periodic signal, measured in meters (m). Amplitude is the maximum value of a periodic signal, measured in volts (V) or decibels (dB). Phase is the angle of a periodic signal relative to a reference point, measured in degrees (°) or radians (rad). Modulation is the process of changing one or more properties of a periodic signal, such as amplitude, frequency, or phase, to encode information. Bandwidth is the range of frequencies that a signal occupies, measured in hertz (Hz) or decibels (dB). Impedance is the ratio of voltage to current at a given point in a circuit, measured in ohms (Ω). Power is the rate of energy transfer, measured in watts (W) or decibels (dBm). Noise is any unwanted or random signal that interferes with the desired signal, measured in volts (V) or decibels (dB).