R = 10 log 10 P 2/ P 1, where P 2and P 1are the amount of power in the two signals. The ratio R of two signals in decibels is: The decibel is 0.10 bel, an obsolete term.
This convention works well because it is how we perceive sound and light. Otherwise, the oscilloscope screen might have to be 30 ft tall.ĭecibels are used to compare two quantities in acoustics and elsewhere. The advantage is that it permits the user to see high-amplitude spikes juxtaposed with the low-amplitude noise floor, both in the same display. The scale is logarithmic rather than linear. This power, moreover, is denoted in decibels relative to the Y-axis. In the frequency domain, when you press Math>FFT or send a signal into the RF input, amplitude along the Y-axis displays as power rather than as volts as in the time domain.
In the time domain, amplitude is usually shown as volts on a linear scale. RMS is defined in mathematics as the square-root of the mean square.
RMS values for non-sinusoidal waveforms are different. The reason we are interested in the RMS value of alternating current that conforms to a sine wave is that it is equal to the amount of direct current that would dissipate the same amount of power in a resistive load. That stands for root-mean-square, and it is used in a variety of disciplines including statistics, water flow, weather forecasting, etc. In an oscilloscope display, for example, we may see branch circuit voltage as 325 V peak-to-peak, but the meaningful figure is 115 to 120 V, depending on your distance from the transformer, wire size and loading. In the time domain, amplitude, the dependent variable, is shown in volts relative to the Y-axis.