mag2db

Convert magnitude to decibels (dB)

Syntax

ydb = mag2db(y)

Description

ydb = mag2db(y) expresses in decibels (dB) the magnitude measurements specified in y. The relationship between magnitude and decibels is $$ydb=20\ast {\log }_{10}(y)$$

Examples

Display Gain Margins in Decibels

For this example, consider the following SISO feedback loop where the system contains multiple gain crossover or phase crossover frequencies, which leads to multiple gain or phase margin values:

Create the transfer function.

G = tf(20,[1 7]) * tf([1 3.2 7.2],[1 -1.2 0.8]) * tf([1 -8 400],[1 33 700]);

Use the allmargin command to compute all stability margins.

m = allmargin(G)

m = struct with fields:
     GainMargin: [0.3408 3.3920]
    GMFrequency: [1.9421 16.4807]
    PhaseMargin: 68.1178
    PMFrequency: 7.0762
    DelayMargin: 0.1680
    DMFrequency: 7.0762
         Stable: 1

Note that gain margins are expressed as gain ratios and not in decibels (dB). Use mag2db to convert the values to dB.

GainMargins_dB = mag2db(m.GainMargin)

GainMargins_dB = 1×2

   -9.3510   10.6091

Input Arguments

y — Input array

scalar | vector | matrix | array

Input array, specified as a scalar, vector, matrix, or an array. When y is nonscalar, mag2db is an element-wise operation.

Data Types: single | double
Complex Number Support: Yes

Output Arguments

ydb — Magnitude measurements in decibels

scalar | vector | matrix | array

Magnitude measurements in decibels, returned as a scalar, vector, matrix, or an array of the same size as y.