Convert Watt or mWatt to dB or dBm

This post is merely to remind me. So, I just copy paste from here.

But, first of all, here is the formula to convert dB to Watt

watts = 10^((dbm-30)/10)

Then, let's read the explation for corveting from Watt or mWatt to dB or dBm.


The dB Conversion block converts a linearly scaled power or amplitude input to dB or dBm. The reference power is 1 Watt for conversions to dB and 1 mWatt for conversions to dBm. The Input signal parameter specifies whether the input is a power signal or a voltage signal, and the Convert to parameter controls the scaling of the output. When selected, the Add eps to input to protect against “log(0) = -inf” parameter adds a value of eps to all power and voltage inputs. When this option is not enabled, zero-valued inputs produce -inf at the output.

The output is the same size as the input.

Power Inputs

Select Power as the Input signal parameter when the input, u, is a real, nonnegative, power signal (units of watts). When the Convert to parameter is set to dB, the block performs the dB conversion

y = 10*log10(u)      % Equivalent MATLAB code

When the Convert to parameter is set to dBm, the block performs the dBm conversion

y = 10*log10(u) + 30

The dBm conversion is equivalent to performing the dB operation after converting the input to milliwatts. 

Voltage Inputs

Select Amplitude as the Input signal parameter when the input, u, is a real voltage signal (units of volts). The block uses the scale factor specified in ohms by the Load resistance parameter, R, to convert the voltage input to units of power (watts) before converting to dB or dBm. 

When the Convert to parameter is set to dB, the block performs the dB conversion

y = 10*log10(abs(u)^2/R)

When the Convert to parameter is set to dBm, the block performs the dBm conversion

y = 10*log10(abs(u)^2/R) + 30

The dBm conversion is equivalent to performing the dB operation after converting the (abs(u)^2/R) result to milliwatts.

Parameters

Convert to

The logarithmic scaling to which the input is converted, dB or dBm. The reference power is 1 W for conversions to dB and 1 mW for conversions to dBm. Tunable(Simulink).

Input signal

The type of input signal, Power or Amplitude.

Load resistance

The scale factor used to convert voltage inputs to units of power. Tunable (Simulink).

Add eps to input to protect against “log(0) = -inf”

When selected, adds eps to all input values (power or voltage). Tunable (Simulink).

Supported Data Types

• Double-precision floating point
• Single-precision floating point