The PSD can be computed by taking the ratio of the signal power to bandwidth.
To compute the Power spectral density, enter
- Signal power Ps in either Watt/Milliwatt/Microwatt/Nanowatt OR dBm
- Bandwidth BW in GHz/MHz/kHz/Hz
Formula
PSD = Ps/BW
Example Calculation
- A total power of 10 Watt across bandwidth of 1 kHz results in a PSD of 10/1000 = 0.01 W/Hz.
- Watt can be converted to dBm and 0.01 Watt is equivalent to 10 dBm.
- The PSD is therefore 10 dBm/Hz.
Starting with a dBm value instead. For instance 30 dBm across a bandwidth of 20 kHz.
- The dBm value is first converted to Watt. 30 dBm = 1 Watt
- The next step is do divide Watt by the bandwidth as above. 1 Watt/20 kHz = 0.00005 Watt/Hz
- Finally convert Watt back to dBm to give -13 dBm/Hz.
Note that the dBm value cannot be divided by the bandwidth. It has to be converted to a linear value before dividing the two numbers.
Background
Power Spectral Density (PSD) is a mathematical tool used to analyze signals in terms of their frequency content. It represents the distribution of power across different frequency components of a signal and as a function of frequency or bandwidth.
Unit for PSD is Watt/Hz or dBm/Hz.
PSD provides valuable information about the strength and distribution of different frequency components in a given signal.
What’s the difference between Power Spectrum and Power Spectral Density?
While power spectrum focuses on the magnitude of the signal in the frequency domain, PSD considers the power per unit frequency.
Where is PSD used?
PSD is used to describe the noise floor of a spectrum analyzer. The units are dBm/Hz. The normalized quantity is called the Displayed Average Noise Level or DANL.
The DANL of the Siglent Spectrum Analyzer for instance is -161 dBm/Hz. The lower this number the more sensitive the analyzer. A sensitive analyzer is able to detect very weak RF signals.
Related Calculators
Thermal noise power depends on the bandwidth and temperature. If the bandwidth is 1 Hz, then it is the power spectral density of noise or noise spectral density.
SNR (in dB) is the difference between the PSD and the noise density.