Note 1: Some manufacturers and authors define NEP as the minimum detectable power per square root bandwidth. When defined this way, NEP has the units of watts per (hertz)1/2. Therefore, the term is a misnomer, because the units of power are watts.
Note 2: Some manufacturers define NEP as the radiant power that produces a signal-to-dark-current noise ratio of unity. The NEP measurement is valid only if the dark-current noise dominates the noise level.
Noise contributions are based on the mixer(s) having symmetrical upper and lower side-band power spectral density characteristics (additional information regarding SysCalc's SSB noise modeling can be found in Mathematical Definitions and Derivations).; therefore, the total mixer noisepower is based on twice the equivalent SSB noisepower in the bandwidth defined by the system.
The equivalent SSB power due to the composite power spectral density of all contributing mixers is computed in the LO Modulation report type.
The power is a function of the integration range and mixer(s) SSB power spectral density.
Noise can originate in the signal itself or in the detection system, but the first step in improving measurement quality is to minimize the noise from the environment.
The NEP is related to the total noise current by the responsivity of the detector at the wavelength being measured, R(l).
So if the noise sources generate a current on the order of, say, tens of nanoamps at the signal wavelength, the signal will be competing with the equivalent of tens of nanowatts of noisepower, in the range of many sensitive applications.
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