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Spectrum analyzer based measurement can show the phase-noise power over many decades of frequency; e.g., 1 Hz to 10 MHz. The slope with offset frequency in various offset frequency regions can provide clues as to the source of the noise; e.g., low frequency flicker noise decreasing at 30 dB per decade (= 9 dB per octave).
Signal analyzers can perform the operations of both spectrum analyzers and vector signal analyzers.A signal analyzer can be viewed as a measurement platform, with operations such as spectrum analysis (including phase noise, power, and distortion) and vector signal analysis (including demodulation or modulation quality analysis) performed as measurement applications.
Spectrum analyzers are widely used to measure the frequency response, noise and distortion characteristics of all kinds of radio-frequency (RF) circuitry, by comparing the input and output spectra. For example, in RF mixers, spectrum analyzer is used to find the levels of third order inter-modulation products and conversion loss.
For perfect reconstruction, the spectrum analyzer must preserve both the amplitude and phase of each frequency component. These two pieces of information can be represented as a 2-dimensional vector, as a complex number , or as magnitude (amplitude) and phase in polar coordinates (i.e., as a phasor ).
S v is directly observable on a spectrum analyzer, whereas S φ is only observable if the signal is first passed through a phase detector. Another measure of oscillator noise is L, which is simply S v normalized to the power in the fundamental. As t → ∞ the phase of the oscillator drifts without bound, and so S φ (Δf) → ∞ as Δf → 0 ...
For thermal noise, its spectral density is given by N 0 = kT, where k is the Boltzmann constant in joules per kelvin (J/K), and T is the receiver system noise temperature in kelvins. The noise amplitude spectral density is the square root of the noise power spectral density, and is given in units such as volts per square root of hertz, V / H z ...
A constellation diagram for rectangular 16-QAM The constellation as received, with noise added Spectrum analyzer software using different views to show a QAM 4096 constellation diagram The number of constellation points in a diagram gives the size of the "alphabet" of symbols that can be transmitted by each sample, and so determines the number ...
The noise current is set by the filament temperature. The current is an exponential function of filament temperature. At low frequencies, there is 1/f noise. At high frequencies, the transit time of the electron becomes an issue. Ott (1976, pp. 218–219) describes using a noise diode to measure noise factor.