Phase and Amplitude Noise Measurements

The Noise Figure (NF) is a common amplifier specification that is used to calculate the noise at Fourier frequencies f that represents the offset from a carrier frequency v₀. In the presence of a carrier signal, the noise level near the carrier is no longer constant but often increases as f decreases. This increase usually changes at a rate of at least 1/f, "flicker" behavior, which often significantly dominates over the white-noise level given by the NF, which in practice is measured in the absence of an actual signal through the amplifier. Furthermore, the flicker-noise level depends on the amplifier's linearity and input power. Because of this signal-induced rise in amplifier noise, many systems do not achieve the performance predicted by using the no-signal NF characterization. A paper published by Archita Hati and others addresses two important issues: (i) it compares the usefulness of phase-modulation (PM) noise measurements vs. NF measurements in characterizing the merit of an amplifier, and (ii) it reconciles a general misunderstanding in using –174 dBc/Hz (relative to carrier input power of 0 dBm) as thermal noise level. The residual broadband (white PM) noise is used as the basis for estimating the NF of an amplifier. It has been observed experimentally that many amplifiers show an increase of 1 to 5 dB in the broadband noise as the signal level through the amplifier increases. This effect is linked to input power through the amplifier's nonlinear intermodulation distortion. Consequently, this effect is reduced as linearity is increased. It is important to note that NF is sometimes used as a selection criterion for an amplifier but yields no information about potentially important close-to-carrier 1/fnoise of an amplifier, whereas PM and amplitude modulation (AM) noise measurements do. It has been verified both theoretically and experimentally that the single-sideband PM (and AM) noise floor due to thermal noise is –177 dBc/Hz, relative to a carrier input power of 0 dBm; this result differs from that found in early literature.