Noise figure meters measure the noise contribution of an amplifier relative to a noise-free amplifier at a reference temperature. For most Ku-band amplifiers, values are expressed in decibels (dB). Noise figure is defined as the ratio of the signal-to-noise (S/N) power ratio at the input to the S/N ratio at the output. It is independent of modulation and demodulation, and is not related to gain. Like sensitivity and bit error ratio (BER), however, noise figure is an important system parameter for receivers that process low-level signals. By determining the noise figure, the sensitivity of the bandwidth can then be estimated. Noise figure meters are used to test entire systems, or to test individual components such as the pre-amplifier, intermediate frequency (IF) amplifier, and mixer.
Noise figure meters carry various specifications for noise figure measurement, gain measurement, input, and electromagnetic compatibility. Measurement range, instrumentation uncertainty, and resolution are important parameters for both noise figure measurement and gain measurement. Input specifications for noise figure meters include frequency range, tuning accuracy, frequency resolution, noise figure for input power levels below -60 dBm, input standing wave ratio (SWR) at a 50-ohm reference impedance, maximum operating input power, and maximum net internal gain. In terms of electromagnetic compatibility, some noise figure meters meet U.S. military standards such as MIL-STD-461B-1980 and MIL-STD-461B. Others meet standards such as IEC 801-2 from the International Electrotechnical Commission (IEC).
Selecting Noise Figure Meters
Selecting noise figure meters requires an analysis of general and supplemental features, as well as functional properties. General features for noise figure meters include noise source drive, compatibility with the general-purpose interface bus (GPIB) or universal serial bus (USB), operating and storage temperature, connector type, power, net weight, shipping weight, and physical dimensions. Supplemental characteristics include bandwidth, sensitivity in dBm, measurement speed, sweep speed and jitter at minimum smoothing, jitter with increased smoothing, maximum safe input level, and audible noise level. As a rule, noise figure jitter is equivalent to Y factor jitter to within 10%. Jitter may limit accuracy at minimum smoothing, but not at maximum smoothing. In terms of functional properties, noise figure display units and noise figure jitter display are important parameters to consider when selecting noise figure meters.