National Physical Laboratory

Noise Standards

RF Noise

Noise measurements at NPL encompass noise sources, amplifier noise and oscillator noise. The commonest of these is measurement of a noise source. Noise sources are typically used to measure the noise of a receiver or amplifier although full characterisation of amplifier noise is a more complex process. Oscillator noise measurement is the measurement of the phase and amplitude noise on a notionally pure signal.

The electrical noise present within receiving systems is normally measured by connecting in turn two known levels of noise power to the receiver input. For accurate measurements the two noise sources must be calibrated, this is accomplished by comparison with a known noise source on an instrument known as a radiometer.

Primary noise standards and radiometer systems are researched and developed at NPL. The former are realised by using a blackbody or thermal radiator held at a fixed temperature, which may be above or below ambient. In the RF and microwave region a blackbody radiator is produced by heating or cooling a resistive termination equal to the characteristic impedance of the transmission line. The power spectral density radiated is calculable from the absolute temperature.

The termination is connected to the ambient temperature measuring system by a length of transmission line. This transmission line will have some loss, which will attenuate the thermal radiation from the termination, and itself be a source of thermal radiation, proportional to its physical temperature and loss. Knowledge of the thermal gradients and scattering coefficients of the transmission line allow calculation of the noise power spectral density at the output.

The transmission line has two sections: a transition which supports the temperature gradient to ambient and an output section which is controlled at a constant temperature close to ambient. A number of such calculable thermal noise standards have been manufactured to cover the frequency range 10 MHz to 110 GHz. All the NPL standards currently used are above ambient although below ambient standards are maintained. The standards are used as reference sources for a range of radiometer systems to provide a calibration service for unknown noise sources. The thermal environments for the above ambient standards are temperature controlled enclosures. At frequencies up to 18 GHz the termination section is heated directly using controlled external heaters. Above 18 GHz the standards are currently all in waveguide and the termination section is heated by immersion in a bed of sand which is agitated and heated. This behaves like a fluid, and a region of constant temperature is established in which the termination sits. One oven enclosure can accommodate several different waveguide sizes.

The radiometers used are either switching or total power types. They are auto-balancing and computer controlled and thus require the minimum of operator intervention during calibrations.

Full characterisation of an amplifier treats the device as a two port and results in measurement of two real and one complex quantity. This process can be carried out using NPL’s coaxial radiometers.

NPL also offers a phase noise characterisation service. This uses commercial equipment with additional measurements to establish traceability to thermal noise standards.

Further information pages


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Last Updated: 24 Jul 2012
Created: 8 Jun 2007


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