Ensuring accurate measurements
Attenuation is a measure of the reduction in power level experienced by a signal as it passes through a circuit. Being a power ratio, attenuation can be directly measured as such over small dynamic ranges. For high dynamic ranges, and high resolution, it is necessary to use systems specifically designed for attenuation measurement.
Attenuation standards are either based on:
- Reference to other parameters such as length and angle (i.e. the Waveguide Below Cut Off and the Rotary Vane attenuator respectively), or
- Inductive Voltage Divider (IVD) systems.
The measurement systems are based on IVDs operating at 10 kHz or 50 kHz. However, low values of attenuation are measured directly using the power ratio technique. At these low reference frequencies all the measurements are made through super-heterodyne systems from the RF or microwave frequency. The inherent accuracy of the standard is high at about 0.0002 dB in 20 dB. The realisable uncertainty obtained at the microwave frequency is determined by the linearity of the super-heterodyne system.
Measurements over the frequency range 0.5 MHz to 50 GHz are based on an audio frequency substitution system using 10 and 50 kHz IVDs. For attenuation measurements up to 90 dB the measurement frequency is down-converted to either 10 or 50 kHz, at which it is referenced to an IVD standard via an AC digital voltmeter. Higher attenuation values can be measured with an audio-frequency substitution method, which may also use noise-balancing or coherent detection to extend the measurement range. By extending the voltage ratio systems to a dual-channel configuration, phase as well as attenuation can be measured.
Between 50 GHz and 110 GHz a receiver based system is used that is a rf version of the above. The measurement resolution can be as high as 0.0002 dB and the uncertainties are typically 0.001 dB per 10 dB or better (at the 95% confidence level).
The lowest measurement uncertainty is offered for transfer standards which operate in an incremental mode, i.e. devices which need not be connected into and disconnected from the measurement circuit to achieve the attenuation change. The rotary vane attenuator and high quality switched coaxial attenuators are good examples of such devices.
NPL offers a calibration service for the wide range of attenuators and signal generators available on the commercial market. The former are assumed to be two-port devices and the latter one-port devices. Attenuation values from 0.001 dB to over 140 dB can be measured, depending on the frequency of measurement.
In addition to attenuation and calibrations NPL is able to offer consultancy in all aspects of attenuation measurement systems and primary standards.
In view of the wide range of options on attenuation range and frequency band, customers are requested to contact us about the details of the measurements or calibrations required.
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