Ensuring accurate measurements
Attenuation is a parameter that quantifies the loss in power experienced by a signal as it passes from one point in a circuit to another. 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.
At the NPL we offer Attenuation Measurements in coaxial line over the frequency range 0.5 MHz to 18 GHz using an Audio Frequency Substitution Technique. Our measurement systems are based on IVD standards operating at 10 kHz. At this low reference frequency all 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.
For attenuation measurements up to 90 dB the measurement frequency is down-converted to 10 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. 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. A high-quality switched coaxial attenuator is a good example of such a device.
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 can 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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