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Radiometer systems for the intercomparison of low temperature noise sources have been under development in the UK RF and Microwave Standards Division at the Royal Radar Establishment since 1968 [1,2] . A simplified block diagram of a typical system is shown in Fig 1. When precision measurements are being performed the system must be carefully analysed for source of systematic error. The most significant of these occur in uncertainties due to attenuation and mismatch. A number of authors have discussed these uncertainties [3-81.Mismatch uncertainties can be reduced to negligible proportions in most cases by the inclusion of tuners at the measurement reference planes, and in the system shown in Fig 1 a minimum of six tuners would be required. The inclusion of tuners restricts measurement to a narrow band and the tuners require adjustment at each measurement frequency. It must also be remembered that tuner losses and their associated uncertainties must be accounted for when dealing with low noise systems, and so tuner losses must be measured at each frequency or a worst-case value measured and used. Consequently we shall show later that the overall noise temperature uncertainty of a narrow-band tuned system is only slightly less than that of an untuned broad-band system in which the magnitudes of reflection coefficients at the reference planes are small, as they should be in a high grade system. A tuned system becomes a necessity when the reference plane reflection coefficient magnitudes are high.Without tuners the mismatch uncertainties can be assessed accurately if the value of the complex reflection coefficients at the reference plane are known at all measurement frequencies. Until recently this was a tedious exercise. However, the advent of the Automatic Network Analyser (ANA) has made the performance of such measurements simple and rapid. Moreover, the majority of the measurements need to be done once only for any system which is left unchanged. The mismatch uncertainties then become a function of how accurately the ANA can perform the measurements.If one adopts this approach it becomes important to consider how the system components may best be grouped in order to reduce the number of measurements, and hence the uncertainties, to a minimum. Reference to Fig 2 illustrates this point. Fig 2(a) shows a source connected to a load by two two-port networks defined by the usual scattering parameters. Fig 2(a) reduces to 2(b) if the two networks are grouped together and measured as one. The number of parameters defining the source-to-load path is thus reduced from 14 to 8, and we have Crown
Radiometer systems for the intercomparison of low temperature noise sources have been under development in the UK RF and Microwave Standards Division at the Royal Radar Establishment since 1968 [1,2] . A simplified block diagram of a typical system is shown in Fig 1. When precision measurements are being performed the system must be carefully analysed for source of systematic error. The most significant of these occur in uncertainties due to attenuation and mismatch. A number of authors have discussed these uncertainties [3-81.Mismatch uncertainties can be reduced to negligible proportions in most cases by the inclusion of tuners at the measurement reference planes, and in the system shown in Fig 1 a minimum of six tuners would be required. The inclusion of tuners restricts measurement to a narrow band and the tuners require adjustment at each measurement frequency. It must also be remembered that tuner losses and their associated uncertainties must be accounted for when dealing with low noise systems, and so tuner losses must be measured at each frequency or a worst-case value measured and used. Consequently we shall show later that the overall noise temperature uncertainty of a narrow-band tuned system is only slightly less than that of an untuned broad-band system in which the magnitudes of reflection coefficients at the reference planes are small, as they should be in a high grade system. A tuned system becomes a necessity when the reference plane reflection coefficient magnitudes are high.Without tuners the mismatch uncertainties can be assessed accurately if the value of the complex reflection coefficients at the reference plane are known at all measurement frequencies. Until recently this was a tedious exercise. However, the advent of the Automatic Network Analyser (ANA) has made the performance of such measurements simple and rapid. Moreover, the majority of the measurements need to be done once only for any system which is left unchanged. The mismatch uncertainties then become a function of how accurately the ANA can perform the measurements.If one adopts this approach it becomes important to consider how the system components may best be grouped in order to reduce the number of measurements, and hence the uncertainties, to a minimum. Reference to Fig 2 illustrates this point. Fig 2(a) shows a source connected to a load by two two-port networks defined by the usual scattering parameters. Fig 2(a) reduces to 2(b) if the two networks are grouped together and measured as one. The number of parameters defining the source-to-load path is thus reduced from 14 to 8, and we have Crown
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