Making noise figure measurements with handheld analyzers

We recently were able to get with Roland Zhang from Keysight who took us through a noise figure measurement system that really comes in handy if you’re doing 5G measurements in the field.

Zhang explains that in the 5G arena there will be a lot of amplifiers or up and down converters to convert signals from 28 or 39 GHz in the millimeter wave range to lower frequencies and vice versa. In field measurements, one of the key parameters of such devices is noise figure which helps evaluate the performance of low-noise amplifiers. A spectrum analyzer serves as the receiver.

The measurements use a technique for noise measurement called Y factor. The Y-factor technique involves the use of a noise source that has a pre-calibrated excess noise ration, ENR. The Y-factor is a ratio of two noise power levels, one measured with the noise source on, the other with the noise source off. The process begins with a calibration step that connects a noise source to the analyzer input. The instrument stores measured calibration values and calculated values, then uses this information to normalize its noise figure display to 0 dB. Then the DUT is inserted in the signal path and the analyzer measures its noise. The Fieldfox instrument has a built-in dc power supply that can be switched on and off for controlling the connected noise sources.

The FieldFox also contains a built-in uncertainty calculator that gives the operator an idea for the accuracy of the measurements.

Zhang says a typical scenario for instrument use would be in satellite communications or 5G where the link budget isn’t what it’s supposed to be. It is typical in those situations to wonder whether the amplifier or up-converter/down-converter is meeting spec. Equipment vendors quite often give typical numbers for noise figures, but not exact figures for the instrument in question, Zhang says. Field measurements can provide definitive answers.