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Using Calibration to Optimize Performance in Crucial Measurements
The most common method for measuring noise figure involves using a noise source with a calibrated excess-noise ratio (ENR). This approach is used with Agilent’s noise figure analyzers and spectrum analyzer-based solutions. Noise figure measurements assume that a perfect 50-ohm source match is presented to the DUT. This is a reasonable (but not perfect) assumption when the noise source is connected directly to the DUT, especially when using low-ENR noise sources which tend to have better matches than high-ENR sources. However, this is usually not a good assumption when the noise source cannot be connected directly to the input of the amplifier or converter under test, as is the case for in-fixture, on-wafer, or automated-test environments. In these instances, the deviation from a perfect 50-ohm source match can contribute a large amount of measurement error.
The PNA-X microwave network analyzer uses the cold-source method of measuring noise figure, a technique that does not rely on a noise source. Using this method on a PNA-X, one can measure noise figure plus S-parameters, gain and phase compression, harmonics, and intermodulation distortion (IMD), all with a single set of connections to the DUT. Using advanced error correction, all of the major sources of noise figure measurement error are removed. In addition to full vector-error correction, a standard ECal electronic calibration module is used as an impedance tuner to overcome the effects of imperfect system source match, yielding the highest accuracy of any noise figure test solution on the market today.
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