Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 84

An offshoot of a spectrum analyzer, a noise figure analyzer (NFA) is a specialized test instrument that is designed particularly for measuring noise figure. NFAs started out as specialized spectrum analyzers, with improvements in the quality of the receiver and with electronically switched gains to allow the noise figure of the test equipment to be minimized relative to the signal being measured. Some of the things needed to accomplish this, such as adding high‐gain LNAs in front of the first converter, reduced the maximum input power of the instrument so that it was no longer suitable for general‐purpose SA applications.

On the other hand, several spectrum analyzer manufacturers have added a noise figure personality to their SA offerings so that there is quite a lot of overlap between the capabilities of the two systems. However, most SA implementations require the use of an added LNA, at least over some of the band. Newer SAs have an IF structure almost as flexible as an NFA to optimize the performance of the system.

All of these systems of NFA utilize the “hot/cold” or “Y‐factor” method of measuring noise figure (more about this in Chapter 9) using as an input to the DUT a noise source that can be turned on and off. From careful measurement of the output noise, the gain and noise figure of the DUT can be discerned.

More recently, VNAs have been modified to operate as NFAs, utilizing an entirely different technique called the cold‐noise method. In this method, the output noise power is measured, along with the gain using the normal VNA measurement of gain, and the noise figure is computed from these values. No noise source is used in the measurement. This has an advantage of being faster (only one noise measurement is needed) but does have the disadvantage of being sensitive to drift in the gain of the VNA noise receiver. The Y‐factor method does not depend upon the gain of the NFA receiver, but this advantage is often offset by the fact that the gain measurements of the NFA are sensitive to match errors as are the noise measurements, and these are not compensated for.

The ultimate in noise figure analysis is a noise parameter test system. This system properly accounts for all mismatch effects. Some systems use both a VNA and an NFA to measure the gain and noise power, respectively. All noise parameter systems include an input impedance tuner to characterize the change in noise power versus impedance value. Recently, tuners have been combined with VNA‐based NFAs to produce compact, high‐speed noise parameter test systems. These newer systems provide the ultimate in speed and accuracy available today.