Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 86

These instruments were some of the earliest implementations of stimulus/response testing and often consisted only of a sweeping signal source (sometimes called a sweeper) and a diode detector, the output of which was passed through a “log‐amplifier” that produced an output proportional the power (in dBm) at the input. This was sent to the y‐axis of a display, with the sweep tune‐voltage of the sweeper sent to the x‐axis, thus producing frequency response trace. Later, the signal from the detector and the sweeper were digitized and displayed on more modern displays with marker readouts and numerical scaling.

Other SNA systems were developed by putting a tracking generator into a spectrum analyzer so that the source signal followed the tuned filter of the SA. This produced a frequency response trace on the SA screen.

SNAs had the attribute of being simple to use, with almost no setup or calibration required. The scalar detectors were designed to be quite flat in frequency response, and a system typically consisted of one at the input and one at the output of a DUT. However, for measurements of input and output match, or impedance, the SNA relied on a high‐quality coupler or directional bridge. If there was any cabling, switching, or other test system fixturing between the bridge and the DUT, the composite match of all were measured. There was no additional calibration possible to remove the effects of mismatch. As test systems became more complex and integrated, scalar analyzers started to fall from favor, and there are virtually none sold today by commercial instrument manufacturers.