Читать книгу Handbook of Microwave Component Measurements - Joel P. Dunsmore - Страница 82

A spectrum analyzer (SA) is a specialized type of receiver, which displays the power of a signal on the y‐axis versus the frequency of the signal on the x‐axis. As such, it could be considered a frequency‐sensitive power meter.

The key attributes of a spectrum analyzer are its displayed average noise level (DANL) and its maximum input power. The maximum input is set by the compression of the input mixer in the SA and can be increased by adding input attenuation. However, adding attenuation degrades the DANL by an equal amount. A further limitation in measuring signals, for example, TOI, is the self‐generated distortion of the input mixer, which will generate TOI signals at the same frequencies as that as the TOI from the signal under test. The data sheet for an SA will typically specify the distortion in dBc relative to some input level at the mixer. This, coupled with the noise floor, will set the measurement range of the SA. Lower‐resolution bandwidth will lower the noise floor at the cost of speed of measurement. Similar effects are present for the measurement of harmonics.

Another key attribute of a spectrum analyzer is its frequency flatness and power linearity specifications. Flatness specifications of a spectrum analyzer are usually quite large, as much as ±2.0 dB for 26 GHz microwave version, although typical performance is much better, and this flatness can be compensated for with an amplitude calibration. Microwave and mm‐wave systems can be even worse. The large value for frequency response comes from the interactions of the pre‐selector (which is usually a swept YIG filter) and the first converter in the SA. To a first order, these are stable and can be corrected for, but there will still be a residual flatness error even after calibration related to the post‐tuning drift of the pre‐selector; that is, it does not always tune its peak value to the same frequency for the same settings. Another source of uncorrected error is the mismatch between the SA input and the output match of the signal source being measured. In some cases, this can be a quite high number, up to ±1 dB or more.

As the name implies, the key role of spectrum analyzers is in determining the quality of unknown spectrums. The use of spectrum analyzers in microwave component test applications is primarily as a means to the frequency response or distortion response of a system as a known stimulus is applied.

In recent times, these applications of spectrum analyzers are being augmented by advanced VNAs, which have higher‐speed receivers, built‐in sources, and advanced calibration capabilities, as well as PXI‐based spectrum analyzers. Spectrum analyzers are available in modular (PXI) formats and can be configured as multi‐channel spectrum analyzers. Further, some spectrum analysis has moved into the mm‐wave range, typically with external down‐converters, but some manufacturers are producing spectrum analyzers with internal mm‐wave converters to provide broadband (up to 110 GHz) capabilities. These newer implementations may not have hardware preselection for image rejection, but rather use advanced forms of digital image‐rejection based on high‐speed FFT processing or other methods. More about this appears in Chapter 8.