Signal Integrity Engineer's Companion: The Wireless Signal--Part III

The Swept Spectrum Analyzer
The swept-tuned, superheterodyne spectrum analyzer is the traditional architecture that first enabled engineers to make frequency domain measurements several decades ago. Originally built with purely analog components, the swept SA has since evolved along with the applications it serves. Current-generation swept SAs now include advanced digital elements such as analog-to-digital conversion (ADC), digital signal processing (DSP), and microprocessors. However, the basic swept approach remains largely the same, and the instrument retains its role as the primary measurement tool for observing controlled RF signals. A clear advantage of a modern swept SA is its excellent dynamic range, whereby it can capture and detect a broad array of RF data.

The swept SA makes power-versus-frequency measurements by down-converting the signal of interest and sweeping it through the passband of an RBW filter. The RBW filter is followed by a detector that calculates the amplitude at each frequency point in the selected passband,as shown in Figure 10-3.

Figure 10-3 shows the measurement trade-off between frequency resolution and time. The local oscillator sweeps through a "span" of frequencies feeding the mixer. Each sweep produces sum and difference frequencies at the mixer output. The resolution filter has a bandwidth that is set to a user-selected frequency, the RBW. The narrower the filter bandwidth, the higher the resolution of the measurement and the greater the exclusion of unwanted instrument-generated noise. The RBW filter feeds the detector, which measures the spectral power at each instant in time to produce a frequency-domain display plotting spectral power against frequency. While this method can provide high dynamic range, its principal disadvantage is that it can calculate the amplitude data for only one frequency point at a time. If the RBW filters are made too narrow, the time taken to complete a sweep of the RF input is too long, and any changes in the RF input go undetected. Sweeping the analyzer over a span of frequencies or number of passbands can take considerable time. This measurement technique is based on the assumption that the analyzer can complete several sweeps without any significant changes to the signal being measured. Consequently, a relatively stable, unchanging input signal is required. If the signal changes rapidly, the change probably will be missed.

For example, the left part of Figure 10-4 shows an RBW logic analyzer sweep that is looking at frequency segment Fa while a momentary spectral event occurs at Fb. By the time the sweep arrives at segment Fb, the event has vanished and goes undetected. The RBW spectrum analyzer sweep fails to provide a trigger for the transient signal at Fb and fails to store a comprehensive record of signal behavior over time. This is an example of the classic trade-off between frequency resolution and measurement time, which is the Achilles' heel of the traditional RBW spectrum analyzer.