Spectrum analysis consists of a wide variety of applications and measurements. Each involves a different set of procedure and measurement sequences. For example, what users must do to determine the occupied spectrum width for a digitally modulated QAM signal differs from the procedure for finding the total harmonic distortion of an oscillator, the percentage modulation for an amplitude modulated signal, the level of EMI emissions for regulatory compliance purposes, and the pulse desensitization factor of a pulsed radar signal. There's literally no end to the list that one can invent.
But, this seemingly endless list of applications and associated measurement needs can be greatly reduced by grouping signals into common-factor classes. There are only three basic classes into which all signals fall. Understanding what these classes are, and how to optimize the spectrum analyzer settings for each signal class, will significantly simplify the measurement process. Before beginning a discussion of signal classes, though, we need to examine the basic spectrum analyzer functions that interact with these signals.
The basic signal-processing elements of a spectrum analyzer include a mixer, bandpass filter, envelope detector, low-pass filter, and peak detectors (Fig. 1). The diagram is condensed to show only the essential aspects of the elements being discussed. Some important elements are omitted, such as preselection and logarithmic amplification.
The system in Figure 1 is a specialized version of the standard superheterodyne RF receiver. Such a receiver is recognized by the mixer whose function is to bring the incoming signal frequency into the passband of an intermediate frequency (IF) amplifier. Signal conditioning and signal processing take place there. Known as the resolution filter, the IF amplifier in the spectrum analyzer has a variable bandwidth that's called the resolution bandwidth (Br).
Next to center frequency, the resolution bandwidth control is possibly the most prominent function of the spectrum analyzer. The chosen setting of the Br has an important, and sometimes essential, impact on measurement results, depending on the signal class involved. This will be discussed in the signal class section of this article.
Additionally, the spectrum analyzer has another variable-bandwidth filter, known as the video filter (Bv). This is a confusing name, but we're stuck with it for historical standardization reasons. A more descriptive name would be post-detection filter. By contrast, the resolution filter would then be a pre-detection filter. Identification of location, as before and after a detector, clarifies the function of these filters and what they do to the signal.
The detector intervening between these filters is termed an envelope detector. Its purpose is to detect the envelope (the outer outline) of the output from the IF amplifier, or resolution bandwidth filter. It's immediately apparent that the resolution circuit acts upon the incoming signal, while the video circuit acts upon the detected envelope of the signal.
This type of post-detection filter is equivalent to an arithmetical averaging function. Furthermore, the averaging isn't of the signal, but the envelope of the spectrum. Any application that needs an average will call for a narrow Bv, while those that shouldn't be averaged require a wide Bv.
The final important factor in this discussion is the choice of detector functions. Many spectrum analyzers have additional detector functions. But we are interested in just two of these, the positive peak envelope detector (Dp) and the negative peak envelope detector (Dn). The default turn-on of the spectrum analyzer has both detectors on. About 70% of the time, the negative detector doesn't provide any useful information. Still, having this detector actuated will only rarely cause any harm. So, having both detectors on is the usual default, although turning the negative detector off also is normally acceptable. Yet it isn't typically acceptable to turn the positive detector off. That's a very rare situation, perhaps occurring 1% or 2% of the time at most. This situation calls for a measurement involving only the negative detector.
A summary of the likelihood of needing these detectors follows. There must be Dp about 98% of the time. Both detectors are required around 30% of the time. A Dn-only situation occurs about 2% of the time. Finally, around 70% of the time, it makes no difference whether the Dn is on or off.
Three signal classes and three spectrum analyzer functions make for a 3-by-3 matrix of relationships. But before we can get to such a matrix, we need to look at the signal classes and understand how these are affected by the previously discussed spectrum analyzer functions.
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