Spectrum Analysis Basics - Part 2: What's in a spectrum analyzer?
2020-05-28 | 5 min read
In my last blog, I talked about types of spectrum analyzers, frequency and time domain basics, and a few applications of spectrum analyzers. In this blog, I’ll go over the block diagram of a spectrum analyzer and describe a few important components of the equipment.
Figure 1 shows a high-level block diagram of a classic superheterodyne spectrum analyzer. “Super” refers to frequencies above the audio range and “heterodyne” means to translate frequency. These days, this design has evolved to contain more digital circuitry, but the fundamentals of the original design help us to understand the analyzer’s function. Below, I’ll go through the blocks of this diagram to explain their function.
An RF attenuator, shown in Figure 2, ensures the signal enters the mixer at an optimum level. If a signal is too high in amplitude, it may overload the mixer, leading to distortion. This component protects the analyzer from damage. The next block in Figure 1, the low pass filter, prevents high frequencies from reaching the mixer. In future microwave spectrum analyzers, this original low pass filter is replaced by a preselector, which we will learn more about later.
The next component, the mixer, is a nonlinear device. It outputs not only the two original signals entering, but also their harmonics and the sums and differences of their harmonics. After the mixer, which helps tune the analyzer based on the LO and IF, is the IF gain. This block helps to adjust the vertical reference level of a signal while remaining coupled to the input attenuator. It ensures the two levels do not change independently so that a constant signal position shows on the display.
Next, an intermediate frequency (IF) filter processes these results and blocks signals that do not fall within the pass band. The signal then undergoes more processing, such as amplification or compression on a logarithmic scale.
Previously, analyzers used an envelope detector, shown in Figure 3, to covert an IF signal to video. This component makes the spectrum analyzer act as a voltmeter, helping it to indicate power levels as it follows the changing amplitudes of peaks of the signal. Newer, digital signal analyzers use digital signal processing instead of an envelope detector, calculating the root sum of the squares of the I and Q data.
Next is a section of the spectrum analyzer called the IF section. The analog filters (video filters) in this section help to maintain a high frequency resolution, which enables the analyzer to separate two sinusoids into distinct responses. Many analyzers also utilize digital filters, which improve bandwidth selectivity. Bandwidth selectivity helps us determine power levels of unequal sinusoids in the form of a ratio of the 60-dB bandwidth to the 3-dB bandwidth.
Lastly, we have our display. The classic display includes 10 horizontal (frequency) and 10 vertical (amplitude) divisions. Using the spectrum analyzer front panel controls, the user can adjust center frequency, span, start frequencies, and stop frequencies.
Now that you know the classic components of a spectrum analyzer, you can dive deeper into the functions they perform. Check out part three of this blog series to learn about detector types and how spectrum analyzers gather data, and Spectrum Analysis Basics to learn more.