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Today's RF test instruments are highly complex products and are therefore characterized by a wide variety of specifications. We have presented a three-part series on a wide array of RF instrument specifications. In part 1, we discussed generic RF
specifications that apply to every instrument. In part 2, we covered the specifications that are particularly important to RF continuous wave and vector signal generators. In this part, we describe specifications that apply to RF analyzers. Because there are two major types of RF analyzers, vector signal analyzers and spectrum analyzers, we first explain the distinctions between each type of instrument. Next, we provide an introduction to specifications that apply specifically to spectrum measurements. These specifications include dynamic range, averaging modes, and displayed average noise floor.
Types of RF Signal Analyzers
When capturing RF signals, engineers are typically interested in signal characteristics such as amplitude, frequency, and phase. Depending on the characteristics you need to analyze, you might choose either a spectrum analyzer or a vector signal analyzer. (Note that a third instrument, the vector network analyzer, also performs analysis, but we do not address it in this paper.) The spectrum analyzer is used to capture only the frequency and power information of an RF signal. The typical output of this instrument is a power versus frequency graph, which we observe later in this article.
A vector signal analyzer, on the other hand, is capable of the same measurements as a spectrum analyzer but with added capabilities. Because a vector signal analyzer captures the time-domain of the RF signal as well, you can acquire phase information to produce a constellation plot, shown in Figure 1.
1. Constellation plot illustrates phase and amplitude transitions of a communications Signal.
Traditionally, spectrum analyzers and vector signal analyzers used different instrument architectures. The traditional spectrum analyzer consists of basic components such as a tunable local oscillator (LO), a mixer, a bandpass filter, and a power sensor. To make spectrum measurements, the traditional analyzer simply tunes the LO to each frequency bin and makes a power-in-band measurement on the resulting signal. By sweeping through each frequency bin, a traditional spectrum analyzer can provide power information through a broad range of frequencies. Some spectrum analyzers still operate in this mode, known as the "swept" mode. The architecture of a traditional spectrum analyzer is shown in Figure 2.
2. Block diagram illustrates traditional spectrum analyzer architecture.
Many modern spectrum analyzers are designed in much the same way as a vector signal analyzer. This architecture, shown in Figure 3, uses a tunable LO mixed with the RF signal to produce a wideband intermediate frequency (IF). Rather than retuning the LO for each frequency bin, however, the analyzer simply performs a fast Fourier transform (FFT) on the IF signal. The FFT can provide power and frequency information that spans a broad frequency range with a single acquisition. As you might expect, the architecture of a more modern vector signal analyzer is actually quite similar to that of a vector signal generator. See Figure 3 for a simplified block diagram of a superheterodyne vector signal analyzer, which uses an IF signal.
3. Block illustrates modern vector signal analyzer architecture
As the diagram illustrates, an analog-to-digital converter (ADC) captures a broader spectrum of data. By acquiring a broader spectrum, you can capture the phase information of the RF signal as well. In addition, this enables a vector signal analyzer to perform spectrum measurements with a simple FFT calculation.
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