The purpose of this article is to provide you with a basic overview of a spectrum analyzer or signal analyzer. You may want to learn more about other topics related to spectrum analysis, you can visit the spectrum analyzer web page. Here we will focus on the basic principles of spectrum analyzer operation. Although today's technology has made it possible to replace many analog circuits with modern digital implementations, it is still very beneficial to start with the classic spectrum analyzer architecture. In the future we will also look at the capabilities and advantages that digital circuitry brings to spectrum analyzers, and discuss the digital architecture used in modern spectrum analyzers.

Figure 2-1 is a simplified block diagram of a superheterodyne spectrum analyzer. "Heterodyne" refers to mixing, which is the conversion of frequencies, while "hyper" refers to superaudio frequencies or the frequency range above audio. From the figure we see that the input signal first passes through an attenuator, then a low-pass filter (you will see why the filter is placed here later), reaches the mixer, and then is combined with the signal from the local oscillator (LO). phase mixing.

Figure 2-1. Block diagram of a typical superheterodyne spectrum analyzer

Since the mixer is a nonlinear device, its output contains not only the two original signals, but also their harmonics, as well as the sum and difference signals of the original signal and its harmonics. If any of the mixed signals fall within the passband of the intermediate frequency (IF) filter, it is further processed (amplified and possibly logarithmically compressed). The basic processing procedures include envelope detection, low-pass filtering and display. The ramp generator produces horizontal movement from left to right on the screen, and it also tunes the local oscillator so that the change in local oscillator frequency is proportional to the ramp voltage.

If you are familiar with a superheterodyne AM radio that receives ordinary amplitude modulation (AM) broadcast signals, you will find that its structure is very similar to the block diagram shown in Figure 2-1. The difference is that the output of the spectrum analyzer is a screen rather than a speaker, and its local oscillator is tuned electronically rather than by a front panel knob.

Since the output of the spectrum analyzer is the XY trace on the screen, let's see what information we can get from it. The display is mapped on a dial consisting of 10 horizontal grids and 10 vertical grids. The horizontal axis represents frequency, with scale values ​​increasing linearly from left to right. Frequency setting is usually divided into two steps: first adjust the frequency to the center line of the dial through the center frequency control, and then adjust the frequency range (span) across 10 grids through the frequency span control. These two controls are independent of each other, so when you change the center frequency, the span does not change. Also, we can set the start frequency and stop frequency instead of setting the center frequency and span. In either case, we can determine the absolute frequency of any displayed signal and the relative frequency difference between any two signals.

The vertical axis scale is divided by amplitude. You can choose a linear scale scaled in voltage or a logarithmic scale scaled in decibels (dB). The logarithmic scale is more often used than the linear scale because it reflects a larger range of values. The logarithmic scale can simultaneously display signals whose amplitudes differ by 70 to 100 dB (voltage ratios from 3200 to 100,000 or power ratios from 10,000,000 to 10,000,000,000), while the linear scale can only be used with amplitudes that differ by no more than 20 to 30 dB (voltage ratios). than 10 to 32) signal. In both cases, we will use calibration technology 1 to give the level of the highest row on the dial, that is, the absolute value of the reference level, and determine other positions on the dial based on the proportion corresponding to each small cell. value. In this way, we can measure both the absolute value of the signal and the relative amplitude difference of any two signals.

The scale values ​​for frequency and amplitude are annotated on the screen. Figure 2-2 is a typical spectrum analyzer display.

Figure 2-2. Typical spectrum analyzer display with parameters set

Now let's return our attention to the spectrum analyzer components shown in Figure 2-1.

RF attenuator

The first part of the analyzer is the RF attenuator. Its role is to ensure that the signal is at the appropriate level when entering the mixer, thereby preventing overload, gain compression and distortion. Because the attenuator is a protection circuit for the spectrum analyzer, it is usually set automatically based on a reference level value, but the attenuation value can also be manually selected in steps of 10 dB, 5 dB, 2 dB, or even 1 dB. Figure 2-3 shows an example of an attenuator circuit with a maximum attenuation value of 70 dB in 2 dB steps.

The DC blocking capacitor is used to prevent the analyzer from being damaged by DC signals or DC bias of signals. However, it will attenuate low-frequency signals and increase the lowest available starting frequency of some spectrum analyzers to 9 kHz and 100 kHz. or 10 MHz.

In some analyzers, an amplitude reference signal can be connected as shown in Figure 2-3, which provides a signal with precise frequency and amplitude for periodic self-calibration of the analyzer.

Figure 2-3. RF attenuator circuit

Low pass filter or preselector

The purpose of the low-pass filter is to prevent high-frequency signals from reaching the mixer. This prevents out-of-band signals from mixing with the local oscillator and producing unwanted frequency response at the intermediate frequency. Microwave spectrum analyzers or signal analyzers replace low-pass filters with preselectors, which are adjustable filters that filter out signals at frequencies other than those of interest. In Chapter 7, we will introduce in detail the purpose and method of filtering input signals.

Analyzer tuning

We need to know how to tune a spectrum analyzer or signal analyzer to the frequency range we want. Tuning depends on the center frequency of the IF filter, the frequency range of the local oscillator, and the frequency range that allows external signals to reach the mixer (allowing them to pass through the low-pass filter). Of all the signal components output from the mixer, there are two signals with the largest amplitude that we are most interested in. They are the signal components produced by the sum of the local oscillator and the input signal and the difference between the local oscillator and the input signal. If we can make the signal we want to observe an IF higher or lower than the local oscillator frequency, then one of the desired mixing components will fall within the passband of the IF filter and will then be detected and produce an amplitude on the screen. response.

In order to tune the analyzer to the desired spectral range, we need to select the appropriate local oscillator frequency and intermediate frequency. Assuming that the required tuning range is 0 to 3.6 GHz, the next step is to select the IF frequency. If we choose an IF of 1 GHz, which is within the desired tuning range, we get a 1 GHz input signal, and since the output of the mixer contains the original input signal, the 1 GHz input from the mixer The signal will have a constant output at the intermediate frequency. So no matter how the local oscillator is tuned, a 1 GHz signal will pass through the system and give a constant amplitude response on the screen. The result is a blank area within the frequency tuning range that cannot be measured because the signal amplitude response in this area is independent of the local oscillator frequency. Therefore, the IF frequency of 1 GHz cannot be selected.

That is, we should select the mid-frequency at a higher frequency than the tuning band. In Keysight's X-Series signal analyzers, which are tunable to 3.6 GHz, the first local oscillator frequency range is 3.8 to 8.7 GHz, and the selected IF frequency is approximately 5.1 GHz.

Now we want to tune from 0 Hz (since the instrument with this structure cannot observe the 0 Hz signal, it is actually from a low frequency) to 3.6 GHz.

By choosing the local oscillator frequency to start at the intermediate frequency (LO - IF = 0 Hz) and tune upward to 3.6 GHz above the intermediate frequency, the LO - IF mixing component can cover the required tuning range. Using this principle, the following tuning equation can be established:

If you want to determine the local oscillator frequency required to tune the analyzer to a low, mid, or high frequency signal (such as 1 kHz, 1.5 GHz, or 3 GHz), you first transform the tuning equation to obtain fLO:

Figure 2-4. To produce a response on the display, the local oscillator must be tuned to fIF + fs

Figure 2-4 illustrates the analyzer tuning process. In the figure, fLO is not high enough to cause the fLO -fsig mixing component to fall within the IF passband, so there is no response on the display. However, if the ramp generator is adjusted so that the local oscillator is tuned to a higher frequency, then at some point on the ramp (sweep) the mixing product will fall within the IF passband and we will see a response on the display.

Because the ramp generator simultaneously controls the horizontal position of the trace on the display and the local oscillator frequency, the horizontal axis of the display can be calibrated based on the frequency of the input signal.

We haven't quite solved the tuning problem yet. What happens if the input signal frequency is 9.0 GHz? When the local oscillator is tuned in the 3.8 to 8.7 GHz range, when it reaches the intermediate frequency (3.9 GHz) away from the 9.0 GHz input signal, it gets a mixing product with a frequency equal to the intermediate frequency and generates a response on the display. In other words, the tuning equation easily becomes: