SPECTRUM ANALYZER; Chapter 2 Spectrum Analyzer

Chapter 2 
Spectrum Analyzer 
Fundamentals This chapter will focus on the fundamental theory of how a spectrum analyzer
works. While today s technology makes it possible to replace many analog circuits with modern digital implementations, it is very useful to understand classic spectrum analyzer architecture as a starting point in our discussion. In later chapters, we will look at the capabilities and advantages that digital circuitry brings to spectrum analysis. Chapter 3 will discuss digital architectures used in modern spectrum analyzers.

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

Figure 2-1 is a simplified block diagram of a superheterodyne spectrum analyzer. Heterodyne means to mix; that is, to translate frequency. And super refers to super-audio frequencies, or frequencies abov the audio range. Referring to the block diagram in Figure 2-1, we see that an input signal passes through an attenuator, then through a low-pass filter ( later w shall see why the filter is here) to a mixer, where it mixes with a signal from the local oscillator ( LO) . Because the mixer is a non-linear d vice, its output includes not only the two original signals, but also their harmonics and th sums and differences of the original frequencies and their harmonics. If any of the mixed signals falls within the passband of the int rmediate-frequency 

( IF) filter, it is further processed ( amplified and perhaps compressed on a logarithmic scale) . It is essentially rectified by the envelope detector, digitized, and displayed. A ramp generator creates the horizontal movement across th display from left to right. The ramp also tunes the LO so that its frequency change is in proportion to the ramp voltage. 

If you are familiar with superheterodyne AM radios, the type that receiv ordinary AM broadcast signals, you will note a strong similarity between them and the block diagram of Figure 2-1. The differences are that the output of a spectrum analyzer is a display instead of a speaker, and the local oscillator is tuned electronically rather than by a front-panel knob.

Since the output of a spectrum analyzer is an X-Y trace on a display, let's see what information we get from it. The display is mapped on a grid ( graticule) with ten major horizontal divisions and generally ten major vertical divisions. The horizontal axis is linearly calibrated in frequency that increases from left to right. Setting the frequency is a two-step process. First w adjust the frequency at the centerline of the graticule with the center frequency control. Then w adjust the frequency range ( span) across the full ten divisions with 

the Frequency Span control. These controls are independent, so if w change the center frequency, w do not alter the frequency span. Alternatively, w can set the start and stop frequencies instead of setting center frequency and span. In either case, we can determine the absolute frequency of any signal displayed and the relative frequency difference between any two signals. 

The vertical axis is calibrated in amplitude. We hav the choice of a linear scale calibrated in volts or a logarithmic scale calibrat d in dB. The log scale is used far more oft n than the linear scale because it has a much wider usable range. The log scale allows signals as far apart in amplitude as 70 to 100 dB ( voltage ratios of 3200 to 100,000 and power ratios of 10,000,000 to 10,000,000,000) to be displayed simultaneously. On the other hand, the linear 

scale is usable for signals differing by no more than 20 to 30 dB ( voltage ratios of 10 to 32) . In either case, we giv the top line of the graticule, the reference lev l, an absolute value through calibration techniques 1 and use the scaling per division to assign values to other locations on the graticule. Therefore, w can measure either the absolute value of a signal or the relative amplitude difference between any two signals. 

Scale calibration, both frequency and amplitude, is shown by annotation written onto the display. Figure 2-2 shows the display of a typical analyzer. Now, let s turn our attention back to Figure 2-1.

Figure 2-2. Typical spectrum analyzer display with control settings

1. See Chapter 4, Amplitude and Frequency Accuracy.