Specifications

10
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 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 above 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 we
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 device, its output
includes not only the two original signals, but also their harmonics and the
sums and differences of the original frequencies and their harmonics. If any
of the mixed signals falls within the passband of the intermediate-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 the
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 receive
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.
Input
signal
RF input
attenuator
Pre-selector, or
low-pass filter
Mixer
IF gain
Local
oscillator
Reference
oscillator
Sweep
generator
Display
IF filter
Log
amp
Envelope
detector
Video
filter
Figure 2-1. Block diagram of a classic superheterodyne spectrum analyzer