GW Instek GRF-1300A - Manual (Page 26)

GRF-1300A User Manual and Teaching Materials
A
N INTRODUCTION to SPECTRUM
ANALYZERS
Spectrum analyzers are one of the most important
instruments for RF microwave measurements. Being familiar
with spectrum analyzers in general is very important for
operating high frequency microwave equipment or for
performing communication measurements. In addition, being
familiar with the basic operating principals will allow you to
quickly understand other related test equipment. In this chapter,
we will briefly introduce the basic working principles of the
spectrum analyzer. After understanding the basic working
principles, you will find that a spectrum analyzer can be a
handy tool to use.
Broadband Receiver
The principal function of a spectrum analyzer is to convert
the input signal frequency down to a frequency (band) that
detection circuitry can handle. For example, a 2.4GHz signal
needs to be down-converted to several MHz before the
Detection & Display unit can process the signal. Therefore a
spectrum analyzer must be able to reduce the frequency band
down to several MHz. The first half of a spectrum analyzer is
called the radio frequency module and its task is to reduce the
input signal frequency. A mixer and a bandpass filter are used to
decrease the frequency (they can raise the frequency as well).
The mixer is a component with three ports: two inputs and one
output. Assume that the two input frequencies on input port are
f
RF
and f
LO
respectively, and then the output frequency will be
f
IF.
f
IF
is made of two signals of different frequencies (f
LO
- f
RF
and
f
LO
+ f
RF
) that appear on the output port at the same time. One
signal is the sum of the input signals and the other is the
difference. Determining which of the IF signals that will be used
depends on the system and subsequent bandpass filter design.
As for why the three ports are named after RF, LO, IF, they are
just the conventional terms that are used.
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