Agilent Spectrum Analysis Basics - Agilent Technologies

In examining Figure 7-5, we find some additional complications. The
spectrum analyzer is set up to operate in several tuning bands. Depending
on the frequency to which the analyzer is tuned, the analyzer display is
frequency calibrated for a specific LO harmonic. For example, in the 6.2
to 13.2 GHz input frequency range, the spectrum analyzer is calibrated for
the 2 – tuning curve. Suppose we have an 11 GHz signal present at the input.
As the LO sweeps, the signal will produce IF responses with the 3 + , 3 – , 2 +
and 2 – tuning curves. The desired response of the 2 – tuning curve occurs
when the LO frequency satisfies the tuning equation:
11 GHz = 2 f LO – 0.3
f LO = 5.65 GHz
Similarly, we can calculate that the response from the 2 + tuning curve
occurs when f LO = 5.35 GHz, resulting in a displayed signal that appears
to be at 10.4 GHz.
The displayed signals created by the responses to the 3 + and 3 – tuning curves
are known as in-band multiple responses. Because they occur when the LO is
tuned to 3.57 GHz and 3.77 GHz, they will produce false responses on the
display that appear to be genuine signals at 6.84 GHz and 7.24 GHz.
30
4+
Apparent location of an
input signal resulting from
the response to the 2 +
tuning curve
Apparent locations of
in-band multiples of an
11 GHz input signal
Signal frequency (GHz)
In-band
multiple
responses
25
20
15
11
10.4
10
7.24
6.84
5
4–
3+
3–
2+
2–
1+
1–
Band 4
Band 3
Band 2
Band 1
0
3
3.57 3.77 5.35 5.65
4
5
LO frequency (GHz)
6
7
Band 0
(lowband)
Figure 7-5. Tuning curves up to 4th harmonic of LO showing in-band multiple responses
to an 11 GHz input signal.
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