Agilent Spectrum Analysis Basics - Agilent Technologies

Next let’s see to what extent harmonic mixing complicates the situation.
Harmonic mixing comes about because the LO provides a high-level drive
signal to the mixer for efficient mixing, and since the mixer is a non-linear
device, it generates harmonics of the LO signal. Incoming signals can mix
against LO harmonics, just as well as the fundamental, and any mixing
product that equals the IF produces a response on the display. In other words,
our tuning (mixing) equation now becomes:
f sig = nf LO ±f IF
where
n = LO harmonic
(Other parameters remain the same as previously discussed)
Let’s add second-harmonic mixing to our graph in Figure 7-3 and see to what
extent this complicates our measurement procedure. As before, we shall first
plot the LO frequency against the signal frequency axis. Multiplying the
LO frequency by two yields the upper dashed line of Figure 7-3. As we did
for fundamental mixing, we simply subtract the IF (3.9 GHz) from and add
it to the LO second-harmonic curve to produce the 2 – and 2 + tuning ranges.
Since neither of these overlap the desired 1 – tuning range, we can again argue
that they do not really complicate the measurement process. In other words,
signals in the 1 – tuning range produce unique, unambiguous responses on
our analyzer display. The same low-pass filter used in the fundamental mixing
case works equally well for eliminating responses created in the harmonic
mixing case.
2+
15
2xLO
Signal frequency (GHz)
10
5
1+
2–
LO
1–
0
4 5 6
LO frequency (GHz)
7
Figure 7-3. Signals in the “1 minus” frequency range produce
single, unambiguous responses in the low band, high IF case
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