Specifications

89
Can we conclude from this discussion that a harmonic mixing spectrum
analyzer is not practical? Not necessarily. In cases where the signal frequency
is known, we can tune to the signal directly, knowing that the analyzer will
select the appropriate mixing mode for which it is calibrated. In controlled
environments with only one or two signals, it is usually easy to distinguish
the real signal from the image and multiple responses. However, there are
many cases in which we have no idea how many signals are involved or
what their frequencies might be. For example, we could be searching for
unknown spurious signals, conducting site surveillance tests as part of a
frequency-monitoring program, or performing EMI tests to measure
unwanted device emissions. In all these cases, we could be looking for
totally unknown signals in a potentially crowded spectral environment.
Having to perform some form of identification routine on each and every
response would make measurement time intolerably long.
Fortunately, there is a way to essentially eliminate image and multiple
responses through a process of prefiltering the signal. This technique is
called preselection.
Preselection
What form must our preselection take? Referring back to Figure 7-4, assume
that we have two signals at 4.7 and 5.3 GHz present at the input of our
analyzer. If we were particularly interested in one, we could use a band-pass
filter to allow that signal into the analyzer and reject the other. However,
the fixed filter does not eliminate multiple responses; so if the spectrum is
crowded, there is still potential for confusion. More important, perhaps, is
the restriction that a fixed filter puts on the flexibility of the analyzer. If we
are doing broadband testing, we certainly do not want to be continually
forced to change band-pass filters.
The solution is a tunable filter configured in such a way that it automatically
tracks the frequency of the appropriate mixing mode. Figure 7-7 shows the
effect of such a preselector. Here we take advantage of the fact that our
superheterodyne spectrum analyzer is not a real-time analyzer; that is, it
tunes to only one frequency at a time. The dashed lines in Figure 7-7 represent
the bandwidth of the tracking preselector. Signals beyond the dashed lines
are rejected. Let’s continue with our previous example of 4.7 and 5.3 GHz
signals present at the analyzer input. If we set a center frequency of 5 GHz
and a span of 2 GHz, let’s see what happens as the analyzer tunes across this
range. As the LO sweeps past 4.4 GHz (the frequency at which it could mix
with the 4.7 GHz input signal on its 1
+
mixing mode), the preselector is tuned
to 4.1 GHz and therefore rejects the 4.7 GHz signal. Since the input signal
does not reach the mixer, no mixing occurs, and no response appears on the
display. As the LO sweeps past 5 GHz, the preselector allows the 4.7 GHz
signal to reach the mixer, and we see the appropriate response on the display.
The 5.3 GHz image signal is rejected, so it creates no mixing product to
interact with the mixing product from the 4.7 GHz signal and cause a false
display. Finally, as the LO sweeps past 5.6 GHz, the preselector allows the
5.3 GHz signal to reach the mixer, and we see it properly displayed. Note in
Figure 7-7 that nowhere do the various mixing modes intersect. So as long
as the preselector bandwidth is narrow enough (it typically varies from
about 35 MHz at low frequencies to 80 MHz at high frequencies) it will
greatly attenuate all image and multiple responses.