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SPECTRUM ANALYSER WITH TRACKING GENERATOR

Most spectrum analysers today come with an inbuilt tracking generator. We
explained how the analyser sweeps or scans between two frequencies determined
by the operator. Suppose we had an oscillator that was synchronised with the
spectrum analyser so that when the analyser was tuning through 140MHz, the
oscillator outputs a signal on 140MHz. As the analyser scans, the oscillator follows
it. When the analyser is on 141MHz, so is the oscillator. As the analyser moves from
140-150MHz (or whatever chosen span) the oscillator moves with it. Such an
oscillator is called a Tracking Generator.

Suppose the tracking generator was set to have an output power of 0dBm (one
milliwatt). What would you see on the display of the spectrum analyser? No matter
what frequency the analyser is scanning through the tracking generator is sending
to the analyser input, a constant signal level of 0dBm. You would see a straight line
on the display.

Figure 39-8. The Spectrum analyser
Figure 39-8 shows the arrangement just discussed. We have connected the RF
input of the spectrum analyser to the tracking generator output using a patch lead.
We will presume the output of the tracking generator is set to 0dBm. Let's call this
our reference level.

What we see on the analyser display is a straight line as shown and it is at a power
level of 0dBm. Does not seem like we have achieved much yet. What if we were to
remove the patch lead and connect a frequency selective device like a filter? The
filter would attenuate some of the frequencies going between the tracking generator
and the analyser. We would know how frequencies were being attenuated, as we
would no longer see a straight line on the display. The display would now show a
selectivity graph of the device under test (DUT). I show this setup in Figure 39-9. The
tracking generator has been connected to a pair of notch filters allegedly tuned to
145MHz.
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