Agilent Spectrum Analysis Basics - Agilent Technologies

Let’s assume that we have some idea of the characteristics of our signal,
but we do not know its exact frequency. How do we determine which is the
real signal? The image-shift process retunes the LO fundamental frequency
by an amount equal to 2f IF /N. This causes the Nth harmonic to shift by 2f IF .
If we are tuned to a real signal, its corresponding pair will now appear at the
same position on screen that the real signal occupied in the first sweep. If we
are tuned to another multiple pair created by some other incorrect harmonic,
the signal will appear to shift in frequency on the display. The ESA spectrum
analyzer shifts the LO on alternate sweeps, creating the two displays shown
in Figures 7-17a and 7-17b. In Figure 7-17a, the real signal (the 14 – mixing
product) is tuned to the center of the screen. Figure 7-17b shows how the
image shift function moves the corresponding pair (the 14 + mixing product)
to the center of the screen.
Figure 7-17a. 14 – centered
Figure 7-17b. 14 + centered
Figure 7-17. Alternate sweeps taken with the image shift function
Let’s examine the second method of signal identification, called image
suppression. In this mode, two sweeps are taken using the MIN HOLD
function, which saves the smaller value of each display point, or bucket,
from the two sweeps. The first sweep is done using normal LO tuning values.
The second sweep offsets the LO fundamental frequency by 2f IF /N. As we
saw in the first signal ID method, the image product generated by the correct
harmonic will land at the same point on the display as the real signal did
on the first sweep. Therefore, the trace retains a high amplitude value. Any
false response that shifts in frequency will have its trace data replaced by
a lower value. Thus, all image and incorrect multiple responses will appear
as noise. This is shown in Figure 7-18.
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