Specifications

In Chapter 2, we used a mathematical approach to conclude that we needed

a low-pass filter. As we shall see, things become more complex in the situation

here, so we shall use a graphical approach as an easier method to see what is

happening. The low band is the simpler case, so we shall start with that. In

all of our graphs, we shall plot the LO frequency along the horizontal axis

and signal frequency along the vertical axis, as shown in Figure 7-2. We know

we get a mixing product equal to the IF (and therefore a response on the

display) whenever the input signal differs from the LO by the IF. Therefore,

we can determine the frequency to which the analyzer is tuned simply by

adding the IF to, or subtracting it from, the LO frequency. To determine our

tuning range, then, we start by plotting the LO frequency against the signal

frequency axis as shown by the dashed line in Figure 7-2. Subtracting the

IF from the dashed line gives us a tuning range of 0 to 3 GHz, the range that

we developed in Chapter 2. Note that this line in Figure 7-2 is labeled “1

–

”

to indicate fundamental mixing and the use of the minus sign in the tuning

equation. We can use the graph to determine what LO frequency is required

to receive a particular signal or to what signal the analyzer is tuned for a

given LO frequency. To display a 1 GHz signal, the LO must be tuned to

4.9 GHz. For an LO frequency of 6 GHz, the spectrum analyzer is tuned

to receive a signal frequency of 2.1 GHz. In our text, we shall round off

the first IF to one decimal place; the true IF, 3.9214 GHz, is shown on the

block diagram.

Now let’s add the other fundamental-mixing band by adding the IF to the

LO line in Figure 7-2. This gives us the solid upper line, labeled 1

, that

indicates a tuning range from 7.8 to 10.9 GHz. Note that for a given LO

frequency, the two frequencies to which the analyzer is tuned are separated

by twice the IF. Assuming we have a low-pass filter at the input while

measuring signals in the low band, we shall not be bothered by signals in

the 1

frequency range.

–

LO frequency (GHz)

Signal frequency (GHz)

–IF

+IF

Figure 7-2. Tuning curves for fundamental mixing in the

low band, high IF case