Specifications

13
We need to pick an LO frequency and an IF that will create an analyzer with
the desired tuning range. Let’s assume that we want a tuning range from
0 to 3 GHz. We then need to choose the IF frequency. Let’s try a 1 GHz IF.
Since this frequency is within our desired tuning range, we could have an
input signal at 1 GHz. Since the output of a mixer also includes the original
input signals, an input signal at 1 GHz would give us a constant output from
the mixer at the IF. The 1 GHz signal would thus pass through the system and
give us a constant amplitude response on the display regardless of the tuning
of the LO. The result would be a hole in the frequency range at which we
could not properly examine signals because the amplitude response would
be independent of the LO frequency. Therefore, a 1 GHz IF will not work.
So we shall choose, instead, an IF that is above the highest frequency to
which we wish to tune. In Agilent spectrum analyzers that can tune to 3 GHz,
the IF chosen is about 3.9 GHz. Remember that we want to tune from 0 Hz to
3 GHz. (Actually from some low frequency because we cannot view a 0 Hz
signal with this architecture.) If we start the LO at the IF (LO minus IF = 0 Hz)
and tune it upward from there to 3 GHz above the IF, then we can cover the
tuning range with the LO minus IF mixing product. Using this information,
we can generate a tuning equation:
f
sig
= f
LO
– f
IF
where f
sig
= signal frequency
f
LO
= local oscillator frequency, and
f
IF
= intermediate frequency (IF)
If we wanted to determine the LO frequency needed to tune the analyzer to
a low-, mid-, or high-frequency signal (say, 1 kHz, 1.5 GHz, or 3 GHz), we
would first restate the tuning equation in terms of f
LO
:
f
LO
= f
sig
+ f
IF
Then we would plug in the numbers for the signal and IF in the tuning
equation
2
:
f
LO
= 1 kHz + 3.9 GHz = 3.900001 GHz,
f
LO
= 1.5 GHz + 3.9 GHz = 5.4 GHz, or
f
LO
= 3 GHz; + 3.9 GHz = 6.9 GHz.
2. In the text, we shall round off some of the frequency
values for simplicity, although the exact values are
shown in the figures.