Specifications

14
Figure 2-4 illustrates analyzer tuning. In this figure, f
LO
is not quite high
enough to cause the f
LO
– f
sig
mixing product to fall in the IF passband, so
there is no response on the display. If we adjust the ramp generator to tune
the LO higher, however, this mixing product will fall in the IF passband at
some point on the ramp (sweep), and we shall see a response on the display.
Since the ramp generator controls both the horizontal position of the trace on
the display and the LO frequency, we can now calibrate the horizontal axis of
the display in terms of the input signal frequency.
We are not quite through with the tuning yet. What happens if the frequency
of the input signal is 8.2 GHz? As the LO tunes through its 3.9 to 7.0 GHz
range, it reaches a frequency (4.3 GHz) at which it is the IF away from the
8.2 GHz input signal. At this frequency we have a mixing product that is
equal to the IF, creating a response on the display. In other words, the
tuning equation could just as easily have been:
f
sig
= f
LO
+ f
IF
This equation says that the architecture of Figure 2-1 could also result in a
tuning range from 7.8 to 10.9 GHz, but only if we allow signals in that range to
reach the mixer. The job of the input low-pass filter in Figure 2-1 is to prevent
these higher frequencies from getting to the mixer. We also want to keep
signals at the intermediate frequency itself from reaching the mixer, as
previously described, so the low-pass filter must do a good job of attenuating
signals at 3.9 GHz, as well as in the range from 7.8 to 10.9 GHz.
In summary, we can say that for a single-band RF spectrum analyzer, we
would choose an IF above the highest frequency of the tuning range. We would
make the LO tunable from the IF to the IF plus the upper limit of the tuning
range and include a low-pass filter in front of the mixer that cuts off below
the IF.
Freq range
of analyzer
Freq range of LO
A
A
f
sig
f
LO
f
LO
– f
sig
f
LO
+ f
sig
f
LO
IF
f
f
f
Freq range
of analyzer
Figure 2-4. The LO must be tuned to f
IF
+ f
sig
to produce a response on the display