Specifications
16
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IF gain
Referring back to Figure 2-1, we see the next component of the block diagram
is a variable gain amplifier. It is used to adjust the vertical position of signals
on the display without affecting the signal level at the input mixer. When the
IF gain is changed, the value of the reference level is changed accordingly to
retain the correct indicated value for the displayed signals. Generally, we do
not want the reference level to change when we change the input attenuator,
so the settings of the input attenuator and the IF gain are coupled together.
A change in input attenuation will automatically change the IF gain to offset
the effect of the change in input attenuation, thereby keeping the signal at a
constant position on the display.
Resolving signals
After the IF gain amplifier, we find the IF section which consists of the
analog and/or digital resolution bandwidth (RBW) filters.
Analog filters
Frequency resolution is the ability of a spectrum analyzer to separate two
input sinusoids into distinct responses. Fourier tells us that a sine wave
signal only has energy at one frequency, so we shouldn’t have any resolution
problems. Two signals, no matter how close in frequency, should appear as
two lines on the display. But a closer look at our superheterodyne receiver
shows why signal responses have a definite width on the display. The output
of a mixer includes the sum and difference products plus the two original
signals (input and LO). A bandpass filter determines the intermediate
frequency, and this filter selects the desired mixing product and rejects
all other signals. Because the input signal is fixed and the local oscillator
is swept, the products from the mixer are also swept. If a mixing product
happens to sweep past the IF, the characteristic shape of the bandpass filter
is traced on the display. See Figure 2-6. The narrowest filter in the chain
determines the overall displayed bandwidth, and in the architecture of
Figure 2-5, this filter is in the 21.4 MHz IF.
So two signals must be far enough apart, or else the traces they make will fall
on top of each other and look like only one response. Fortunately, spectrum
analyzers have selectable resolution (IF) filters, so it is usually possible to
select one narrow enough to resolve closely spaced signals.
Figure 2-6. As a mixing product sweeps past the IF filter, the filter shape is traced on the display