Intel 2920 Design Handbook page 95

APPLICATION EXAMPLES
the signal energy at any frequency. Practically speaking,
it is nearly impossible to design a complex tunable
analog filter which can cover a 10 to 1 range of frequen-
cies·, especially near DC. When tuning is required, even
digital implementation becomes very complex and hard-
ware inefficient. It is therefore easier to realize the
equivalent of the scanning filter by sweeping the signal
past a fixed tuned narrow and bandpass filter. This is
accomplished by the system illustrated in the block
diagram of Figure 7-10.
The input signal spectrum is first shaped by the input
low pass filter (LPF) (in addition to the anti-aliasing
filter shaping) to avoid overlapping spectral com-
ponents after mixing. The filtered signal then is
multiplied (mixed) by the sweeping local oscillator
(SLO) to generate upper and lower sidebands centered
about the SLO frequency. The spectral characteristics
of the system are shown in Figure 7-11. The bandpass
filter (BPF) is centered at 4.5 KHz with a 100 Hz band-
width. Figure 7-lla shows the filter characteristics.
The SLO sweeps from 1.3 KHz to 4.3 KHz as seen in
Figure 7-11 b.
After mixing, the upper and lower sidebands are seen in
Figures 7-llc and 2d for SLO frequencies of 1.3 and 4.3
KHz, respectively. Only the upper sideband is of
interest, however, as it is swept across the BPF and the
signal energy is extracted. When the SLO is at 1.3 KHz,
the BPF is looking at the high band (3.2 KHz). As the
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INPUT
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SIGNAL
T'------'
MIXER
SLO frequency increases, at a SLO frequency of 4.3
KHz, the BPF "sees" the signal energy at 200 Hz (4.5
KHz minus 4.3 KHz).
The block diagram shows that the BPF output is then
passed through a full wave rectifier (FWR) and lowpass
filter to extract the envelope from the 4.5 KHz carrier,
which is generated when signal energy is present. The
resulting signal spectrum is centered at DC and shown in
Figure 7-lle.
The sweep output provides a horizontal sweep voltage
for an X-Y display. The purpose of the delay shown in
Figure 7-10 is to synchronize the sweep output with the
amplitude response output. This delay should approx-
imately equal the propagation delays of the BPF and
output LPF.
I/O-The input to the spectrum analyzer is the analog
signal to be analyzed. There are two outputs identified
in Figure 7-10. These include the frequenoy sweep out-
put which becomes the horizontal axis drive to a scope,
the VCO output, the the BPF amplitude response (both
linear and logarithmic) output which becomes the ver-
tical axis drive to the scope.
The block diagram shows the basic functions or sub-
systems which must be implemented to operate the spec-
trum analyzer. In the digital implementation there must
VERTICAL
OUTPUT
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SAWTOOTH
OSCILLATOR
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Figure 7-10. Block Diagram of a Spectrum Analyzer
7-8