Intel 2920 Design Handbook page 96

APPLICATION EXAMPLES
INPUT LOWPASS FILTER
AMP (8)
HIGH RESOLUTION
BANDPASS FILTER
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4.5
FREQ (kHz)
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1.3
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4.3
FREQ (kHz)
1.3
FREQ (kHz)
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4.3
FREQ (kHz)
AMP ( )
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wm REen"ER
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LPF OUTPUT
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FREQ (kHz)
Figure 7-11. Spectrum Analyzer Design Frequency Domain Analysis
also be an input anti-aliasing filter, sample and hold,
AID converter, and the corresponding output DI A con-
verter and reconstruction filter. The functions in the
figure are implemented by the 2920 signal processor
program.
7.4.3
Sampled Data System Considerations
An expansion of the frequency axis in Figure 7-11 to
include the sampling frequency at 13 KHz shows the
first order aliasing spectra as seen in Figure 7-12. From
this figure the limitations and requirements for filter
rolloff, bandwidths, and center frequencies become
clearer.
Bandpass Filter-The location of the bandpass filter
is determined by the input lowpass filter bandwidth and
rolloff (Figure 7-12a) and the aliased spectrum of the
lower sideband resulting when the SLO is at 4.3 KHz
(Figure 7-12c). The BPF must have enough rolloff to
7-9
eliminate both the baseband and alia sed out-of-band
signal components that are present. Analysis shows that
a 3 pole Bessel filter will suffice if the input LPF is
designed properly. The Bessel filter also has ideal tran-
sient response (no overshoot), so that the resulting out-
put will not have overshoot and ringing.
Input LPF-This filter determines not only the base-
band (centered about DC) spectrum but also that of the
aliased lower sideband of the SLO. It was found that a 4
pole, 2 zero filter provides adequate rolloff to keep
spurious signal (and aliased) components of signifi-
cant amplitude (less than 48 dB down) out of the BPF
passband.
Output LPF-This filter is used to remove the har-
monic content of the FWR output (and the associated
aliased components) before the signal is converted back
to analog and output.