Intel 2920 Design Handbook page 97

APPLICATION EXAMPLES
AMP (8)
AMP (b)
AMP (d)
INPUT
SIGNAL
1..--+-
veo
--+---
4.3
I
I
43
SAMPLING
SPURIOUS RATE
/
FILTER~R~ES~P~ONs:S~E~~~~~ L
10 11 12
13
FREQ
kHz
FREQ
FREQ
14 FREQ
Figure 7-12. Aliasing Analysis of Aliasing Components
Anti-Aliasing Filter-The basic requirement of the
anti-aliasing filter is to assure that out-of -band input
signal components at the filter output are at least 50 dB
down (based on dynamic range specification of 49 dB)
before entering the passband of the input digital
lowpass filter shown in Figure 7-10. From Figure 7-12 it
is evident that with a 13 KHz sampling frequency (cor-
responding to a full 2920 program and a 10 MHz clock),
the aliasing components must be below -50 dB at 3.2
KHz or 9.8 KHz from the sampling frequency. There-
fore, the anti-aliasing filter attenuation characteristics
are: relatively little rolloff by 3.2 KHz
(1
dB) and 50 dB
by 9.8 KHz. Filter curves readily available in the
literature (see also Chapter 5) show that this would
require a 5 pole 0.5 dB ripple Chebyshev, or equivalent.
Note that this filter is only needed if the input signal has
significant frequency components above about 7 KHz.
If a controlled signal is to be processed by the spectrum
analyzer (such as sine waves or narrow-band signals), no
anti-aliasing filter is needed.
7-10
7.4.4 Complete Spectrum Analyzer Assembly
Listing
The spectrum analyzer program listed in Figure 7-13
was coded in a structure form, with each functional
block coded separately and the blocks arranged to
follow the signal paths shown in the block diagram of
Figure 7-10. This was done for clarity in describing the
program. It is not necessary to implement the code one
functional block at a time or in any specific order as
long as the relationships between the inputs and outputs
of the functional blocks remain unchanged. In fact, it is
usually more efficient to program the 2920 in a less
structured form. For example, because each functional
block is executed in its entirety before proceeding to the
next functional block, it was not possible to execute all
input and output instructions simultaneously with
digital instructions. To take advantage of the fact that
analog and digital instructions can execute simultan-
eously, portions of the program could be rearranged,
and these analog instructions combined with digital
instructions, thus reducing the program length.