Intel 2920 Design Handbook page 89

APPLICATION EXAMPLES
Voltage Controlled Oscillator-The
VCO is
developed similarly, except that the decrement value is
not a constant, but rather is determined by a scaled ver-
sion of the SRG input waveform. Assuming a sweep
from DC to 1.3 KHz, an offset would be determined by
the low frequency and the scaling factor by the high fre-
quency. The net result would be a sawtooth wave with
a
period varying as a function of time.
This high frequency sawtooth wave (DC to 1.3 KHz) has
significant harmonic content, which will be reflected by
the sampling frequency harmonics and will cause alias-
ing distortion of the SLO output. Digital filters cannot
be used to compensate for this because they are also
susceptible t? the aliasing components.
Some means must therefore be found to reduce the har-
monic content of this signal. One approach is to filter
the
veo
output using an external filter. This would
involve additional hardware, plus many extra instruc-
tions for
110
and A/D conversion. An alternative is to
shape the waveform in the time domain to look more
like the desired sinusoid.
Investigation of the Fourier Transforms of various sym-
metric waveforms reveals that a trapezoidal waveform
can be adjusted so that even harmonics are eliminated
and the first odd harmonic is the fifth. This adjustment
is done by selecting the top of the trapezoid to be
2/3
of
the peak of a corresponding triangle wave. The flow
diagram to accomplish this transformation is shown in
Figure 7-2.
The final, correctly assembled 2920 program is shown in
Figure 7-3. This listing gives the correct assembly code
with comments, the hexadecimal object code, a symbol
table with a list of errors or warnings, and RAM/ROM
sizes. This program requires 18 instructions and 5 RAM
locations.
7.2 Piecewise Linear Logarithmic
Amplifier
The purpose of the logarithmic amplifier is to amplify
low level signals with a higher gain than high level
signals to reduce the overall output dynamic range. Fur-
thermore, the log amplifier described here provides an
example of the use of 2920 code to implement a
piecewise linear approximation of a general function.
The input dynamic range of the amplifier is 50 dB with
an error of less than 1 dB for signal levels to -30 dB.
The transfer characteristic is shown in Figure 7-4.
'5~"
" "
I
-0.5
V'VVV
-
t
..
..
t
..
1.0
rJ\/\I\I\J .
-1.0
Figure 7-2. Waveform Shaper
7-2