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CHAPTER 7
APPLICATION EXAMPLES
7.0 APPLICATION EXAMPLES
This chapter will emphasize signal processing applica-
tions. The corresponding 2920 software necessary to
implement the particular functions will be generated in
this chapter. Some of the functions demonstrated
include waveform generation, filtering, piecewise linear
approximation, and a complete spectrum analyzer
implemented on a single 2920 device.
7.1 Sweeping Local Oscillator
As an example of using digital processing techniques to
implement a typical 'analog circuit, consider the
development of a sweeping local oscillator (SLO). This
circuit is made up of three building blocks:
1) a sawtooth wave sweep rate generator, (SRG),
2)
a voltage-controlled oscillator, (VeO), and
3) a waveform modifier (to reduce harmonic content
of the yeO).
Each of these building blocks is discussed below, and
the final coding is developed and displayed.
AMP
1.0
I
fool
.1t----'1.sEC---~·1
Sweep Rate Generator-This subsystem controls the
minimum frequency of the yeO, its frequency range,
and the rate of change of frequency. It does so by pro-
ducing a sawtooth wave whose slope determines the rate
of change, whose voltage excursion is proportional to
the frequency range, and whose offset represents the
minimum of that range.
The sawtooth wave is simple to generate: continuous
decrementing of a register by a fixed value produces a
linear negative slope. When the register voltage changes
sign (crosses zero), a constant equal to the desired peak
amplitude of the sawtooth is added. This is accom-
plished using a load (LDA) instruction conditioned on
the sign bit (Section 4.2).
The first step is to generate the slope constant. In
this example, assume that four sweeps per second
are desired. The resulting calculations are given in
Figure 7-1.
TIME
ASSUME 1= 76.8 USEC (192 INSTRUCTIONS AT A 10 MHz CLOCK RATE)
THEN
51
=
lIT
=
1.2288' 10. 3
IN BINARY
51
=
[0.10100001]' 2. 11
WHICH EQUALSsl
=
[(0.101
+
(0.001)'2. 5 ]'2. 11
REWRITTEN AS sl
=
[KP5
+
KPl '2.
5
]'2. 11
WHICH, IN 2920 ASSEMBLY LANGUAGE, IS WRITTEN
OPCODE
DEST
SOURCE
~
sr--;
~
ADD
Sl
KPl
LOA
Sl
Sl
SUB
Hl
Sl
LOA
DAR:
Hl
ADD
Hl
KP4
SHIFT
COND
ROO
R05
Rll
ROO
ROO
LOl
CNDS
Figure 7-1. Sawtooth Sweep Rate Generator
7-1
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