7.1 ANALOG CIRCUIT
A square wave at the desired frequency is generated by a
timer
internal
to the microprocessor.
This square wave is applied to
the input of switched capacitor filters for smoothing into a sine
wave.
The filter
section is programmed as Low
Pass
for
all
frequencies
except
8
KHz where it is configured for
Band
Pass.
Since
there are two filter output configurations, a
multiplexer
(switch) is used to route the sine wave to the attenuator block.
The
attenuator
is a voltage controlled amplifier set to
.5
dB
resolution
by an 8 bit Digital to Analog Converter
(DAC).
The
DAC,
which is processor driven, controls the attenuator in
the
Normal
Mode at a resolution of 5.0
dB.
The
.5
dB
resolution is
used only for calibration purposes in the Calibration Mode.
The
total
dynamic range of the attenuator is 112
dB.
required to
This range is
provide
the Normal Mode HL range as
well as
the
overhead
dB
range required for calibration per frequency.
Following
the
attenuator
is a
=10
dB
range
extension
block.
Range extension is accomplished by increasing the gain of the
OP-
Amp,
which is
sourced
by the attenuator, by 10
dB.
The range extender circuit applies the pure tone sine wave to the
input of
the output routing switches.
The switches
apply
the
signal
to either the left or right power amplifier, however, it
is
never
applied to both simultaneously.
The
left and
right
power amplifiers, in turn,
drive the left and right earphones.
7.2 DIGITAL CIRCUIT
The
heart of
the
digital
circuitry is a MC
68HC11A1
microcomputer.
This CPU was chosen for its flexibility and
low
power consumption.
Functions available from the device are (5) 8
bit programmable I/O ports,
(A/D),
an
8
bit analog to digital
converter
8
K Bytes of ROM, 256 Bytes of RAM, 512 Bytes of EEPROM,
Counter/Timer,
and Serial Port Interface.
As a result, there are
very few external
support devices required.
If there is an
external EPROM
(32 K Bytes), a programmable
timer,
and
miscellaneous decodes, line multiplex, etc.
The
CPU executes the commands stored in ROM and
EPROM.
These
instructions
are
the
result of a
front
panel
command in
conjunction
with
look-up tables, and such,
for
applying
the
pertinent
calibration data for the selected parameters.
Such
parameters
could be:
HL calibration for a
specific
frequency,
clock
frequencies for the switched capacitor
filters,
display
instructions,
or as simple as an ON/OFF command to a CMOS switch.
In
summary,
the digital circuitry in the GSI
17 controls
the
analog
and
display circuitry in such a manner
that
the
front
panel selections are processed and acted upon accordingly.
57
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