6500-CX Hearing Aid Test System - Frye Electronics

stays up. The 100 pF cap holds it up if the FF1 is a little slow in getting up.
RAM\ is generated when (U3D) is armed by FF1 and the second pulse goes high. It
also causes the generation of the first CE\ pulse through (U3E) 74LS04 and (U4D)
74LS02.
The falling edge of the second pulse pulls FF1 low and terminates the R/C\ pulse.
Thus when the last pulse comes through, it passes through the enabled gates (U4B
and U4D) to generate the second and last CE\ pulse and starts the conversion process
on the digitizer board. The falling edge of this pulse sets FF2 low again, disarming
the gate.
The drive signal, RAM\, enables the RAM chips on the digitizer board to receive
information from the A/D converter. It is also used to latch through the data from the
signal generator RAM to generate the next programmed level. Examination of the
action of the flip-flops will show that the RAM\ clock is exactly one clock cycle later
than the cycle that caused an address change, making sure that the data will be solid
and stable.
4.4.5 Digital to Analog Converter
All of the latch lines on the DAC811 (U1E) except the WR\ input are tied down,
because the eight bit and four bit load features are not used in this design. A 12-bit
word is available all at once, so the single instruction RAM\ to load the data will
make a clean signal transition possible.
4.4.6 Low Pass Filter
This filter is used to remove the higher order frequency components above 8 kHz
that can cause possible difficulties with the signal being processed by the 6500-CX’s
power amplifier and the device under test. It is formed by a series of op-amps (U1J
and U1L) forming two 2-pole filter sections and one 2-pole filter, respectively.
4.5 Digitizer Board (060-0023-XX)
This board contains circuits to preamplify the incoming signals with prescaling
controlled by the 68000, a brick wall filter to reduce the high frequency components
above 8 kHz, a switch to allow either audio signals or battery current signals to be
measured, a sample and hold circuit, an A/D converter, RAM for holding the
sampled data in 12-bit words, a counter for deriving addresses for the RAM in data
collection, and logic circuits to control the board. A circuit is also included that
supplies a battery equivalent voltage and a current voltage converter that ranges up
to –5 volts for a current drain of 25 mA. A number of signals are supplied from the
Chapter 4: Circuit Description
35