re-trigger and the output will remain 1. If the pulses stop or the
frequency is too low the one-shot output will go low.
K.3.6.3
A/D Converter Monitor Circuit (U13-A)
The signal DAV at TP5 comes from the A/D converter after each
conversion. The period of this signal is 1.2 to 2.5 to microsec-
onds. This is the input to pin 2 of one-shot U13-A. The output of
the one-shot is labeled A/D ERROR-L. The one-shot output
pulse is 3.6 microseconds long. As long as the DAV signal is
present, the one-shot will continue to re-trigger and the output
will remain 1. If the pulses stop or the frequency is too low, the
one-shot output will go low.
K.3.6.4
Conversion Error Indicator (U14-B, U11, DS1)
The signals CLK ERROR-L and A/D ERROR-L go into AND
gate U15-A. The output of this gate is the signal CONVERSION
ERROR-L at TP8. If the signal CONVERSION ERROR-L goes
low, it triggers one-shot U14-B. The output of this one-shot will
be a low pulse at pin 12 for 10 microseconds. This low propagates
through U15-B and U15-C and causes the signal CLEAR-L to
go low. This ensures that any error will cause the bits that are
driving the PA modules to be cleared for at least 10 microsec-
onds.
Operational amplifier U11B functions as a comparator with the
inverting input level set at about +1.4 Volts by the R28-R29
voltage divider. If there is an error, then U15 pin 6 will have a
lower voltage then pin 5. U11 pin 7 will be -15 Volts. This will
cause bicolor LED DS1 to indicate RED. If there is no error, then
U15 pin 6 will have a higher voltage than pin 5. U11 pin7 will
be +15 Volts. This will cause bicolor LED DS1 to indicate green.
The signal CONVERSION ERROR-H goes to the LED board
A32 and is high if an error has occurred.
K.3.7
Big-Step Sync Circuit
The big step sync circuit produces a pulse each time a big step
occurs in the transmitters output. In the DX 10 and the DX 25 a
big step occurs whenever a change occurs in any of the six most
significant bits from the A/D chip. In the DX 50 a big step occurs
whenever a change occurs in any of the seven most significant
bits from the A/D chip.
The big step sync pulse synchronizes the “dither” oscillator on
the analog input board. Also the big step sync adds a small
amount of voltage to the analog input signal that goes into the
A/D chip. This small amount of voltage is to minimize undesired
switching back and forth between the big steps.
K.3.7.1
Big Step Sync Circuit D/A Converter (U22)
A digital to analog converter is used to convert the bits of the
digital audio signal back into an analog signal. Switch S2 deter-
mines the number of bits that go into the D/A. Section A of S2
is between pins 1 and 4. Section B of S2 is between pins 2 and
3. Section A and B are open for DX25 operation so the 6 MSB’s
go to the D/A. Section A of S2 is closed in the DX50 operation
so the 7 MSB’s go to the D/A.
The D/A converter output, at U22 pin 4, is a current level which
goes into R35 and produces a 0 to -1 Volt signal.
K.3.7.2
Amplifier Stage (U24, U25, U26)
The output of the A/D converter is amplified by U24 and U25.
The gain of the amplifier stage is slightly over 5. U24 is an
operational amplifier and U25 is a current amplifier that’s used
to increase the current output capability of the amplifier to drive
the next stage without degrading the stepped waveform.
The low-pass filter R53-C93 removes any high frequency com-
ponents. U26 is a buffer stage. The output of U26 is added,
through R70, to the analog input signal. The output of U26 also
drives a differentiator.
K.3.7.3
Differentiator and Buffer (U27)
R55 and C92 form a differentiator which produce a pulse each
time a transition occurs. The pulses can be observed at the output
of U27 pin 6 or at R63. The output signal from U27 is the big
step sync pulses which go to the dither oscillator circuit on the
Analog Input Board (A35).
K.3.8
Reconstructed Audio Circuit
An audio signal is reconstructed on the A/D board by sending
the bits into D/A converters U8. Another audio signal comes
from the envelope detector at the transmitter’s output. These two
audio signals are compared at the envelope error circuit on LED
Board A32. (Refer to Section Q, LED Board, for a discussion of
the envelope error circuit).
K.3.8.1
Reconstructed Audio Circuit D/A converter (U8)
The 12-bit digital audio signal is converted back to an analog
signal by D/A converter chip U8, operational amplifier U9 and
current amplifier U10. The unfiltered D/A converter circuit’s
output is at U10 pin 8 and is available for viewing at test point
TP9. Voltage divider R31-R30 isolates the D/A converters output
from any loading by test equipment. The unfiltered output at TP9
varies between 0 and 5 Volts when observed using a high-imped-
ance probe .
K.3.8.2
Reconstruction Filter (L1, L2, L3, C47, C48, C49)
The D/A converters output is stepped. The reconstruction filter
is a low-pass filter which passes the audio components and
removes the higher frequency components in the steps. This
smooths the output (a D/A reconstruction filter is also sometimes
referred to as a “smoothing” filter). The response of this filter is
approximately the same as the output network’s response, thus
allowing the audio from the two filters to be compared in the
envelope error circuit LED board A32. Operation amplifier U11
isolates the filter output from any load variations.
K.3.8.3
Grounds A, AA, B and Chassis
There are four grounds being used on this board. Ground A is
used in the digital signal sections. Ground B is used in the analog
signal sections. Ground AA is used in the reconstructed audio
section. Chassis ground is used where the two RF samples
sections. Ground A and ground B are connected through JP1 as
instructed on the data sheet for the AD1671. Ground A and
ground AA are connected through JP2. Chassis ground is con-
nected to the transmitter chassis through mounting hole 2 by
using JP3.
Care must be used when connecting test equipment to avoid
ground loops or other ground connections through test equip-
Section K - Analog to Digital Converter (A34)
Rev. X: 05-13-98
888-2297-002
K-3
WARNING: Disconnect primary power prior to servicing.
