Section K
Analog To Digital Converter (A34)
K.1
Introduction
This section describes the Analog to Digital Converter board and
includes a “silkscreen” of component locations.
The schematic diagram can be found in the Drawing Package and
the parts list can be found in Section VII. “Analog to Digital is
also referred to as ”A/D" or “A to D”. The A/D board is located
in the center control compartment.
K.2
Principles of Operation
An analog audio signal from the Analog Input board goes to the
A/D board where it is converted to a 12-bit digital audio signal
by an A/D chip. The rate of this conversion is 1.2 to 2.5 micro-
seconds depending on the transmitter frequency. The A/D con-
version process is synchronized with the RF signal so that PA
modules are switched on and off when the RF driver current
crosses through zero and the PA transistors are not conducting,
called the “zero crossing”. The digital audio signal from the A/D
is stored in latches.
The latch outputs go to the Modulation Encoder board where
they are used to turn on PA modules. The latch outputs also go
to the reconstruction audio circuit and to the big step sync circuits
on the A/D board. The reconstruction audio signal goes to the
envelope error circuit on the Controller board (A38). The big step
sync signal goes to the dither oscillator on the Analog Input
board.
The following description refers to the schematic diagram for the
Analog to Digital Converter board (drawing 839-7855-177).
Refer to SECTION V, Maintenance, for adjustment and pc board
maintenance procedures.
Refer to SECTION IV, Overall System Theory, for a block
diagram and overall descriptions of the audio and digital audio
sections of the transmitter.
K.3
Circuit Description
K.3.1
Converting a PA Sample to the A/D ENCODE
Pulse (T1, U29, Q9)
There are two RF sample inputs to the A/D converter board. One
is the Splitter Sample Frequency Input from the RF Splitter
(A15) on pins J3-1 and J3-2.
PA modules must be switched on and off when the RF drive
crosses through zero.
The RF input goes to the primary winding of wide-band toroidal
RF transformer T1. Resistor R18 and an L-C network with
components selected by section of DIP switch S1 provide adjust-
able, frequency-determined phase shift (refer to the Tuning/Fre-
quency change procedure in Section 5, Maintenance for infor-
mation on setting S1).
Schmitt Trigger U12C converts the RF input to TTL level pulses.
Diodes CR14 and CR15 limit the voltage at the Schmitt trigger’s
input to b0.7 and +4.3 Volts.
K.3.2
Frequency Divider (U29, Q9 )
The frequency output at TP6 is at the RF input frequency (from
J3 pin 1) if the jumper plug is installed between JP10 pins 5 and
6,. The output at TP6 is at one-half of the RF input frequency if
the jumper is installed between pins 1 and 2. The output at TP6
is at one-third of the RF input frequency if the jumper is installed
between pins 3 and 4.
The position of the jumper plug JP10 depends on the transmit-
ter’s operating frequency. Refer to the note on the A./D converter
schematic diagram or to the Frequency Determined Components
chart.
K.3.3
ENCODE Signal Pulse Width (Q9)
The timing diagram labeled A/D Converter Board Signals shows
the interaction of signals on this board. The signal on TP6 goes
through C106. The base of Q9 is held at about 0.7 V. The falling
edge of the pulse from TP6 causes Q9 to turn off. This allows the
collector of Q9 to increase in voltage. R78 and R79 charge up
the base of Q9, turn it on again and cause the collector to drop
0.3 V. The end result is a pulse at TP3. The length of this pulse
depends on the value of resistors R78 and R79. This pulse width
should be between 20 and 50 nanoseconds. This is the ENCODE
signal that goes into the A/D and starts the conversion process.
K.3.4
Analog to Digital Converter Circuit
K.3.4.1
Analog Input Circuit (U28)
The analog input signal (J4-10) to the A/D converter is the Audio
+ DC from the Analog Input board (actually the negative Audio
+ DC). The DC component determines the unmodulated trans-
mitter power output (“carrier” level) by turning on a constant
number of PA modules. The audio component amplitude modu-
lates the output by turning PA modules on or off to vary the
instantaneous RF output voltage.
Rev. X: 05-13-98
888-2297-002
K-1
WARNING: Disconnect primary power prior to servicing.
