Motorola Saber Theory Maintenance 68p81044c05 O Manual

							can be controlled by the coded/clear selector switch,
or by “strapping” on a per-channel basis using the
Radio Service Software. Regardless of the
coded/clear selector switch position or the channel
strapping, the radio will receive both coded and clear
transmissions.
4. PRINTED CIRCUIT BOARDS AND FLEXIBLE 
CIRCUITS
a. General
Functional circuits in the SABER radio are con-
tained on the main radio circuit board, and, in SABER
II and III radios, the display circuit board. Flexible cir-
cuits are used to eliminate discrete wiring.
b.  Main Radio Board
The main radio board is a six-layer printed circuit
board containing the rf, i-f, frequency generation, con-
trol, power, and audio portions of the radio. With the
exception of the circuit modules, most of the boards
components are mounted on its top side.
c. Display Board
The display board is a four-layer (six-layer in mid-
band radios) printed circuit board containing the dis-
play circuitry, additional control circuitry,  and the
EEPROM for the radio.
d. Flexible Circuits
The SABER radio uses several flexible printed 
circuits for interconnection. These include:
•  PTT/Controls Flex
•  Speaker/Microphone Flex
•  Universal Connector Flex
•  LCD interconnect Flex (SABER II and III radios)5.  BATTERIES
The rechargeable nickel-cadmium batteries avail-
able for the SABER radio are listed in Table 1. Battery
choice is governed by duty cycle, operating time, and
maximum height and weight desired.
Table1. SABER Radio Batteries
MODEL BATTERY CHARGE
NUMBER CAPACITY  TIME
NTN4537C LIGHT 1 HR
NTN4538C MEDIUM 1 HR
NTN4592C LIGHT 1 HR
NTN4593C MEDIUM 1 HR
NTN4595C ULTRA-HIGH 1 HR
NTN4596C ULTRA-HIGH 1 HR
NTN4657A MEDIUM 1 HR
NTN4671A MEDIUM 1 HR
NTN4992A ULTRA-HIGH 1 HR
NTN5155A MEDIUM 1 HR
NTN5156A MEDIUM 1 HR
2 
						

							3
BATTERY CHARGING
1. AVAILABLE CHARGERS
Available chargers include a single-unit desk top
charger, a single-unit Porta-Pocket charger, and multi-
unit chargers that may be mounted on a wall or a
bench. The multi-unit chargers will charge up to six
nickel-cadmium batteries at once.
The single-unit desktop and multi-unit chargers are
rapid-charge models, while the porta-pocket is a slow-
charge model. The slow-charge model will charge any
of the batteries, with or without the radio attached, in
16 hours. The rapid-charge models will charge any of
the batteries in approximately one hour.
Refer to the ACCESSORIES page at the beginning
of this manual for a list of the available battery charg-
ers and their applications. For further information, con-
tact your Motorola sales representative.
2. BATTERY CONSTRUCTION
(See Figure 1)
The SABER rapid-charge battery has four charger
contacts, two of which receive the charging current. A
third contact connects the internal capacity resistor
(R
C) to the charger, automatically setting the charging
current output to match the capacity of the battery.
The fourth contact connects an internal thermistor to
the charger. The thermistor senses battery tempera-
ture and automatically controls the charger output to
permit maximum charger output without overheating
the battery.
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							a. Obtain a Radio Housing Adapter (Motorola part
number RTL-4225A) from your nearest Area Parts
Office.
b. Connect the appropriate 20-ohm or 11-ohm load
resistor (see note above) between the gold (+) ter-
minal and a solder lug (-) screw and nut of the
housing adapter.
c. Connect a voltmeter across the load resistor and
slide a fully charged battery onto the housing
adapter.
d. Monitor the voltmeter as the battery discharges
through the load resistor, until the voltage is 6.0
volts. 
e. Disconnect battery from the housing adapter
(resistor load) when the cell pack reaches 6.0
CAUTION
Discharging the battery down to 4.0 volts can
cause permanent cell pack damage.
4volts.
f. Recharge the battery to a complete charge. This
will require a 1-hour rapid charge followed by a 
16-hour standard charge.
g. Reattach the battery to the housing adapter (resis-
tor load) and measure the elapsed time until the 
						

							THEORY OF OPERATION
1. INTRODUCTION
This section of the manual provides a functional
description of the SABER radio. First, basic functions
are discussed, with each circuit and its relationship to
other parts of the radio described. Then, detailed cir-
cuit descriptions are given for each circuit and module
used in the radio.
2. BASIC FUNCTIONAL DESCRIPTION
a.  DC Voltage Distribution
(See Figure 2)
Operating power for the radio is derived from a
7.5-volt battery. This 7.5 volts (B+), is fed, via the uni-
versal connector flex, to P4, pins 4 and 6, on the radio
board. B+ is next routed through 5-amp fuse  F900, to
pin 11 of J2. Then, via the PTT/controls flex, B+ is
applied one side of the on/off switch, S800. Raw B+
from the battery (identified on the schematic by the

À symbol) is also applied directly to the power
amplifier (PA), U202, pins 6 and 12 (vhf) or pins 6 and
8 (uhf). 
When the radio is turned on, the voltage sources
required to operate the various stages of the radio are
distributed as shown on the main board schematic
diagram in the applicable service manual.
SWITCHED B+ from S800 enters the main radio
board via interconnect J2, pin 4. From this point it is
distributed throughout the radio to most of the ICs, to
OPTION B+ on the universal connector (through
R433), to the display board (via jack J1, pin 4), to mul-tifunction LED CR40 (through Q405), and to regulator
U103. SWITCHED B+ (source and destination) can be
identified by the Á symbol. Note that SWITCHED B+
is also provided to the emitters of Q204 (base bias to
the PA), Q1 (which is connected to the 5-volt regulator
contained within U100), Q206 (provides RX 5V), and
Q203 (provides TX 5V). Additionally, in the uhf radio,
SWITCHED B+ is also supplied to the collector of
Q200.
No.1A REGULATED 5V (mid-band and vhf radios
only) originates at inductor L5, and is identified by the
 symbol. No.1A REGULATED 5V is distributed to
the following ICs: U101, pins 1, 18, and 39; U700, pins
1 and 22; and U102, pin 14.
No.1 REGULATED 5V (uhf radios only) originates
at U100, pin 14, and is identified by the  symbol.
No.1 REGULATED 5V is distributed to the following
ICs: U101, pins 1, 18, and 39; U700, pins 1 and 22;
U102, pin 14; U301, pin 4; and U200, pin 1.
No.2 REGULATED 5V, identified by the à sym-
bol, is provided by regulator U103 (pin 2). This voltage
is distributed to various circuits and ICs within the
radio, including pins 2, 4, 19, 28, 29, 32, and 64 of
microcomputer U400, and pin 7 of jack J2.
TX 5V, identified by the Ä symbol, is provided by
U201s internal TX/RX 5V regulator. This voltage is
distributed to many of the transmitter circuits, including
the internal microphone biasing, the temperature-
5
Figure 2.  DC Voltage Distribution Block Diagram
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							sensing circuit of PA U202, and (uhf radios only) the
base of Q200.
RX 5V, identified by the Å symbol, is also provid-
ed by U201s internal TX/RX 5V regulator. This voltage
is distributed to the following circuits: pin 22 of U201;
pin 7 of U1 and pin 2 of T1 (vhf) or pin 3 of U2  (uhf).
No.1B REGULATED 5V (vhf radios only) origi-
nates at U100, pin 14, and is identified by the Æ
symbol. No.1B REGULATED 5V is distributed to
U200, pin 1, and U301, pin 4.
b.  Frequency Generation and 
Distribution Circuits (U300, U301)
The SABER radio uses a coherent synthesizer
(traditional voltage-controlled oscillators [VCO] and
phase-locked loop [PLL]) with state-of-the-art designs
to generate frequencies that support a dual-conver-
sion radio with unlimited capabilities in the mid-band,
uhf, and vhf ranges with operating splits of up to 30
MHz.
The rf frequency generation circuits include the
reference oscillator, U301, and the synthesizer, U300.
The synthesizer has three major subassemblies: oscil-
lator, controller (PLL/divider), and buffer/amplifier. To
provide superior system performance, each sub-
assembly is broken down into a separate TX and RX
section. The synthesizer (U300, pin 1) uses the 16.8
MHz signal from the reference oscillator (U301, pin 3)
in conjunction with its own internal dividers and VCOs
to generate and synthesize the following frequencies:
·TX carrier (U300, pin 14),
·local oscillator (1st injection) (U300, pin 15),
·2nd local oscillator (both high- and low-side 
injection) (U300, pin 32),
·2.1 MHz (U300, pin 17), and
·300 kHz (internal only).
The audio in the SABER synthesizer is simultane-
ously modulated at two different ports. The audio is
first conditioned (pre-emphasis and limiting) externally
by audio filter U101, then sent, via the VCO MOD and
REF MOD lines, to two different ports on the synthe-
sizer module, U300.
The reference modulation port (U300, pin 19)
accepts low-frequency audio (70Hz) and modulation is pro-
duced by varying the control voltage of the VCO in
proportion to the high frequency audio input. The dual-
modulation scheme allows for a flat deviation
response for all desirable signals that readily support
Motorolas PL channels and sensitive SECURENET
radios.The following generic (TX or RX) description of the
SABER synthesizer is used because of the symmetri-
cal hardware and operational systems for both the TX
and RX sections. The VCO becomes active and gen-
erates an output frequency, which is compared to the
desired frequency. If the frequencies differ, an error
ramp voltage is generated to the VCO that brings the
output frequency to the desired frequency. When the
output and desired frequencies match, the VCO is
locked. The locked state of the synthesizer can be
observed externally by looking for zero volts on the
LOCK DETECT line of the synthesizer (U300, pin 16).
c.  Antenna Switch and Bias Circuits
Steering of rf between receiver and transmitter,
and standard and remote antennas, is accomplished
electronically by a 4-port PIN diode switch located in
the filter/detector/switch module, U203. This module
also contains a directional coupler and power detector
that supply the system with an indication of transmit
output power. Low-pass filters are also included to
attenuate transmitter and receiver (mid-band only)
harmonics.
d.  Display Circuitry (SABER II and III radios only)
The display circuitry for the SABER II and III
radios includes the liquid-crystal display (LCD) and the
display circuit board. This board, mounted on the
radios front shield, provides SABER II and III radios
with additional and expanded capabilities. Two basic
types of display boards are available: the standard 8k
board, and the optional 2k board (not available on
mid-band models). Both boards have four ICs in 
common: 
·An MC68HC11 microprocessor, U502. This IC is
also called the COPE (control of peripheral 
electronics).
·An electrically-erasable, programmable read-only
memory (EEPROM), U501. This ICs memory size
is either two kilobytes (2k board) or eight kilobytes
(8k board).
·A liquid-crystal display (LCD) driver, U504.
·A serial-to-parallel shift register, U503.
The 8k board has one additional IC, the dual-tone,
multi-frequency (DTMF) generator, U505.
The display board communicates with the radio
board via the 8-wire LCD interconnect flexible cable
(J8); this cable provides both power and signal paths.
There is also (8k board only) a 3-wire connection to
the speaker/microphone flex (J9) that is used as a
DTMF signal path.
e. SECURENET Circuitry 
(SECURENET radios only)
The SECURENET module (U900) requires an
encryption key, or key variable, to perform its
encode/decode function. This key is a digital
sequence that is loaded into the radio, via the radio’s
6 
						

							er/prescaler (U300), and the signalling IC (U700).
The CORE uses its serial peripheral interface
(SPI) subsystem to program these ICs. The micro-
processor lines that make up the SPI subsystem
include the MISO (pin 28), MOSI (pin 29), and
SCK (pin 30) lines. In conjunction with the SPI, the
CORE uses dedicated output ports to select each
individual IC. Examples of when the ICs can be
programmed include channel changes, volume
changes, transitions from receive to transmit, and
transitions from transmit to receive.
·
Serial Bus. The SABER radio can have more than
one processor in its system; these multiple pro-
cessors communicate over the serial bus, which
runs at a rate of 9600 baud. The CORE processor
communicates on the serial bus via its serial com-
munications interface (SCI) subsystem (RD1, pin
22 and TD1, pin 27) and the BUSY line (pin 14).
The BUSY line indicates whether the serial bus is
active; when the BUSY line is low, the bus is
active. Examples of when the serial bus can be
active include switch changes, channel changes,
and transitions from receive to transmit and trans-
mit to receive.
·
Analog-to-Digital (A/D) Subsystem. The CORE
processor has four A/D inputs for processing ana-
log data. The voltage from the volume potentiome-
ter (R800) is fed to one of the A/D lines (PE5, pin
56). The OPTION SELECT line (PE7, pin 62) is
the second A/D input, and the battery voltage
(PE4, pin 54) is the third input. The last input
(PE6, pin 60) is the SIDE CONTROL line, which
has the PTT switch (S803), monitor switch (S805),
and RAT 1 (S806) and RAT 2 (S807) switches
connected to it. When the PTT switch is pressed,
this line is grounded; the PTT switch has the high-
est priority, followed by the monitor switch, RAT 1
switch, and RAT 2 switch.
·
Frequency Switch. The CORE processor reads
the output of the frequency switch (S823) via four
input lines (PE0 through PE3; pins 53, 55, 59, and
61 respectively). The emergency switch (S801) is
also connected to the frequency switch. When the
emergency switch is pressed, all four input lines
are grounded. If the radio is turned on while the
emergency switch is pressed, the radio cannot
power up because it does not have a valid chan-
nel on which to power-up.
·
PL Encoding. The PL encoder is part of the audio
filter IC (U101), but is controlled by the CORE pro-
cessor. The CORE processor feeds (pin 39) a
pulse train to the audio filter IC (U101, pin 33) dur-
ing tone PL encoding; the frequency of the pulse
train is 12 times the desired tone PL frequency.
For digital PL encoding, U101 is sent bursts of six
pulses of every DPL transition.
·
PL Decoding. The PL filter and hard limiter are universal connector, from a hand-held key variable
loader (such as the T3010BX DVP Keyloader, which
is suitable for all radios with the DVP algorithm). In
order for two SECURENET radios to communicate
with each other in the secure mode, both must have
the same encryption key loaded.
3. DETAILED CIRCUIT DESCRIPTION
The circuit descriptions contained in the following
paragraphs are intended to help the service technician
understand the signal processing in various parts of
the radio. Refer to the complete schematic diagram in
the applicable service manual when repairing a radio.
a. DC Switching
In the receive mode, after a dekey, channel
change, or at the end of a power-up sequence, the
microcomputer, U400, starts a receiving sequence.
The R/T line is set to receive (RX = 1).
The following voltages determine the options
selected via pin 7 of the universal connector: 1.235V =
external speaker/microphone, 2.5V = public safety
microphone, and 3.735V = external antenna only.
When the R/T line is set to receive (1), the transmit
automatic level control IC, U201, switches the
filter/detector/switch (U203) PIN diodes to enable the
rf from either the standard antenna or the remote
antenna to the receiver front end (for mid-band and
vhf radios).
In uhf radios, if the standard antenna path is to be
activated, then Q207 is saturated; if the remote anten-
na is selected, then Q208 is saturated. In either case,
the current is directed to pin 10 of U203, supplying all
the current/voltage for the receiver front end.
In the transmit mode (PTT switch pressed), pin 60
of microcomputer U400 is grounded. This sets the
reprogramming of the chip set (audio filter IC,
digital/analog converter IC, and the signalling IC) in
motion without changing the R/T line status (RX = 1;
TX = 0). The internal/external microphone is selected
and enabled. The microphone itself will not be
enabled until the TX 5V is active. The last chip pro-
grammed is the audio filter IC, U101; this will change
the status of the R/T line to transmit (0). Once the R/T
line status changes, the transmit automatic level con-
trol IC, U201, changes several outputs simultaneously,
providing the required TX 5V to the transmitter 
circuits.
b.  CORE Microcomputer (U400)
The control of radio electronics (CORE) micro-
computer, U400, directly controls many of the SABER
radios functions. The major functions of the CORE
include:
·
IC Programming. The CORE processor is respon-
sible for programming the radios support
ICs, including the audio filter (U101), the digital-
to-analog (D/A) converter (U200), the synthesiz-
7 
						

							also part of the audio filter IC (U101). The demod-
ulated, filtered, hard-limited signal is sent (U101,
pin 28) over the PL DECODE line to the CORE
processor (pin 41). At the instant that the CORE
wants to sample this line, it sends (pin 39) a latch-
ing pulse, via the PL SAMPLE/CLK line, back to
U101 (pin 33). This pulse latches the sample,
which can then be read by the CORE processor.
The frequency of the pulse is 1071 Hz for TPL or
537 Hz for DPL.
·
MDC Encode.The MDC encoder is part of the sig-
nalling IC (U700), but is controlled by the CORE
processor. The CORE sends pulses to the sig-
nalling IC that clock the signalling ICs encoder.
The encoded MDC signal contains instantaneous
frequencies of 1200 Hz and 1800 Hz.
The signals and levels to be expected at various
pins of the CORE microcomputer (U400) are as
follows:
* 1 Vss Ground
* 2 Mode B 5V
* 3 Mode A Ground
4 PD6 5V
5, 6 No connection Dont care
* 7 E XTAL 7.3728MHz signal (high-
impedance)
8, 9 No connection Dont care
* 10 XTAL 7.3728MHz signal
11 AFIC select 0V when AFIC is being 
programmed; 5V otherwise
12 No connection Dont care
13 XMIT power ind. 0V or 5V
14 Busy 5V = Serial bus inactive;
0V = Serial bus active
15 Squelch 5V = Squelch detect;
0V = No squelch detect
16 Lock detect 5V = Synthesizer not locked;
0V = Synthesizer locked
17 Fast squelch 5V = Squelch detect;
0V = No squelch detect
18 Option switch 0V or 5V
* 19 Reset 0V = Reset mode;
5V = Otherwise
* 20 XIRQ 5 Volts
* 21 IRQ 5V
22 Serial bus data 5V = Bus inactive; Toggles 
between 0V and 5V at 9600 
baud when active
23, 24 No connection Dont care
* 25 Vss Ground
26 No connection Dont care
27 Serial bus data 5V = Bus Inactive; Toggles 
between 0V and 5V at 9600 
baud when active
28 MISO 5V =ICs being programmed; 
Toggles between 0V and 5V
at 115.2 kHz when ICs are 
not being programmed
29 MOSI 5V =ICs being programmed; 
Toggles between 0V and 5V
at 115.2 kHz when ICs are 
not being programmed
30 SCK 5V =ICs being programmed; 
Toggles between 0V and 5V
at 115.2 kHz when ICs are 
not being programmed
31 No connection Dont care
Pin No. Function Signal
* 32 Vdd 5V
* 33 Vss Ground
34 PASF Dont care
35 Tone clock out Toggles between 0V and 5V
when MDC or tone signalling 
is being transmitted
36 DTMF clock out 0V or 5V
* 37 AFIC watchdog 5V = Normal operating mode;
disable 0V = Radio reset in progress
38 No connection Dont care
39 PL sample clock Toggles between 0V and 5V 
at 1071Hz when TPL decode 
is enabled; 537Hz when DPL 
decode is enabled. 12 times 
TPL frequency (in transmit)
40 MDC reference Dont care
41 PL decode Toggles between 0V and 5V
42 No connection Dont care
43 Limiter in Toggles between 0V and 5V 
in receive mode
44 No connection Dont care
45 Adapt 5V = During channel change;
0V = Otherwise
46 D/A IC select 0V = When D/A IC is being 
programmed; 5V = Otherwise
47 Synthesizer IC 0V = When synthesizer IC is 
select being programmed;
5V = Otherwise
48 Prescaler IC 0V = When prescaler IC is 
select being programmed;
5V = Otherwise
49 Signalling IC 0V = When signalling IC is 
select being programmed;
5V = Otherwise
50 Red LED 5V = LED on; 0V = LED off
51, 52 No connection Dont care
53 Freq. select 0 This is the least-significant bit 
of the frequency switch. 0V or 5V
54 Battery voltage 1/2 of the battery voltage
55 Freq. select 1 0V or 5V
56 Volume sense 0V through 5V
57, 58 No connection Dont care
59 Freq. select 2 0V or 5V
60 Side control 0V = PTT switch pressed;
Å1.23V = Monitor button 
pressed
61 Freq. select 3 This is the most-significant bit 
of the frequency switch. 0V or 5V
62 Option select 5V = No option connected;
Pin No. Function Signal
Note:Ground = 0 volts
* = Needed for processor to power-up correctly.
8 
						

							c.  Digital-to-Analog (D/A) Converter (U200)
The digital-to-analog (D/A) converter, U200, is a
multifunction CMOS integrated circuit containing two
7-bit D/A converters, one 4-bit D/A converter, six con-
trol outputs, two SPDT transmission gates, and a
microcomputer interface.
The output (U200, pin 11) of the first 7-bit D/A con-
verter supplies the tuning voltage for the reference
oscillator, U301. When the R/T line is low (0V), the
output of the second 7-bit D/A converter is routed, via
an internal switch, to pin 9. This provides the power
control reference voltage for the TX ALC IC, U201,
during transmit operation.
In vhf radios only, when the R/T line is high (5V),
the second D/A converters output is switched to pin 8,
providing tuning voltage for the vhf 2-pole filter, U1. A
combination of resistors R218 and R219, and a
microcomputer-interface-controlled switch (connected
internally between pins 15 and 16 of U200) allows
extension of the 2-pole tuning voltage range beyond
that of the 7-bit D/A converter.
In mid-band and uhf radios there is no receive tun-
ing adjustment, so the second 7-bit D/A converter is
used only for transmit.
The 4-bit D/A converter is not used in SABER
radios, but its four pull-down resistors are used. These
resistors, which connect internally to U200, pins 4
through 7, are connected externally to the BCD fre-
quency switch, S823, and U400.
Three of U200s control outputs are used in
SABER radios:
·Pin 2 is the REMOTE ANTENNA ENABLE line; 
a high output on this line enables the remote
antenna.
·Pin 3 is the low-power range enable line (normally
low); a high on this line enables the very-low
power tuning range.
·Pin 20 is the clock shifter enable line; a low on this
line enables the clock shifter.
d.  Antenna Switch (U201, U203)
(1) Mid-Band, VHF
When the PTT switch is pressed, or the OPTION
SELECT line is brought to 0 Vdc, the microcomputer
(U400) sends data to U101, which sets the R/T line
low (0V). A logic low on U201, pin 9, causes U201, pin
28 to go high (Å6Vdc). This voltage is applied to the
anode side of a series-connected pair of PIN diodes,
internal to U203 (pin 8), which control the
transmit/receive rf steering. The cathode side of the
diode pair is connected to U203, pin 9.
During transmit operation, the PIN diodes are for-
ward biased and a low-impedance path connects
U203, pin 1, to the selected antenna. When biased for
transmit operation, the voltage dropped between pins
8 (+) and 9 of U203 should be two diode drops orapproximately 1.5 volts.
During receive operation, the R/T line goes high
(5V), and U201, pin 28 (anode bias), should go to
approximately 0Vdc. U201, pin 26 (cathode bias),
should pull-up to approximately 7.5V, reverse-biasing
the T/R PIN diode pair, resulting in a low-impedance rf
path from U203, pin 10, to the selected antenna.
The standard/remote antenna switch position is
determined by the voltage on the OPTION SELECT
line (U400, pin 62). When the OPTION SELECT line is
at 5V or 1.24V, the microcomputer commands U200 to
bring the REMOTE ANT ENABLE line (U201, pin 23)
low (0V), selecting the standard antenna. When U201,
pin 23, is low, U201, pin 24, is also low, and U201, pin
20, is high (7.5V). This reverse-biases the PIN diode
pair that makes up the standard/remote antenna
switch in U203 (U201, pin 24, is the anode; U201, pin
20, is the cathode). When the diodes are reverse-
biased, a low-impedance rf path exists between U203,
pin 14 (standard antenna) and the transmitter or
receiver. Additional filtering is provided in vhf radios by
capacitors C206, C207, and C208, and inductor L201,
and in mid-band radios by capacitors C207 and C208,
and inductor L201.
Setting the OPTION SELECT line to 3.74V or 2.5V
causes the microcomputer to instruct U200 to bring
the REMOTE ANT ENABLE line high (5V). This caus-
es U201, pin 24, to go high and U201, pin 20, to go
low, forward-biasing U203s standard/remote antenna
switch PIN diodes, and forming a low-impedance path
from U203, pin 12, to the receiver or transmitter.
When the PIN diodes are forward-biased, the volt-
age dropped between pins 12 (+) and 13 of U203
should be two diode drops or approximately 1.5 volts.
In vhf radios, capacitors C222, C223, C224, and
C225, and inductor L205 are for rf decoupling; C229 is
a dc block and C241 is a matching element. In mid-
band radios, capacitors C222, C223, and C224, and
inductor L205 are for rf decoupling; C229, C231, and
L206 provide additional filtering on the remote antenna
path. Mid-band radios also have a reverse-bias circuit,
consisting of capacitor C250, inductors L250 and
L254, and diode CR250. This circuit prevents the PIN
diodes from turning on and starting to generate har-
monics at critical levels.
Proper operation of bias circuits in U201 is depen-
dent on correct voltages being present on the TX 5V
and RX 5V regulators, and resistors R211 through
R213. Proper operation of U203 is dependent on cor-
rect installation of the 4205577Q01 grounding clip.
(2) UHF
Although the filter/detector/switch module is func-
tionally equivalent in both vhf and uhf radios, the elec-
trical realization of the two 4-port PIN diode rf switches
are somewhat different, and require slightly different
biasing circuits.
9 
						

							As in the vhf models, the TX/RX antenna switch-
ing is controlled by the R/T line (U201, pin 9). When
the R/T line is high (5V), the RX 5V regulator in U201
is on and supplying current to receiver U2. The supply
current for the RX 5V regulator is drawn from U203,
pin 10 (receive path PIN diode cathode). Current flow
through the receive path PIN diode causes a 
low-impedance rf path from U203, pin 9, to the select-
ed antenna. When the R/T line is high, the voltages 
at pin 26 of U201 and pin 7 of U203 should be approx-
imately 7.5Vdc. 
When the R/T line goes low, U201, pin 13, should
go high (7.5V), turning off Q206 and bringing pin 10 of
U203 high (7.5V). The receive path PIN diodes in
U203 are now reverse-biased, turning off the receive
rf path. With the R/T line in the low state, U201, pin
26, goes low (Å4.7Vdc), allowing dc current to flow
through the selected transmit path PIN diodes, form-
ing a low-impedance path from the selected antenna
to U203, pin 1.
Selection of the standard or the remote antenna is
determined by the state of switching transistors Q207
and Q208. When the REMOTE ANT ENABLE line is
low (the standard antenna has been selected), U201,
pin 20, is high (7.5V) and Q208 is turned off, causing
U203, pin 11, to go low. When U201, pin 20, is high,
U201, pin 17, goes low (0V). This turns on Q207,
bringing U203, pin 8, high (7.5V) and selecting the
standard antenna (U203, pin 14).
When the REMOTE ANT ENABLE line goes high
(5V), U201, pin 20, goes low and U201, pin 17 goes
high (7.5V), turning off Q207 and turning on Q208.
U203, pin 11, is now high (7.5V), and the remote
antenna (U203, pin 12) is selected.
When the radio is transmitting, the voltage
dropped between the selected antenna enable (U203,
pin 8 or 11) and the TX SINK line (U203, pin 7) should
be about 2.5V. The receive sink line (U203, pin 10)
should be high (7.5V).
When the radio is receiving, the voltage drop from
the selected antenna enable (U203, pin 8 or 11) to
receive sink line (U203, pin 10) should be about 1.0V.
The TX SINK line (U203, pin 7) should be high (7.5V).
Resistor R225 is necessary for proper RX 5V reg-
ulator power-up, C62 is an audio frequency bypass
capacitor, and C222 through C225 are rf bypass
capacitors.
Operation of the switching circuits in U201
depends on proper operation of the TX 5V and RX 5V
regulators, and resistors R212 and R219. Proper
operation of U203 is dependent on correct installation
of the 4205577Q01 grounding clip.
e.  Power Detector Circuit (U200, U203)
The detector circuit in U203 provides a dc voltage
that is proportional to the transmitter power output.
The detector output voltage appears at U203, pin 5, in
10mid-band and vhf models, and U203, pin 4, in uhf
models. Normally, this voltage should range from
2.4Vdc to 4.0Vdc. Bias for the detector is supplied to
U203, pin 6 (all models).
During normal operation, U200, pin 3 is at 0Vdc
and diode CR201 is reversed-biased, allowing no cur-
rent flow, so all bias current is sourced from the TX 5V
regulator through R203 (mid-band and vhf) or R218
(uhf).
For low-power operation, U200, pin 3, goes high
(Å5V), forward-biasing CR201, and raising the bias
level at U203, pin 6. This alters the operating range of
the power detector circuit, allowing the system to oper-
ate at lower power levels.
On mid-band and vhf models, C230 and C217 rf
bypass the detector output and bias lines. On uhf
models, L210, C230, and C228 perform the same
function.
f.  Signalling IC (U700)
The signalling IC, U700, has analog and digital cir-
cuitry to aid the encoding and decoding functions pro-
vided by the radio. The CORE microcomputer, U400,
programs the signalling IC via the SPI interface.
•
MDC Encode. The signalling IC is fed a digital line
from U400 that controls U700s MDC encoder.
The encoding signal is filtered within U700 before
being sent to the audio filter IC, U101.
•
DOS Detection. The digital-operated squelch
(DOS) algorithm is in the CORE microcomputer,
but the support hardware is in U700. The radio
discriminator output from U100 (pin 31) is fed to
U700 (pin 31), where it is filtered and hard limited.
This hard limiter signal is then fed to an input cap-
ture port on U400 (pin 43).
The signals and levels to be expected at various
pins of the signalling IC, U700, are as follows:
1 5 volts 5V
2 Bias resistor Dont care
3 No connection Dont care
4 PASF 5V
5 MDC reference Dont care
6 No connection Dont care
7 DTMF clock in 0V or 5V
8 Trunking data in Dont care
9 Tone clock in Toggles between 0V and 5V
when MDC or tone signalling 
is being transmitted
10 Clock 5V =IC is being programmed; 
Toggles between 0V and 5V
at 115.2 kHz when IC is not 
being programmed
11 Data 5V =IC is being programmed; 
Toggles between 0V and 5V
at 115.2 kHz when IC is not 
being programmed
12 Chip select 0V when signalling IC is 
being programmed; 5V 
otherwise
13 No connection Dont care
14 TX mod out This line has the analog tone 
signalling during transmit of 
MDC or tone signalling
15, 16 No connection Dont care
17 Side tone out This line has the analog tone 
Pin No. Function Signal 
						

							g. Receiving
The signal received at the antenna is routed
through the filter/detector/switch module (U203) and
applied to the receiver rf front end module for filtering,
amplification, and mixing down to the first i-f.
(1) RF and 1st I-F
(a) Mid-Band (U1, U4)
In the mid-band receiver string, rf enters U1, the rf
filter/amplifier module. This module consists of a dis-
crete-component, 3-pole bandpass filter, designed to
cover either the low (68-84 MHz) or the high (74-88
MHz) bandsplit, and a common-base, transformer
feedback amplifier. The entire module provides about
9dB of gain.
The rf signal leaves U1 on pin 9 and enters the
receiver front-end module, U4, on pin 2. Within U4 the
signal first enters another 3-pole bandpass filter of the
same type as in U1 (about 2dB insertion loss). Next,
the signal moves into a double-balanced mixer, where
it is mixed with the local oscillator (LO) signal from
U300 (pin 15). The LO signal enters the mixer (U4, pin
3) at a level of +4.5 to +5dBm, and one i-f (53.55MHz)
above the channel (rf) frequency.
The resultant first i-f signal (53.55MHz) from the
mixer then passes through U4s i-f amplifier and crys-
tal filter before exiting the module (pin 4). There is a
loss of about 6 to 7dB through the mixer, the i-f ampli-
fier provides about 13B of gain, and the crystal filter
has about 3.5dB insertion loss. The crystal filter sup-
plies about 35dB of attenuation at the adjacent chan-
nel and 80dB of attenuation at the second image. The
bandwidth of the i-f signal leaving U4 is typically 10 to
19kHz, centered on 53.55MHz, with a typical gain of 0
to 2dB. The first i-f signal now moves through match-
ing components C49 and L2 before entering the i-f IC,
U100.
(b) VHF (U1, Q1, T1, U2, U4)
18 No connection Dont care
19 Ground Ground
20 2.1 MHz in 2.1 MHz signal
21 No connection Dont care
22 Digital Vdd 5V
23, 24 No connection Dont care
25 Limiter out Toggles between 0V and 5V 
in receive mode
26 HS bypass Dont care
27 LS bypass Dont care
28 No connection Dont care
29 VAG bypass Dont care
30 No connection Dont care
31 RX audio in The analog demodulated 
signal
32-34 No connection Dont care
35 Ground Ground
Pin No. Function SignalIn the vhf receiver string, rf enters U1, the 2-pole
filter module; this module has about 2dB of insertion
loss. For low split radios (136-150.8 MHz), a fixed-
tuned filter is used; for high split models (146-178
MHz), a tunable filter design is used. This filter has a
bandwidth of about 16MHz, and can be tuned to cover
the entire 146-178 MHz band, depending upon the
applied voltage from the digital/analog converter IC,
U200.
The rf signal leaves U1 (pin 11) and enters the rf
amplifier, Q3. This is a common-base, transformer
feedback amplifier, with the output signal leaving
through the center tap of transformer T1 (pin 1). The
amplifier provides about 10dB of gain over the entire
vhf frequency band.
The rf signal next passes through matching com-
ponents C54 and L52, and into the 5-pole filter, U2
(pin 1). A 32MHz-bandwidth, stripline filter module
containing some discrete components, U2 has a typi-
cal insertion loss of about 3.5dB.
After leaving U2 (pin 2), the rf signal enters the
front end module, U4 (pin 2), which is mounted directly
above U2. Within U4 the signal first moves into the
double-balanced mixer, where it is mixed with the local
oscillator (LO) signal from U300 (pin 15). The LO sig-
nal enters the mixer (pin 3) at a level of +4.5 to
+5dBm, and one i-f (53.55MHz) above the channel (rf)
frequency.
The resultant first i-f signal (53.55MHz) from the
mixer then passes through U4s i-f amplifier and crys-
tal filter before exiting the module (pin 4). There is a
loss of about 6 to 7dB through the mixer, the i-f ampli-
fier provides about 10B of gain, and the crystal filter
has about 3.5dB insertion loss. The crystal filter sup-
plies some 40dB of attenuation at the adjacent chan-
nel and 80dB of attenuation at the second image. The
bandwidth of the i-f signal leaving U4 is typically 12 to
16kHz, centered on 53.55MHz, with a typical gain of 0
to 3dB. The first i-f signal now moves through match-
ing components C49 and L2 before entering the i-f IC,
U100.
(c) UHF (U2)
After leaving FDS module U203 (pin 9), the rf sig-
nal enters the front end module, U2 (pin 2). Within U2
the signal first passes through a 30MHz-wide stripline
filter, an rf amplifier, and another 30MHz-wide stripline
filter. The rf amplifier supplies 10dB of gain over one of
two bandsplits: 403 - 470MHz or 450 - 520MHz. Next,
the rf signal enters a double-balanced mixer, where it
is mixed with the local oscillator (LO) signal from syn-
thesizer U300 (pin 15). The LO signal enters the mixer
(pin 4) at a level of +4.5 to +5dBm, and one i-f
(73.35MHz) below the channel (rf) frequency.
The resultant first i-f signal (73.35MHz) from the
mixer then passes through U2s i-f amplifier and crys-
tal filter before exiting the module (pin 4). There is a
11