Motorola Gp350 6880904z07 0 Manual

							 
2-2 
6880904Z07-O August, 1996 
Intrinsically Safe Radio Information GP350 Portable Radios Service Manual
Repair of FMRC Approved Products
 
Repair
 
A repair constitutes something done internally to the unit 
that would bring it back to its original condition Approved by 
FMRC. A repair should be done in an FMRC Approved facil-
ity.
Items not considered as repairs are those in which an action 
is performed on a unit which does not require the outer cas-
ing of the unit to be opened in a manner which exposes the 
internal electrical circuits of the unit. You do not have to be 
an FMRC Approved Repair Facility to perform these actions. 
Relabeling
 
The repair facility shall have a method by which the replace-
ment of FMRC Approval labels are controlled to ensure that 
any relabeling is limited to units that were originally shipped 
from the Manufacturer with an FM Approval label in place. 
FMRC Approval labels shall not be stocked by the repair 
facility. An FMRC Approval label shall be ordered from the 
original manufacturer as needed to repair a specific unit. 
Replacement labels may be obtained and applied by the 
repair facility providing satisfactory evidence that the unit 
being relabeled was originally an FMRC Approved unit. Ver-
ification may include, but is not limited to: a unit with a dam-
aged Approval label, a unit with a defective housing 
displaying an Approval label, or a customer invoice indicat-
ing the serial number of the unit and purchase of an FMRC 
Approved model. 
Do Not Substitute Options or Accessories
 
The communications equipment package that Motorola sub-
mits to FMRC for testing and approval is tested as a system 
that consists of the communications unit itself and the bat-
tery, antenna and other options or accessories that make up 
the rest of the package to be approved. This approved pack-
age must be strictly observed and there must be no substitu-
tion of items, even if the substitute you wanted to consider 
appears as an approved accessory elsewhere in the Guide for 
some other communications equipment unit. Approved con-
figurations are listed by FMRC Approved Product in the 
annual Approval Guide published by FMRC. That guide, and 
the Approval Standard Class Number 3605 document, can be 
ordered from the following address.
Training Resource Center
Publications-Order Processing Dept.
Factory Mutual Engineering and Research
1151 Boston-Providence Turnpike
PO Box 9102
Norwood, MA, 02062
telephone (617) 762-4300 
						

							 
August, 1996 6880904Z07-O 
3-1 
Remove Battery
 
1. Locate the battery latch on the bottom of the radio.
Push the battery latch toward the front of the radio
and hold it in the open position as shown in Figure
3-1.
2. While holding the battery latch in the open posi-
tion, slide the battery down approximately 1/2 inch
and then off the radio housing as shown in Figure
3-2. 
Remove Belt Clip from Battery
 
1. Push in on tab of belt clip with small flat 
-
 
bladed
screwdriver, and at the same time slide belt clip
toward top of radio (Figure 3-3). 
Remove Chassis
 
1. Pull both control knobs straight off to remove.
2. Unscrew antenna counterclockwise to remove.3. Using a flat-blade screwdriver, carefully pry chas-
sis up on both sides of slot at bottom center of
radio (Figure 3-4).
CAUTION  
Lift the chassis approximately
 
 
 
half way out
of the front cover, because you must discon-
nect the flex cable before completely remov- 
ing the chassis.
4. Disconnect the flex cable connector using a flat
blade screwdriver, as shown in Figure 3-5.
MAEPF-25233-OFigure 3-1.
MAEPF-25234-OFigure 3-2.
MAEPF-25235-OFigure 3-3.
MAEPF 25236 O
Chassis to Front Cover
Snaps are Located Here
Figure 3-4.
 
Section 3
Disassembly/Reassembly 
						

							 
3-2 
6880904Z07-O August, 1996 
Disassembly/ReassemblyGP350 Portable Radio Service Manual
Remove the Main Board
 
5. Pull the chassis out and away from the housing as
shown by the arrow in Figure 3-5. 
Remove the Main Board
 
1. The main board is sandwiched between the front
shield and the chassis. Four chassis clips hold the
sandwiched assembly together. Remove the chassis
gasket and place the radio shield side down on a
flat surface.
2. Using a small flat blade screwdriver, unlock the
four chassis clips while pressing down on the chas-
sis directly above each clip. (Refer to Figure 3-6.)3. After all four chassis clips have been removed,
separate the main board from the chassis as shown
in the exploded view Figure 3-7. 
 Re-assembly of Radio
 
Reverse the disassembly 
 
 
procedure.
IMPORTANT  
Be sure to reinstall the chassis gasket. This
gasket helps keep the main board free of
 
unwanted dirt, dust, and water.
MAEPF-25237-O
Flex Cable
Figure 3-5.
Chassis Clips
Figure 3-6.Figure 1-7.
MAEPF-25239-OFigure 3-7. 
						

							August, 19966880904Z07-O4-1Section4
Theory of Operation
Overview
This section provides a detailed theory of operation for the
GP350 and its components: the microcomputer, the receiver,
the transmitter, and the frequency generation circuitry. 
Microcomputer
The GP350 VHF and UHF radios use the Motorola
68HC11A8 microcomputer, U401, which utilizes:
•7.9488 MHz clock rate
•Multiplexed 8-bit address/data lines
•16-bit addressing
•Internal watchdog circuitry
•Analog to digital conversion input ports
The microcomputer’s operating program is permanently
written or “masked” within the microcomputer. Included in
U401 is an EEPROM memory which stores channel, signal-
ling, and scan list information.
Microcomputer Power-Up and Reset Routine
On power-up U401’s reset line (pin 43) is held low by the
AFIC (U402) until the synthesizer (U201) provides a stable
2.1 MHz output. When U402 releases its control, U401’s
hardware holds the reset line low until it verifies that clock
Y401 is operational. When the reset line goes “high,” U401’s
hardware delays briefly to allow Y401 to stabilize, then the
software begins executing port assignments, RAM checking,
and initialization. A fixed delay of 100 ms is added to allow
the audio circuitry to settle. Next, an alert beep is generated
and the steady state software begins to execute (buttons are
read, radio circuits are controlled).
U401’s reset line can be controlled directly by the 5V regu-
lator (U411), the AFIC, and the microcomputer, and indi-
rectly by the synthesizer. U411 drives the reset line low (via
pin 3) if it loses regulation. This prevents possible latch-up
or overwriting of registers in the microcomputer because the
reset line is higher in voltage than pin 55 of U401 (VDD).
U401 can drive the reset line low if it detects a fault condition
such as an expired watchdog timer, software stuck in an infi-
nite loop, unplanned hardware inputs, static zaps, etc.
The AFIC and synthesizer can control the reset line during
power-up, as outlined above.Receiver
The receiver of the GP350 UHF and VHF radios consists of
4 major blocks each: the front-end module, the double bal-
anced mixer, the 45.1 MHz IF and the back-end IF IC.
The UHF and VHF front-end modules consist of three blocks
of circuitry each: A pre-selector, RF amplifier and a post-
selector filter. These three items are located on a receiver
module pc-board that stands perpendicular to the main radio
pc-board. This module is enclosed in a shield to prevent radi-
ation into and out of the module. All filters on the UHF and
VHF modules are fixed tuned designs to eliminate the need
for factory tuning and to provide wide-band operation.
The shunt and series coupled resonator topology yields a
more symmetrical frequency response to guard against
strong out of band signals that could produce IM products.
The worst case image frequency in the VHF band is
90.2 MHz above the filter passband. The 3 db bandwidth is
approximately 35 MHz, centered at 160 MHz. The center of
the band insertion loss is approximately 1.9 db. The 4-pole
filter is designed to operate with a 50 ohm input termination,
while the output termination is the input impedance of the
RF amplifier that follows it.
The UHF pre-selector filter is a 3-pole,.01 db Chebyshev
bandpass design implemented in a shunt coupled resonator
topology. This topology maximizes the attenuation at the
worst case image frequency for this receiver, which is 90.2
MHz below the filter passband. The 3 db bandwidth is
approximately 45 MHz, centered at 454 MHz. The center of
the band insertion loss is approximately 2.2 db. The 3-pole
filter is designed to operate with a 50 ohm input termination,
while the output termination is the input impedance of the
RF amplifier that follows it.
The RF amplifier, Q1, is a Motorola MMBR571 NPN device
biased in a common emitter configuration. The amplifier is
stabilized by the shunt feedback resistor R3, and has approx-
imately 16.5 db of gain with a noise figure of about 3.0 db
(VHF) and 2.2 db (UHF). The amplifier draws 4 ma of cur-
rent and is supplied by the receiver 5 volt supply (indicated
as “+5R” on the schematics and block diagrams).
Terminating the RF amplifier is the post-selector filter. This
filter is a 3-pole for VHF and a 4-pole for UHF,.01 db Che-
byshev design which is also implemented in a series coupled
resonator topology for maximum image attenuation. The 3
db bandwidth is approximately 38 MHz centered at 160
MHz for VHF and 42.5 MHz centered at 454 MHz for UHF. 
						

							4-26880904Z07-OAugust, 1996Theory of OperationGP350 Portable Radios Service ManualTransmitterThe insertion loss of this filter is approximately 1.9 db for
VHF and 3.5 db for UHF. The filter is designed to be termi-
nated with the amplifier output impedance on one side, and
50 ohm on the other.
The net gain from the receiver module is about (12.2 db
VHF) (10.8 db UHF) in the center of the band and about
(10.7 db VHF) (9.5 db UHF) at the band edges. The net cen-
ter of the band noise figure is approximately (5.5 db VHF)
(5.2 db UHF). This is sufficient to achieve a typical center of
the band sensitivity of 12 dbs.
The double balanced mixer is composed of the two baluns,
T1 and T2, and the ring diode IC, CR2. The mixer operates
with a local oscillator (LO) level of +6 dbm and the conver-
sion loss is approximately 7.5 db. The double balanced type
mixer (DBM) provides excellent isolation between any two
ports. And since a DBM can operate over a large bandwidth,
the same mixer can be used for UHF and VHF radios. The
DBM also provides excellent protection against receiver
spurs due to non-linearizes, such as IM and Half-IF. The
received signal mixes down to the frequency of the first IF,
45.1 MHz, and enters the IF circuitry.
Intermediate Frequency (IF)
The Intermediate Frequency (IF) section of the portable
radio consists of several sections including, the “high” IF, the
second LO, the second IF, and the IF IC chip. The first LO
signal and the RF signal mix to the IF frequency of 45.1
Mhz, and then enters the IF portion of the radio.
The signal first enters the “high” IF, passes through a crystal
filter, is then amplified by the IF amplifier, and then passed
through another crystal filter. The first crystal filter provides
selectivity, second image protection, and intermodulation
protection. The amplifier provides approximately 16 dB of
gain to the signal. The signal then passes through the second
crystal filter which provides further selectivity and second
image protection. The “high” IF has an approximate 3 dB
bandwidth of 7 KHz for 20/25/30 KHz models and 4 KHz
for 12.5 KHz models.
The filtered and amplified IF signal then mixes with the sec-
ond local oscillator at 44.645 MHz. The second LO uses an
amplifier internal to the IF IC, an external crystal and some
external chip parts. The oscillator presents an approximate
level of -15 dBm to the second IF mixer, internal to the IF IC.
The output of the mixing of the IF signal and the second LO
produces a signal at 455 KHz (second IF). This signal is then
filtered by external ceramic filters and amplified. It is then
passed back to the IF IC, sent to a phase-lock detector, and
demodulated. The resulting detected audio output is then
sent to the AFIC to recover the audio.
The IF IC also controls the squelch characteristics of the
radio. With a few external parts the squelch tail, hysteresis,
attack and delay times were optimized for the radio. The
AFIC allows the radio’s squelch opening to be electronically
adjusted.Transmitter
The GP350 VHF and UHF transmitters contain five basic cir-
cuits: a power amplifier, an antenna switch, a harmonic filter,
an antenna matching network, and a power control Refer to
the block diagram and the schematic for more information.
The power amplifier for VHF contains three stages of ampli-
fication. For UHF, the power module contains four stages.
Both modules require an input signal of 1 mW, a supply volt-
age of 7.5 volts, and are capable of supplying, at least, 7
Watts of output. The power out of both modules can be var-
ied by changing the voltage on their second stage.
The antenna switch circuit consists of two PIN diodes
(CR101 and CR102), a pi network (C119, L112, and part of
C112), and at least, one current limiting resistor (R102 for
UHF; and R102, R103, and R108 for VHF). In the transmit
mode, TX B+ is applied to the circuit to bias the diodes “on”.
The shunt diode (CR102) shorts out the receiver port, and the
pi network, which operates as a quarter wave transmission
line, transforms the low impedance of the shunt diode to a
“high” impedance at the input of the harmonic filter. In the
receive mode, the diodes are both off, and hence, there exists
a low attenuation path between the antenna and receiver
ports. 
The harmonic filter consists of part of C112, and L107,
C113, L108, C114, L109, and C115. The design of the har-
monic filter for both VHF and UHF is that of a Zolotarev
design. This particular design is similar to that of a Cheby-
shev filter except for a large amplitude first ripple (near dc).
This type of filter has the advantage that it can give greater
attenuation in the stop-band for a given ripple level.
Another feature of this type of filter is that the coils tend to
be smaller than with a Chebyshev design. The design of the
VHF filter was modified from the Zolotarev design by
slightly changing its capacitor values to yield a filter having
an input impedance which optimized the efficiency of the
power module.
To optimize the performance of the transmitter and receiver
into an antenna, a network is used to match the antenna’s
impedance to the harmonic filter. For VHF the network con-
sists of C117, L111, and C122. For UHF the network is made
up of C117 and L111. Note that, in order to measure the
power out of the transmitter, one must remove the antenna
and screw in its place a special BNC-to-Phono adapter.
The power control circuit consists of the networks associated
with U151, Q156, Q151, Q152, Q155, and U152. The Op
Amp U151A and Q156, along with resistor R101, make up a
current-to-voltage amplifier whose gain is mainly dependent
upon the ratio of R179 to R153. The current to the final stage
of the power module is supplied through R101 (0.1 Ohms),
which provides a voltage proportional to the current drain.
This voltage is amplified and applied to the input of U151B.
The resistors at the input of U151A (R151, R152, R154, and
R155) keep the voltages at the inputs of U151A below its
maximum allowable. These resistors are 1% tolerance parts 
						

							August, 1996                                  6880904Z07-O4-3GP350 Portable Radios Service ManualTheory of Operation Frequency Generation Circuitryto minimize the error produced at the emitter of Q156 result-
ing from the voltage offset at the input of U151A.
The voltage at the other input of the summing amplifier,
U151B, is supplied from two DACs contained within U152.
These DACs are controlled by the microprocessor, and pro-
vide the reference voltage for the control loop. One of the
DACs, that connected to Pin 9 of U152, provides a coarse
tune voltage, while the other provides a fine tune voltage.
Since the output of the DACs is not zero when they are set to
their lowest level, resistor R169 is provided to bias up the
minus input of the summing amplifier to compensate for the
bias resulting from the DACs.
The error voltage at the input of U151B produces a voltage
at its output, which is in turn applied to the series pass tran-
sistor, Q152, through its driver, Q151. The voltage at the col-
lector of Q152 is applied to the controlled stage of the power
module, which for both VHF and UHF is the module’s sec-
ond stage. The feedback from the collector of Q152 to the
emitter of Q151 through R166 is provided to keep the two
stages stable. Likewise, the feedback from the collector of
Q152 to the minus input of the summing amplifier is to keep
the whole control loop stable.
The purpose of Q155 and its associated circuitry is to keep
the control voltage on the module below 7.0 volts, which is
the maximum allowed for the UHF module.
The purpose of R173 was originally that of providing com-
pensation to the control loop for changes in the supply volt-
age, TX B+. However, experimentation has shown that this
compensation is not really required. Also, thermistor, R170,
was provided to enable the shut back of the PA in the event
that it would get too hot. This has also been shown to not be
required
Frequency Generation Circuitry
The frequency generation circuitry is composed of two main
IC’s, the Fractional-N synthesizer (U201) and the VCO/
Buffer IC (U251). Designed in conjunction to maximize
compatibility, the two IC’s provide many of the functions
which normally would require additional circuitry. The
block diagram illustrates the interconnect and support cir-
cuitry used in the design. Refer to the schematic for refer-
ence designator.
The supply for the synthesizer is from Regulated 5 volts
which also serves the rest of the radio. The synthesizer in
turn generates a superfiltered 5 volts (*actually 4.65 volts)
which   powers U251.
In addition to the VCO, the synthesizer must interface with
the logic and AFIC circuitry. Programming for the synthe-
sizer is accomplished through the data, clock, and chip
enable lines (pins 5, 6, and 7) from the microprocessor,
U401. A serial stream of 98 bits is sent whenever the synthe-
sizer is programmed. A 5 volt dc signal from pin 2 indicates
to the microprocessor that the synthesizer is locked while
unlock is indicated by a low voltage on this pin. Transmit
modulation from the AFIC is applied to pin 8 of U201. Inter-nally the audio is digitized by the Fractional-N and applied
to the loop divider to provide the low-port modulation. The
audio is also run through an internal attenuator for modula-
tion balancing purposes before being outputted at pin 28 to
the VCO. A 2.1 MHz clock for the AFIC is generated by the
Fractional-N and is routed to pin 11 where it is filtered and
attenuated from 2.5 volts to approximately 2 volts.
Synthesizer
The Fractional-N synthesizer uses a 16.8 MHz crystal
(Y201) to provide the reference frequency for the system.
The other reference oscillator components external to the IC
are C205, C206, R207, and CR203. The 16.8 MHz signal is
divided down signal from the VCO. The loop filter, com-
prised of R201, R202, R205, C201, C214, C215, and C216,
provides the necessary dc steering voltage for the VCO as
well as filtering of spurious signals from the phase detector.
For achieving fast locking of the synthesizer, an internal
adapt charge pump provides higher current capability at pin
31 than when in the normal steady-state mode. Both the nor-
mal and adapt charge pumps receive their supply from the
voltage multiplier which is made up of C202, C203, C204,
C231, CR201, and CR202. By combining two 5 volt square
waves which are 180 out-of-phase along with Regulated 5
volts, a supply of approximately 12.6 volts is available at pin
32 for the charge pumps. The current for the normal mode
charge pumps is set by R203. The pre-scaler for the loop is
internal to U201 with the value determined by the frequency
band of operation.
VCO
The VCO (U251) in conjunction with the Fractional-N syn-
thesizer (U201) generates rf in both the receive and the trans-
mit modes of operation. The TRB line (U251 pin 5)
determines which oscillator and buffer will be enabled. A
sample of the rf signal from the enabled oscillator is routed
from U251 pin 23, through a low pass filter, to the pre-scaler
input (U201 pin 20). After frequency comparison in the syn-
thesizer, a resultant CONTROL VOLTAGE is received at the
VCO. This voltage is a DC voltage between 3 and 10 volts
when the PLL is locked on frequency.
In the receive mode, U251 pin 5 is grounded. This activates
the receive VCO by enabling the receive oscillator and the
receive buffer of U251. The rf signal at U251 pin 2 is run
through a low pass filter. The rf signal after the low pass filter
is the LO RF INJECTION and it is applied to the first mixer
at T2.
During the transmit condition, PTT depressed, five volts is
applied to U251 pin 5.  This activates the transmit VCO by
enabling the transmit oscillator and the transmit buffer of
U251.  The rf signal at U251 pin 4 is run through a low pass
filter and an attenuator to give the correct drive level to the
input of the PA module (U101 pin 1).  This rf signal is the TX
RF INJECTION.  Also in transmit mode, the audio signal to
be frequency modulated onto the carrier is received by the
transmit VCO modulation circuitry at AUDIO IN. 
						

							4-46880904Z07-OAugust, 1996Theory of OperationGP350 Portable Radios Service ManualFrequency Generation CircuitryWhen a “high” impedance is applied to U251 pin 5, the VCO
is operating in BATTERY SAVER mode. In this case, both
the receive and transmit oscillators as well as the receive,
transmit, and pre-scaler buffer are turned off. In the Frac-
tional-N, the battery saver mode places the   A/D and the
modulation attenuator in the off state. This mode is used to
reduce current drain on the radio.
GP350 receive (RX) and transmit (TX) circuits are common
to both the VHF and UHF models. Most of the radio process-
ing for RX and TX is accomplished in U402, the Audio Filter
IC. The Audio Filter IC performs the following functions:
•Tone/Digital PL encoding and decoding
•PL rejection filter (RX audio)
•TX pre-emphasis amplifier
•Limiter
•Post-limiter filter
•TX deviation digital attenuators
•MIC gain attenuator
•Noise squelch digital attenuator
•Microcontroller port expanders (output only)
•2.5 Vdc reference source
U402 parameters are programmed from U401 microcontrol-
ler ROM and EEPROM data via the serial CLOCK and
DATA lines. Unless otherwise indicated, all signal levels
refer to standard carrier modulation, 1kHz tone at +/-3kHz
deviation.
TX Audio Path
Internal MIC Bias Switch and External PTT Sense 
Circuits
PNP switch transistor Q407, resistors R453, R454, and
capacitor C463 control the operating bias for internal MIC
MK401. Q407 is controlled by microcontroller U401 via
U402-40, the Audio Filter IC expanded output port. On con-
necting an external MIC through the side connector adapter,
the external PTT sense transistor (Q408) switches “on” when
the external PTT is closed. In PTT-equipped accessories, the
PTT switch is series-connected with the MIC element. When
this PTT is closed, 5-volts “high” is produced on the collec-
tor of Q408 and monitored by U401-14. When the collector
voltage is “high” (5 volts), the microcontroller configures the
radio for transmit mode.
There is no series-wired PTT within the headsets. These
accessories always keep the collector of Q408 “high.” With
headsets, the radio must be programmed for headsets or
Audio Sense. When programmed for Audio Sense, on
power-up the microcontroller (U401) reads that line 14 is
“high” and interprets that there is a headset attached. When
the radio is programmed for headsets, the microprocessor
ignores line 14 for PTT operation and it “looks” to the VOXdetect line on U401-19, or to the internal PTT (U409-42), to
transmit the headset audio.    
MIC Amplifier
There are two MIC amplifiers inside U409. The MIC-enable
line, U409-18, is always biased “on” for VOX applications.
The amplifiers are selected according to the bias on U409-
20, which is the collector voltage on Q408. The external
audio amplifier, U409-21, is active when U409-20 is “high”
(5 volts), and the internal audio amplifier, U409-22, is active
when U409-20 is low (0 volts). The audio signal then exits
U409-19 and proceeds through a low pass network (C516,
C517, and R516) into U409-12 and out through U409-11,
with R515 providing feedback. This circuit supplies a low
frequency “roll off” for improved audio clarity. Capacitor
C519 and resistor R518 provide the output bias for the MIC
amplifiers. 
TX Audio Mute Gate
PNP transistor Q409, and resistors R462 and R463 comprise
the TX audio mute gate. The audio Filter IC expanded output
port (U402-40), controls Q409 as well as the internal MIC
bias switch (Q407). When U402-40 is logic LO state, a small
dc current flows from U409-11 MIC amplifier output into
Q409 emitter, through Q409, and out of the collector through
R462. A fraction of the emitter current flows out of the base
through R463 to ground (Vss of Audio Filter IC). MIC audio
at U409-11 passes through the TX audio mute gate. When
U402-40 is logic “high,” Q409 base voltage is 4Vdc (typical)
and emitter voltage is 2.4 Vdc, biasing the device well into
cut-off. No current flows through emitter to base/collector,
and no MIC audio passes. The mute function is enabled
(Q409 is “OFF”) when modulating DTMF or 5/6 tone (Euro-
pean) Signalling.
Pre-emphasis Amplifier (standard models)
U402, the Audio Filter IC, contains a TX audio pre-emphasis
amplifier, with external gain setting resistor R504, and pre-
emphasis elements R506 and C462. Connections are made at
each end of resistor R506 to provide interconnection of
“front cover” option board TX audio through connector P1
(below). Pre-emphasis is 6 dB/octave.
Option Interface Connector P1 (Keypad/Display 
models)
P1 provides interconnection of “front cover” option PC
boards to the GP350 radio main board. MIC audio output is
available from P1-5 at a level of 45 mVrms and 10k ohm out-
put impedance. Option TX Audio input to the GP350 radio is
available at P1-4 with sensitivity of 40 mV rms, pre-empha-
sized at 6 dB/octave, and less than 200 ohm output imped-
ance (from option board). If “flat” audio response is
required, the audio output from the option board must be de-
emphasized at a -6 dB/octave rate, 300Hz to 3kHz, with 0 dB
gain at 1kHz. The low option board output impedance is
required to achieve better than 40 dB isolation between main
board input (P1-4) and output (P1-5) audio. 
						

							August, 1996                                  6880904Z07-O4-5GP350 Portable Radios Service ManualTheory of Operation Frequency Generation CircuitryLimiter (Audio Filter IC)
The audio filter IC U402 contains the limiter circuit, which
prevents over-deviation of the RF carrier by symmetrically
clipping the peaks of the modulating voltage. Audio from the
pre-emphasis amplifier circuit is coupled to the limiter. Gain
of the limiter stage is adjustable in four 3 dB steps, from -3
dB to +6 dB. Therefore, TX audio path gain, or MIC gain,
can be adjusted to compensate for different sound environ-
ments through the Radio Service Software.
Post-Limiter Filter (Audio Filter IC)
Clipped modulating voltage from the limiter output is cou-
pled to the post-limiter filter. Filtering attenuates the spuri-
ous products generated by the limiter. The post-limiter filter
is programmable to operate in the following modes:
•CEPT/EIA mode
•Japan mode
•FTZ (Germany) mode
PL Encoder
Private Line (CTCSS) is generated by the PL encoder circuit
in U402, the Audio Filter IC. Tone PL or Digital PL data is
programmed for each mode from the Radio Service Soft-
ware. On entering transmit mode, TPL or DPL data is pro-
grammed to U402 via the serial DATA and CLOCK lines.
U401-35 microcontroller output strobes &402-32 PL clock
input at a constant rate during DPL encoding, or at a rate
determined by the PL encoder algorithm in the microcontrol-
ler for TPL encoding corresponding to tone frequency. The
encoded PL is summed with MIC audio at the post-limiter
filter input. Digital attenuators are employed to adjust the
balance of MIC radio and PL to prevent over-deviation of the
carrier. PL deviation is adjustable in three “coarse” steps of
500 Hz, 750 Hz, and 1 kHz, for 25 KHz models and steps of
250 Hz, 375 Hz, and 500 Hz for 12.5KHz models with com-
pensation of MIC audio level.
DTMF Encoder
Resistors R424, R425, R426, R428 and R484, and summer
U405A form the DTMF encoder. U405A-1 is coupled to
U402-13 Audio Filter IC auxiliary TX modulation input.
DTMF encoded signals pass from this input to the post-lim-
iter filter input. U405A-1 is also coupled to U402-12 and
coupled through RX audio path to the audio PA for sidetone
audio.
Deviation Attenuators (Audio Filter IC)
Carrier deviation is set by programming the digital deviation
attenuators of the Audio Filter IC. Deviation data for each
mode is entered through the Radio Service Software, and
then programmed into U402 from microcontroller U401 on
entering transmit mode. U402-19 and U402-20 deviation
attenuator outputs are combined through resistors R478 andR479 and dc-coupled to U201-8, the synthesizer modulation
input. Capacitor C218 provides a “high” frequency roll-off
corner at 20 kHz to further attenuate spurious signals from
U402. The dc voltage at the combined attenuator outputs sets
the center frequency for the modulated carrier. Any transient
(R x C) voltages in the TX audio path must settle within 1
millisecond of PTT activation to prevent center frequency
offset.
RX Audio Path
PL Rejection Filter (Audio Filter IC)
The recovered RX audio from the IF detector IC U51 is cou-
pled through capacitor C435 to U402-7 and U402-8 on the
Audio Filter IC. RX audio at U402-7 is processed first by the
PL rejection filter, which is characterized by a two pole, 300
Hz corner frequency “high-pass” response. Audio then
passes through the digital volume attenuator and buffer
amplifier output to U402-23. Unattenuated RX audio is cou-
pled to U402-22 and fed to the center-slicer circuit for detec-
tion of 5/6 tone (European) signals. For standard test
modulation, the audio level at U402-7 is 255 mVrms, and
output audio level at U402-23 is 765 mVrms with the digital
volume attenuator set to minimum attenuation.
PL Decoder
Recovered RX audio at U402-8, the PL decoder input, first
passes through the Tone PL filter, or the Digital PL filter,
depending on the PL option selected for the current operat-
ing mode. Filtered PL is then coupled to the PL detector cir-
cuit, with detected PL output at U402-27. The detected PL
signal is coupled from U402-27 to microcontroller U401-64
where algorithms perform the final PL decoding. Data for the
Tone PL frequency or Digital PL code for each mode is pro-
grammed through the Radio Service Software.
Center-Slicer 
The center-slicer circuit U406A detects Quick-Call and 5/6
tone signals. Unattenuated RX audio from U402-22 is dc
coupled to the two inputs of U406A. The non-inverting input
U406A-3 is fed through resistor R433. Capacitor C415 sets
a low-pass corner frequency of 3.3 kHz. The inverting input
U406A-2 is fed through resistor R434. Capacitor C416 sets
a low-pass corner frequency of 16 Hz. During operation,
R434 / C416 establish an averaged dc offset level at U406A-
2 dependent on the average dc level of the undetected signal
to set the “trigger” threshold of U406A. R433 / C415 provide
“high” audio frequency roll-off to improve falsing immunity.
The detected output from the center slicer circuit is coupled
to microcontroller U401-43 where algorithms perform the
final data decoding.
Option Interface Connector P1 (Keypad/Display 
Models)
P1 provides interconnection of “front cover” option pc
boards to the GP350 radio main board. Filtered “flat” RX
audio output is available at P1-7, at a level of 765 mVrms at 
						

							4-66880904Z07-OAugust, 1996Theory of OperationGP350 Portable Radios Service ManualFrequency Generation Circuitry15k-ohm impedance. P1-7 is always unmuted, not affected
by the receiver with squelch circuit. Option RX audio input
to the GP350 radio is available at P1-6, with a sensitivity of
100 mVrms at less than 200 ohm output impedance from
option board.
RX Audio Mute Gate 
PNP transistor Q406, the RX audio mute gate, with resistors
R458 and R459, and capacitors C432 and C433, provide
receiver audio muting. The RX audio mute gate circuit func-
tions in a similar manner to Q409, the TX audio mute gate
circuit. Muting is controlled by microcontroller U401 via
U402-39, an Audio Filter IC expanded output port. Q406 is
saturated and RX audio unmuted by programming U402-39
to a logic “LO” state. Q406 is placed well into cut-off and
RX audio muted by programming U402-39 to a logic “high”
state.
Audio Power Amplifier
Variable resistor R460 and resistor R461 provide RX audio
volume adjustment. R461 sets the minimum volume level.
R466 and R464 form a resistor divider to set the audio input
amplitude into the amplifier (U409-10), which is ac-coupled
by C518. Fixed level Alert Tone audio is generated by micro-
controller U401-56 and coupled through capacitor C437 and
resistor R465 into the audio path. The audio amplifier
(U409) has three amplifiers designed to differentially drive
its load. Two of the three amplifiers simultaneously drive the
16- ohm speaker. All the amplifiers are enabled with “high”
(5 volts) on U409-23, which is activated by the AFIC (U402-
3). The common amplifier (U409-31 and U409-32) is always
on, and either the external amplifier (U409-4 and U409-5) or
the internal amplifier (U409-27 and U409-28) is on, depend-
ing on the bias of the logic circuits (U409-24). If U409-24 is
“high,” the internal amplifier (U409-27 and U409-28) is on;
if U409-24 is low, the external amplifier (U409-4 and U409-
5) is on. This is how the audio is switched between the inter-
nal speaker and the accessories.
Noise Squelch Attenuator
The Audio Filter IC U402 contains a 16 step programmable
digital squelch attenuator between U402-16 and U402-18.
Noise squelch is set using the Radio Service Software, with
open squelch at step 0, and tight squelch at step 15.
Vox Circuit Operation
As mentioned above, with VOX option enabled, a VOX
(non-PTT) accessory can be plugged into the adaptor for
voice-activated transmit operation. The external MIC ele-
ment is always supplied with operating bias through resistor
R451 and external PTT sense transistor Q408. The external
PTT sense at microcontroller U401-14 is therefore, always
“enabled.” A second output circuit of MIC amplifier U409-
11 couples MIC audio through capacitor C445 to U406B, the
VOX detector circuit. Resistors R492 and R493, and capaci-
tor C451 form a syllabic filter which reduces VOX circuittriggering by “high” frequency ambient noise. Resistors
R442, R443, R444, R445, and R491, capacitor C423, recti-
fier diode CR404 and U406B form a linear peak detector cir-
cuit. MIC audio causes capacitor C423 to charge to a
potential related to the relative amplitude of ambient noise.
Microcontroller U401-19 monitors the potential of C423 and
establishes a threshold for non-voiced ambient noise. When
a positive rise in potential above threshold or voice is
detected by an algorithm in the microcontroller ROM, the
radio is configured to transmit mode. 
						

							August, 19966880904Z07-O 4-7INSERT PAGE SIZE AND RADIO BLOCK DIAGRAM FROM
MANUAL 6880902Z30-D, PAGE 2-7GP350 Portable Radio
Functional Block Diagram