Motorola Gp68 Part1 6881086c09 O Manual

							 
March, 1997 6881086C09-O
 
2-1
 
Section 2
Theory of Operation 
Overview 
This section provides a detailed theory of operation for the
GP60 Series Radios and its components.
The GP60 Series radio consists three main boards; Control-
ler board, rf board and display board. The controller board is
connected to the rf board through a 20 lines flex ribbon cable.
The display board is connected to the controller board
through a 14 pins board-to-board connector. The top key
pads are embedded on a flex that makes connection to the
controller via 20 pins connector. 
Controller Board 
Controller board is the heart of the radio. It contains micro-
computer (U401), AFIC (audio processor IC, U451), general
5 volt regulator (U302), 5 volt regulator with reset to power
up the microcomputer (U456), Unswitch ram back up 5 volt
regulator (U457), audio power amplifier IC (U454), MIC
amplifier (U452-1)and rf power control circuitry/APC
(U152, U150).
Microcomputer
The GP60 Series VHF and UHF radios use the Motorola
68HC11E20 microcomputer, U401, which utilizes:
• 7.9488 MHz clock rate
• Single chip mode operation
• 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 (U451) until the synthesizer (U201, on the rf board)
provides a stable 2.1 MHz output. When U451 releases its
control, U401’s hardware holds the reset line low until it ver-
ifies that clock Y401 is operational. When the reset line goes
high, U401’s hardware delays briefly to allow Y401 to stabi-
lize, then the software begins executing port assignments,
RAM checking, and initialization. A fixed delay of 100 mSis added to allow the audio circuitry to settle. Next, an alert
beep is generated and the steady state software begins to exe-
cute (buttons are read, radio circuits are controlled).
U401’s reset line can be controlled directly by the 5 V regu-
lator (U456), the AFIC, and the microcomputer, and indi-
rectly by the synthesizer. U456 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. 
Transmit and Receive Audio 
Circuitry 
The GP60 Series Radios receive (Rx) and transmit (Tx) cir-
cuits are common to both the VHF and UHF models. Most
of the radio processing for Rx and Tx is accomplished in
U451, 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
U451 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, 1 kHz tone at   
±  
3 kHz
deviation. 
						

							 
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6881086C09-OMarch, 1997
 
Theory of OperationGP68 Portable Radios Service Manual
Transmit and Receive Audio Circuitry
 
Tx Audio Path 
Internal PTT, MIC Bias Switch and External PTT 
Sense Circuits  
The internal PTT switch (SW402) is connected direct to the
input port of the microcomputer (pin 42) to ground. This port
is an active low. One of the R415 resistor is used to pull up
the line to 5 volt. While PTT (internal) this line will be pulled
low. Internal MIC bias is supplied from the +5VTX (switch
by microcomputer) through R467 and R468. Internal MIC is
connected to the controller board via top keypad flex.
When connecting an external MIC through connector J406,
the external PTT sense transistor Q403(pins 2, 3, and 4)
switches “ON” when the external PTT switch is closed. Col-
lector voltage Q403-4 is monitored by U401-34. When col-
lector is logic “HI” state, the microcontroller configures the
radio for transmit mode. In PTT equipped accessories, the
PTT switch is series connected with the external MIC ele-
ment. 
MIC Amplifier  
MIC audio from internal MIC MK401 is coupled through
C468, J406, and L402 to the MIC amp circuit U452-1. Exter-
nal MIC plug insertion mechanically disconnects the internal
MIC. External MIC audio is coupled through L402 to the
MIC amp input. Capacitors C458, C460 and C461, and resis-
tors R458, R459 and R460 provide a low audio frequency
roll off with a high-pass corner frequency of 1 kHz to
improve transmit audio clarity. Crossover gain is 12 dB (at 1
kHz). Reference deviation is obtained with 11.0 mV rms
input to the external MIC connector J406.  
Tx Audio Mute Gate 
Pins 1, 5, and 6 of dual PNP transistor Q403 and resistors
R465 and R456 comprise the Tx audio mute gate. Audio Fil-
ter IC U451-40, expanded output port, controls PNP transis-
tor Q403 (pins 1, 5, and 6). When U451-40 is logic LO state,
a small dc current flows from U452-1-1 MIC amp output into
Q403-6 emitter, through Q403, and out of the collector
(Q403-1) through R456. A fraction of the emitter current
flows out of the base (Q403-5) through R465 to ground (Vss
of Audio Filter IC). MIC audio at U452-1-1 passes through
the Tx audio mute gate. When U451-40 is logic “HI”, Q403-
6 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 (Q403 pins 1, 5, and 6
is “OFF”) when modulating DTMF Signalling. 
Pre-emphasis Amp  
The Audio Filter IC U451, contains a Tx audio pre-emphasis
amp, with external gain setting resistor R451 and C452. Con-
nections are made at each end of resistor R451 to provide
interconnection of option board Tx audio through connector
J403. Pre-emphasis is 6 dB/octave with a corner frequency of
6600 Hz. Crossover gain is 0 dB at 1 kHz, with passband
gain (head-room) of 17.5 dB. 
Limiter (Audio Filter IC)  
The audio filter IC U451 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, 0 dB, +3 dB, and +6 dB. Therefore, Tx audio path gain,
or MIC gain, can be adjusted to compensate for different
sound environments 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:
•EIA mode  
PL Encoder  
Private Line (CTCSS) is generated by the PL encoder circuit
in the Audio Filter IC U451. Tone PL or Digital PL data is
user programmable (see user manual) for each mode. On
entering transmit mode, TPL or DPL data is programmed to
U451 via the serial DATA and CLOCK lines. U401-57
microcontroller output strobes & U451-32 PL CLOCK input
at a constant rate during DPL encoding, or at a rate deter-
mined by the PL encoder algorithm in the microcontroller
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.  
DTMF Encoder  
Resistors R423, R424, R425, and R427, and summer U452-
2 form the DTMF encoder. Output from U452-2 pin 7 is cou-
pled to U451-13 Audio Filter IC auxiliary Tx modulation
input. DTMF encoded signals pass from this input to the
post-limiter filter input.
Output from U452-2 pin 7 is also coupled to U451-6 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 U451 from microcontroller U401 on
entering transmit mode. U451-19 and U451-20 deviation
attenuator outputs are combined through resistors R454,
R455 and R457 and dc-coupled to U201-5 (on rf board), the
synthesizer modulation input. Capacitor C218 provides a
high frequency roll-off corner at 20 kHz to further attenuate
spurious signals from U451. The dc voltage at the combined
attenuator outputs sets the center frequency for the modu-
lated carrier. Any transient (R x C) voltages in the Tx audio 
						

							 
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GP68 Portable Radios Service ManualTheory of Operation 
Transmit and Receive Audio Circuitry
 
path must settle within 1 millisecond of PTT activation to
prevent center frequency offset. 
Rx Audio Path 
Audio Processing (Audio Filter IC)  
The recovered Rx audio from the rf board (IF detector IC
U51) is coupled through to U451-7 and U451-8 on the Audio
Filter IC. Rx audio at U451-7 is processed first by the PL
rejection filter, which is characterized by a 2-pole, 300 Hz
corner frequency high-pass response. Audio de-emphasis is
a single pole low pass filter with a corner frequency of 231
Hz. Audio then passes through the digital volume attenuator
and buffer amplifier output to U451-23. For standard test
modulation, the audio level at U451-7 is 255 mV rms, and
output audio level at U451-23 is 765 mV rms with the digital
volume attenuator set to minimum attenuation. 
PL Decoder 
Recovered Rx audio at U451-8, 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 U451-27 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 programmed through the Radio Service Software.  
Rx Audio Mute 
The Rx audio mute is controlled by microcontroller U401
via U451-3. The chip disable U454-1 is used to power down
audio PA to conserve standby current and mute. When at a
logic “0” (0 V to 0.8 V), the U454 is enabled for normal oper-
ation. When pin 1 (U454-1) is at a logic “1” (2.0 V to Vcc V),
the U481 is disabled (muted). 
Audio Power Amplifier  
U454 (MC 34119) is a low amplifier capable of low voltage
operation (Vcc=2.0 V minimum). The circuit provides a dif-
ferential output (U454 pins 5 & 8) to the speaker (24 ohms)
to maximize the available voltage swing at low voltages. The
internal configuration consists of two identical operational
amplifiers. Open-loop gain is above or equal to 80 dB (at f
						

							 
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Theory of OperationGP68 Portable Radios Service Manual
 
Adaptive Power Control
 
TM
 
 Technology
 
J403-6 
J403-6 is interfaced to pin 4 of voltage regulator U456. This
is the microprocessors 5 V source from the main board to the
option board. Maximum current sourcing is 100 mA. Regu-
lation is  
± 
0.2 Vdc.  
J403-7 
J403-7 interfaces with pin 52 of U401. Pin 52 of the micro-
controller is bi-directional port D bit 3. In the GP60 Series
Radio, this connection is for serial data out of the microcon-
troller. This controls loading of the various electrical sub-
systems internal to the radio in addition to data required by
option boards installed into the radio. For option connector
purposes this pin is used to shift multi-byte messages from
the radio to an option board. When used for this purpose, pin
J403-2 option board enable, is driven low by the radio micro-
controller to enable a serial byte transfer to an option board. 
J403-8 
J403-8 interfaces with pin 51 of U401, the radio microcon-
troller. Pin 51 of the microcontroller is bi-directional port D
bit 2. In the GP60 Series Radio, this is decoder data into the
radio. On a DTMF decoder board this would be the serial
input for the 4-bits of tone data. On other option boards this
input is used as the serial input for a multiple byte message. 
J403-9 
This option interface pin is connected to the Rx Out signal,
pin 23 of the Audio Filter IC, U451 through coupling capac-
itor C450. In the GP60 Series Radio, this signal de-empha-
sizes Rx audio and output is always unmuted audio in the
radio. This pin may be used as the receive audio to a decoder
option board such as DTMF, Two Tone Sequential, or MSK
signalling decode. An audio scrambler option board may
also use this input for receive audio in. Any option board
requiring pre-emphasized audio would have to include the
necessary filtering. The level of this de-emphasized audio is
450 mV rms at 15 ohm impedance. 
J403-10 
This is the Rx audio output of the option board. This connec-
tion may be used for option boards that need to enable Rx
audio on signaling decodes or to descramble audio as
required by the option board descrambling technique. Option
board Rx audio input is available at J403-10 with a sensitiv-
ity of 100 mV rms at less than 200 ohm output impedance
from an option board. R480, a 30k ohm resistor between
option board pins J403-9 and J403-10 requires design con-
sideration on the part of any option board using Rx audio
output. The Rx audio output level is controlled by the GP60
Series Radio volume control before the audio amp. 
J403-11 
J403-11 interfaces with pin 53 of U401. Pin 53 of the micro-
controller is bi-directional port PD5. In the GP60 Series 
Radio, this is the CLOCK output from the microcontroller
for loading all internal subsystems as well as option boards
that require synchronous serial transfers. Option boards
requiring a multi-byte transfer may use this output as the
CLOCK source for uploading internal option board registers
on power-up, channel change, or for reading serial control
requests. 
J403-12 
J403-12 interfaces with pin 63 of U401. Pin 63 of the micro-
controller is an input on port A bit 2 of the radio microcon-
troller. In the GP60 Series Radio, this connection is used for
a variety of input signals from an option board. In a simple
option board configuration, a falling edge on this pin connec-
tion signals that a selective call has been decoded by the
option board. For DTMF decoder boards or other simple
BCD decoder boards, a falling edge on this pin indicates that
a digit decode has been completed and is ready to be shifted
into the microcomputer for concatenation and comparison to
an ID string. In more complex option boards, a falling edge
on this pin indicates that an option board requests a serial
transfer on J403-8 and J403-1 or an acknowledgment of data
received on J403-7 in a multiple byte transfer. 
Adaptive Power Control 
TM
 
 
Technology 
The GP60 Series Radio power control is specially designed
to handle alkaline battery voltage and current characteristics,
without compromising output power variation when used
with NiCd batteries. 
Basically there are three sections of the power control cir-
cuitry. Digital to analog converter, voltage to current con-
verter and the cut back circuitry that react on alkaline
batteries.
Digital to analog converter consists of shift register U152,
R166, R167, R168, R169 and R170. These are the discrete
components that make the resistor ladder digital to analog
converter. The output of the DAC is in a form of a voltage.
Since the power levelling on the rf board requires current as
a reference, this voltage has to be converted into current.
Voltage to current converter consists of U150-2, and Q101.
This is a standard voltage to current converter.Since the oper-
ational amplifier cannot work at zero volt input, reference
zero level has been shifted to around 1.5 volt on operational
amplifier input by R173 and R174. The DAC voltage also is
shifted accordingly by R172. The output of this section will
go to the power levelling circuit on the rf board. A delay
capacitor is added (C169) to ensure that DAC voltage will
appear only after TXB+. A fast discharge transistor (Q404)
is needed to ensure that the capacitor is fully discharged
before transmitting.
The cut back circuitry (U150-1 circuitry) is used to protect
the radio from operating beyond the capability of the supply
voltage especially when radio is powered by alkaline batter-
ies. Alkaline batteries have higher internal resistance and 
						

							 
March, 19976881086C09-O
 
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GP68 Portable Radios Service ManualTheory of Operation 
RF Board
 
have difficulty to source high current (2.1 ampere at 5 watts
operation). If there are forced to do so, the voltage will drop
and when the voltage hits 5 volts, the radio will automati-
cally reset by it self. It means the batteries cannot be used at
all for transmitting even though there are still a lot of power
inside the batteries. With this circuitry, the user will be able
to enjoy the radio operation at any battery condition, as long
as the batteries are able to source current sufficient to support
100 mW output.
What the circuit does is just protecting the supply voltage
from dropping below 5.5 volt by reducing the output power
by means of reducing the programmed current to the power
levelling circuitry. The circuit is inactive when the voltage is
higher than 5.5 volt.
The threshold voltage is tapped from the +5VTX and the
supply voltage is sensed on the SWB+. C167 is a compensa-
tion capacitor and C165 is a speed up capacitor to ensure that
this circuitry can react faster than the power levelling cir-
cuitry. 
RF Board 
RF board consists of synthesizer, VCO, receiver section, five
watts power amplifier, harmonic filter with antenna switch
and rf power levelling circuitry. 
Receiver 
The receiver of the GP60 Series UHF and VHF radios con-
sists of 4 major blocks each: 
•the front-end module, 
•the double balanced mixer, 
•the first IF stage (45.1 MHz for VHF and 73.35 MHz
for UHF), and
•the back-end IF IC.
The UHF and VHF front-ends consist of three blocks of cir-
cuitry each: 
•a pre-selector,
•an RF amplifier, and 
•a post-selector filter.
All filters are fixed-tuned designs to eliminate the need for
factory tuning and to provide wide-band operation.
The VHF design uses both shunt and series coupled topology
while the UHF design incorporates only shunt coupled topol-
ogy. The UHF design is optimal for attenuating undesired
signals on its lower side while the VHF design is more
heavily attenuated on its upper side. The worst case image
frequency for VHF is 90.2 MHz above 136 MHz, while the
worst-case of UHF is 146.7MHz below 430MHz. The UHF pre-selector filter is a 2-pole, 0.1 dB Chebyshev
bandpass design implemented in a shunt coupled resonator
topology. The 3 dB bandwidth is approximately 45MHz,
centered at 450 MHz. The center of the band insertion loss is
approximately 1.8 dB. The 2-pole filter is designed to oper-
ate with a 50 ohm input termination, while the output termi-
nation is the input impedance of the RF amplifier that
follows it.
The VHF pre-selector is also a 2-pole, 0.1 dB Chebyshev
bandpass design but with shunt series coupled resonator
topology. This topology provides fairly symmetrical attenu-
ation around the center frequency of 155 MHz. The 3 dB
bandwidth is approximately 60 MHz. Center of band inser-
tion loss is about 1.5 dB. The input is matched to 50 ohms
while the output is matched to the proceeding RF amplifier.
The RF amplifier, Q1, is a Motorola MMBR941L NPN
device biased in a common emitter configuration. The amp is
stabilized by the shunt feedback resistor R1 with a gain of
approximately 19 dB at VHF and 16 dB at UHF. The noise
figure is about 3.5 dB and 3.0 dB at VHF and UHF, respec-
tively. The VHF amplifier draws 2.5 mA of current while the
UHF amplifier draws 3.0 mA of current Both are supplied by
the receive 5 Volt supply (indicated as “+5V Rx” on the
schematics and block diagrams).
Terminating the RF amp is the post-selector filter. This filter
is a 3-pole 0.1 dB Chebyshev design for both bands. The
VHF design is series coupled topology while the UHF is
shunt coupled. The 3 dB bandwidth is approximately
58 MHz centered at 155 MHz for VHF and 25 MHz centered
at 460 MHz for UHF.
The insertion loss of this filter is approximately 2.0 dB for
VHF and 2.7 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 front-end is about 14.0 dB
(VHF) and 10.8 dB (UHF) in the center of the band. The net
center 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 0.25   
m  
V for 12 dBs.
The double balanced mixer is a module. Internal to it is the
two baluns and ring diode. The mixer operates with an LO
level of about +5 dBm for both VHF and UHF. The mixer
conversion loss is approximately 6 dB. The double balanced
type mixer provides excellent isolation between any two
ports. 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-linearization, such as IM and Half-IF. The purpose of
the mixer is to translate the received signal down to the fre-
quency of the first IF, 45.1 MHz for VHF and 73.35 MHz for
UHF, where it then 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 
						

							 
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6881086C09-OMarch, 1997
 
Theory of OperationGP68 Portable Radios Service Manual
Transmitter
 
second LO, the second IF, and the IF IC chip. The first LO
signal and the RF signal mix to the IF frequency (45.1 MHz
for VHF and 73.35 MHz for UHF) which then enters the IF
portion of the radio.
The signal first enters the high IF, passes through a crystal fil-
ter and is then amplified by the IF amp. The crystal filter pro-
vides selectivity, second image protection, and
intermodulation protection. The amplifier provides approxi-
mately 10 dB of gain at VHF and 18 dB of gain at UHF to
the signal. The high IF has an approximate 3 dB bandwidth
of 18 kHz.
The filtered and amplified IF signal then mixes with the sec-
ond local oscillator at 44.645 MHz for VHF and 72.895 MHz
for UHF. 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 can be optimized for the radio. The AFIC
(on the controller board) allows the radio’s squelch opening
to be electronically adjusted. 
Transmitter 
The GP60 Series Radio VHF and UHF transmitters contain
five basic circuits:
•a power amplifier, 
•an antenna switch, 
•a harmonic filter, 
•an antenna matching network, and
•a power levelling circuit.
Refer to the block diagram and the schematic for more infor-
mation.
The PA of both the VHF and UHF transmitters consists of
four stages of amplification with the corresponding stages
using the same transistors. The first two stages of both PA
line-ups utilize the MMBR951L transistor, while the third
stage uses a Phillips BLT50 transistor, and the last stage uses
the Motorola MRF873 transistor. The VHF PA line-up is
capable of supplying 5 watts or more of output power, while
the UHF PA line-up is capable of more than 4 watts at the
antenna port. The power out of each line-up can be varied by
changing the voltage (VCTL) on their second (MMBR951L)
stages.The antenna switch circuit consists of two PIN diodes
(CR101 and CR102), a pi network (C145, L115, and part of
C140), and a current limiting resistor (R115). The UHF cir-
cuit contains one additional capacitor (C149), which is used
to tune out CR102’s lead inductance. In the transmit mode,
TxB+ is applied to the circuit to bias the diodes “on”. To
enable the Tx signal to go to the antenna rather than the input
of the receiver, 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 there
exists a low attenuation path between the antenna and
receiver ports.
The harmonic filter consists of C141, C142, C169, C165,
C166, C168, L112, L113, L114 and part of C140. The design
of the harmonic filter for both VHF and UHF is based on a
10-pole, 0.1 dB ripple Chebyshev filter. The antenna output
required a 50 ohm termination.
Note that to measure the power out of the transmitter, one
must remove the antenna and screw in its place a special
BNC adapter, HLN9087A. 
Power levelling 
The GP60 Series Radios utilize a current comparator auto-
matic level control to control its output power. Incident
power (power going out into the antenna) and reflected
power (power reflected back into the radio due to antenna
mismatch) are detected by two doublers on the 50 db coupler.
The detected current is compared with programmed current
at the current comparator transistor Q153. The error current
then will be amplify by a dc amplifier (Q152, part of U151
and Q155) to generate a control voltage (VCTL). The system
will always maintain the detected current to be the same pro-
grammed current. The programmed current (supplied by the
controller board) is used to set the output power.
C154 on the VCTL and C153 is the compensation capacitors
to ensure system stability. 
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. 
						

							 
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GP68 Portable Radios Service ManualTheory of Operation 
The GP60 Series Radio Alignment Procedures
 
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) which are driven by the micro-
processor, U401. A serial stream of 98 bits is sent whenever
the synthesizer is programmed. A 5 Volt dc signal from pin 2
of U201 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. Internally 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 atten-
uator for modulation balancing purposes before being out-
putted 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 approxi-
mately 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, R211, R207, and CR203. The loop filter,
comprised of R202, R204, R205, 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 STEERING LINE VOLTAGE is
received at the VCO. This voltage is a DC voltage between 3
and 11 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. On VHF radios, the rf signal at U251
pin 2 is run through a low pass filter. On UHF radios, the rf
signal is run through a buffer amplifier before it is passedthrough the low pass filter. This is to provide additional iso-
lation to the receive VCO from high level received rf signals.
The rf signal after the low pass filter is the LO RF INJEC-
TION and it is applied to the first mixer at U41 pin 3.
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. This rf signal is the Tx RF INJECTION.
Also in transmit mode, the audio signal to be frequency mod-
ulated onto the carrier is received by the transmit VCO mod-
ulation circuitry at AUDIO IN.
When 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 mod-
ulation attenuator in the off state. This mode is used to reduce
current drain of the radio. 
Display Board 
The display driver (U801) is powered up by the +5V line
from the controller. Pin 21 and 49 of the U801 should have
the voltage of +5V. The clock frequency of the LCD driver is
determined by R814, R815, and C801. This frequency is
approximately 1.61 kHz.
The +5V line to the U801 also provides bias voltages to pins
23, 24, and 26 of U801 through R811, R812, and R813.
The LEDs are biased, using R802 and R803, through the
+5V line. The switch Q801 is controlled by the
LCD_BCK_LIGHT_EN line. When this line goes high (i.e.
5V), Q801 is turned on and the LEDs lights up. 
The GP60 Series Radio Alignment 
Procedures 
The following procedures are to be done together with the
RSS. 
RSSI Threshold Adjustment 
Tuning Frequency:  
Automatic - Frequency displayed on Tuning screen.
•Apply a standard reference level signal of -47 dBm,
1 kHz tone with 3 kHz deviation.
•Adjust the audio output of the radio to rated level
(0.25W), i.e. 2.45 V rms.
•Reduce the generator level until 10 dB SINAD is
obtained.
•While the radio is in the 10 dB SINAD mode, press
the up-arrow key once to program the correct RSSI
setting into the radio. 
						

							 
2-8
 
6881086C09-OMarch, 1997
 
Theory of OperationGP68 Portable Radios Service Manual
The GP60 Series Radio Alignment Procedures
 
Low Port Modulation 
Tuning Frequency: 
Automatic - Frequency displayed on Tuning screen. 
Deviation Setting: 
375 Hz +/- 40 Hz for 12.5 kHz channel spacing,
750 Hz +/- 40 Hz for 20/25/30 kHz channel spacing.
•Set the radio into TX low power mode. The Modula-
tion Analyzer should be set as follows:
•Use the up/down arrow keys to change the low port
deviation setting and the F6 key to toggle the PTT.
NOTE
 
The low port tuning tone is automatically
generated internally by the radio. No external 
modulation injection is required. 
VCO Deviation Adjustment 
Tuning Frequency: 
Automatic - Frequency displayed on Tuning screen. 
Deviation Setting: 
2.2 kHz +/- 100 Hz for 12.5 kHz channel spacing,
4.6 kHz +/- 200 Hz for 20/25/30 kHz channel spacing.
•Set the radio into TX low power mode.
•Inject a 110 mV rms, 2 kHz audio signal into the
external mic using the radio test box.
•The Modulation Analyzer should be set as follows:
•Use the up/down arrow keys to change the deviation
setting and the F6 key to toggle the PTT. 
Transmitter Power Adjustment 
Tuning Frequency: 
Automatic - Frequency displayed on Tuning screen. 
Power Level: 
VHF - 1 W and 5 W,
UHF - 1 W and 4 W.•For power tuning, it is important to ensure that the
DC Voltage  
MUST 
 be maintained under load at 7.5
V +/- 0.1 V (3 A is the current limit).
•Use the up/down keys to change the power setting
and the F6 key to toggle the PTT.
•Tune the radio according to the specification above.
NOTE
 
To avoid heating the radio too much, do not
leave the radio in TX mode continuously and
leave a 30 second interval between tuning 
points. 
Reference Oscillator Warp Adjustment 
Tuning Frequency: 
Automatic - Frequency displayed on Tuning screen.  
Frequency window: 
VHF = +/- 300 Hz,
UHF = +/- 400 Hz.
•Set the radio into TX low power mode.
•Use the up/down arrow keys to change the frequency
setting and the F6 key to toggle the PTT. FM
PEAK+
15 kHz LP Filter “ON”
All HP Filters “OFF”
De-emphasis “OFF”
FM
PEAK+
15 kHz LP Filter “ON”
All HP Filters “OFF”
De-emphasis “OFF” 
						

							 March, 1997 6881086C09-O 2-9 
Squelch Attenuator
Rx Filter
Tone
Generator
Lowspeed
Generator
Deviation
Attenuator
Splatter
FilterLimiter
De-emphasis
5 Volt
Regulator
Audio
PA
Volume
PA Enable
Speaker
Mic Enable
Mic
PreampliÞer Pre-
Highspeed
Data
Generator
Channel
Switch
Push
Buttons Lowspeed
Control
Battery
MonitorFrom
BatterySCI BUS
Harmonic
FilterReceiver1st
MixerIF2nd Mixer
Demodulator
Synthesizer
VCOReference
Oscilator
RF
PA
Analog
Control
Steering/
VCO
ModWarp
Prescaler
Squelch Adjust
Rx
Audio
Fast
Detect
Carrier
Detect
Adapt
Rx/Battery Saver
Lock
Detect
ClockShift
Tx SPI
BUS
Tx
Audio
LEDs Antenna
From
Battery
Reset 5V
 
m 
P
2.1 MHz
Lowspeed
Detector
Audio
Filter IC
5 Volt
Regulator
5V   
m
 
P
 
The GP60 Series Portable 
Radio Functional Block 
Diagram
 
Power
Referenceemphasis 
						

							 
2-10
 
6881086C09-O March, 1997
 
     
 RECEIVER  BLOCK  DIAGRAM 
IF 
Amp 
2-Pole
Crystal
 Filter
15+ Lo  +6dbm     
     RF
Amp    2-Pole
Bandpass
    Filter     Filter  Bandpass     3-Pole
Detected  audio From
Antenna
Switch
 5V RX5V RX
 
2nd LO 
4-Pole
Ceramic 4-Pole
Ceramic 
 TRANSMITTER BLOCK DIAGRAM 
Pin +5V Tx
Pre-driver
Driver
 
TX B+
Final
CouplerAntenna
    SwitchHarmonic
  Filter
 Power 
ControlTo Receiver
 
IF IC  
Buffer
VCTL
B+
IDAC 
Block Diagram for Receiver and Transmitter