Transmitter; Power Levelling; Frequency Generation Circuitry - Motorola GP68 Service Manual

Theory of Operation

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-
v i d e s s e l e c t iv i t y, s e c o n d i m a g e p r o t e c t i o n , a n d
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.
2-6
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.
6881086C09-O
GP68 Portable Radios Service Manual
March, 1997