Rf Board; Receiver; Intermediate Frequency (If) - Motorola GP68 Service Manual

GP68 Portable Radios Service Manual

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.
March, 1997
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 µ 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
6881086C09-O
Theory of Operation
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