Motorola Gp328 Gp338 Detailed 6804110j64 F Manual

Transmitter6B-53.1.2 Antenna SwitchThe antenna switch circuit consists of two PIN diodes (D3521 and D3551), a pi network (C3531,
L3551 and C3550), and three current limiting resistors (R3571, R3572, R3573 ). In the transmit
mode, B+ at PCIC (U3502) pin 23 will go low and turn on Q3561 where a B+ bias is applied to the
antenna switch circuit to bias the diodes on. The shunt diode (D3551) shorts out the receiver port,
and the pi network, which operates as a quarter wave transmission line, transforms the...

6B-6Receiver4.0 Receiver4.1 Receiver Front-End(Refer toVHF Receiver Front End Schematic Diagramon page 6B-20,VHF Receiver Back End
Schematic Diagramon page 6B-21, andVHF Transmitter Schematic Diagramon page 6B-24)
The RF signal is received by the antenna and applied to a low-pass filter. For VHF, the filter consists
of L3531, L3532, C3532 to C3563. The filtered RF signal is passed through the antenna switch. The
antenna switch circuit consists of two PIN diodes(D3521 and D3551) and a pi network (C3531,...

Receiver6B-7The output of the post-RF amplifier filter is connected to the passive double balanced mixer which
consists of T3301, T3302 and CR3301. Matching of the filter to the mixer is provided by C3317,
C3318 and L3308. After mixing with the first LO signal from the voltage controlled oscillator (VCO)
using high side injection, the RF signal is down-converted to the 45.1 MHz IF signal.
The IF signal coming out of the mixer is transfered to the crystal filter (Y3200) through a resistor pad
(R3321 -...

6B-8Receiver4.3 Automatic Gain Control Circuit(Refer toVHF Receiver Front End Schematic Diagramon page 6B-20 andVHF Receiver Back End
Schematic Diagramon page 6B-21)
The front end automatic gain control circuit provides automatic reduction of gain, of the front end RF
amplifier via feedback. This action is necessary to prevent overloading of backend circuits. This is
achieved by drawing some of the output power from the RF amplifier output. At high radio
frequencies, capacitor C3327 provides the low...

Frequency Generation Circuitry6B-95.0 Frequency Generation CircuitryThe Frequency Generation Circuitry is composed of two main ICs, the Fractional-N synthesizer
(U3701), and the VCO/Buffer IC (U3801). Designed in conjunction to maximize compatibility, the two
ICs provide many of the functions that normally would require additional circuitry. The synthesizer
block diagram illustrates the interconnect and support circuitry used in the region. Refer to the
relevant schematics for the reference designators....

6B-10Frequency Generation Circuitry5.1 Synthesizer(Refer toVHF Synthesizer Schematic Diagramon page 6B-22)
The Fractional-N Synthesizer uses a 16.8MHz crystal (Y3761) to provide a reference for the system.
The LVFractN IC (U3701) further divides this to 2.1MHz, 2.225MHz, and 2.4MHz as reference
frequencies. Together with C3761, C3762, C3763, R3761 and D3761 , they build up the reference
oscillator which is capable of 2.5ppm stability over temperatures of -30 to 85°C. It also provides
16.8MHz at pin 19 of...

Frequency Generation Circuitry6B-115.2 VCO - Voltage Controlled Oscillator(Refer toVHF Voltage Controlled Oscillator Schematic Diagramon page 6B-23)
The VCOBIC (U3801) in conjunction with the Fractional-N synthesizer (U3701) generates RF in both
the receive and the transmit modes of operation. The TRB line (U3801 pin 19) determines which
oscillator and buffer will be enabled. A sample of the RF signal from the enabled oscillator is routed
from U3801 pin 12, through a low pass filter, to the prescaler...

6B-12Frequency Generation CircuitryThe RF section of the VCOBIC(U3801) is operated at 4.54 V (VSF), while the control section of the
VCOBIC and Fractional-N synthesizer (U3701) is operated at 3.3V. The operation logic is shown in
Ta b l e 6 - 1 .
In the receive mode, U3801 pin 19 is low or grounded. This activates the receive VCO by enabling the
receive oscillator and the receive buffer of U3801. The RF signal at U3801 pin 8 is run through a
matching network. The resulting RF signal is the LO RF...

Notes For All Schematics and Circuit Boards 6B-136.0 Notes For All Schematics and Circuit Boards* Component is frequency sensitive. Refer to the Electrical Parts List for value and usage.
1.Unless otherwise stated, resistances are in Ohms (k = 1000), and capacitances are in picofarads
(pF) or microfarads (µF).
2.DC voltages are measured from point indicated to chassis ground using a Motorola DC multime-
ter or equivalent. Transmitter measurements should be made with a 1.2 µH choke in series with
the...

6B-14 Notes For All Schematics and Circuit Boards