Motorola Gp328plus Gp338plus Gp338xls Detailed 6804112j28 G Manual

Transmitter5A-5
4.1.2 Antenna Switch
The antenna switch circuit consists of two PIN diodes (D3521 and D3551), a pi network (C3531, 
L3551 and C3550), and two 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...

5A-6Receiver
5.0 Receiver
5.1 Receiver Front-End
(Refer to VHF Receiver Front End Schematic Diagram on page 5A-22, VHF Receiver Back End 
Schematic Diagram on page 5A-23, and VHF Transmitter Schematic Diagram on page 5A-26)
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...

Receiver5A-7
The 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...

5A-8Receiver
5.3 Automatic Gain Control Circuit
(Refer to VHF Receiver Front End Schematic Diagram on page 5A-22 and VHF Receiver Back End 
Schematic Diagram on page 5A-23)
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 back end circuits. This is 
achieved by drawing some of the output power from the RF amplifier output. At high radio 
frequencies, capacitor C3327 provides...

Frequency Generation Circuitry5A-9
6.0 Frequency Generation Circuitry
The 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...

5A-10Frequency Generation Circuitry
6.1 Synthesizer
(Refer toVHF Synthesizer Schematic Diagram on page 5A-24)
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...

Frequency Generation Circuitry5A-11
6.2 VCO - Voltage Controlled Oscillator
(Refer toVHF Voltage Controlled Oscillator Schematic Diagram on page 5A-25)
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...

5A-12Frequency Generation Circuitry
The 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  
5-1.
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 5A-13
7.0 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...

5A-14 Notes For All Schematics and Circuit Boards
THIS PAGE INTENTIONALLY LEFT BLANK