Motorola Gp328plus Gp338plus Gp338xls Detailed 6804112j28 G Manual

Transmitter5C-5
4.1.2 Antenna Switch
The antenna switch circuit consists of two PIN diodes (CR101 and CR102), a pi network (C107, L104 
and C106), and two current limiting resistors (R101, R170).  In the transmit mode,  B+ at PCIC (U102) 
pin 23 will go low and turn on Q111 where a B+ bias is applied to the antenna switch circuit to bias the 
diodes on.  The shunt diode (CR102) shorts out the receiver port, and the pi network, which 
operates as a quarter wave transmission line, transforms the low...

5C-6Receiver
5.0 Receiver
5.1 Receiver Front-End
(Refer to UHF Band 2 Receiver Front End Schematic Diagram on page 5C-22 and UHF Band 2 
Transmitter Schematic Diagram on page 5C-26)
The RF signal is received by the antenna and applied to a low-pass filter. For UHF, the filter consists 
of L101, L102, C102, C103, C104. The filtered RF signal is passed through the antenna switch. The 
antenna switch circuit consists of two PIN diodes(CR101 and CR102) and a pi network (C106, L104 
and C107).The signal is...

Receiver5C-7
The output of the post-RF amplifier filter which is connected to the passive double balanced mixer 
consists of T301, T302 and CR306. Matching of the filter to the mixer is provided by C381. After 
mixing with the first LO signal from the voltage controlled oscillator (VCO) using low 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 (FL301) through a resistor pad 
and a diplexer (C322 and...

5C-8Receiver
5.3 Automatic Gain Control Circuit
(Refer to UHF Band 2 Receiver Front End Schematic Diagram on page 5C-22)
The front end automatic gain control circuit  is to provide automatic gain reduction 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’s output. At high radio 
frequencies, capacitor C331 provides the low impedance path to ground for this purpose....

Frequency Generation Circuitry5C-9
6.0 Frequency Generation Circuitry
The Frequency Generation Circuitry is composed of two main ICs, the Fractional-N synthesizer 
(U201), and the VCO/Buffer IC (U241). 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...

5C-10Frequency Generation Circuitry
6.1 Synthesizer
(Refer to UHF Band 2 Synthesizer Schematic Diagram on page 5C-24)
The Fractional-N Synthesizer uses a 16.8MHz crystal (FL201) to provide a reference  for the system. 
The LVFractN IC (U201) further divides this to 2.1MHz, 2.225MHz, and 2.4MHz as reference 
frequencies. Together with C206, C207, C208, R204 and CR203 , 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...

Frequency Generation Circuitry5C-11
6.2 VCO - Voltage Controlled Oscillator
(Refer to UHF Band 2 Voltage Controlled Oscillator Schematic Diagram on page 5C-25)
Figure 5-5: VCO Block Diagram
 The VCOBIC (U241) in conjunction with the Fractional-N synthesizer (U201) generates RF in both 
the receive and the transmit modes of operation. The TRB line (U241 pin 19) determines which 
oscillator and buffer will be enabled. A  sample of the RF signal from the enabled oscillator is routed  
from U241 pin 12,...

5C-12Frequency Generation Circuitry
The VCOBIC(U241) is operated at 4.54 V (VSF) and Fractional-N synthesizer (U201) at 3.3V. This 
difference in operating voltage requires a level shifter consisting of Q260 and Q261 on the TRB line. 
The operation logic is shown in Table 5-1.
In the receive mode, U241 pin 19 is low or grounded. This activates the receive VCO by enabling the 
receive oscillator and the receive buffer of U241. The RF signal at U241 pin 8 is run through a 
matching network. The resulting...

Notes For All Schematics and Circuit Boards 5C-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...

5C-14 Notes For All Schematics and Circuit Boards
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