Uhf2 Transmitter; Figure 2-8. Transmitter Block Diagram - Motorola PR860 Detailed Service Manual

Theory of Operation: UHF2 Transmitter
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
Desired Mode
Tx
Rx
Battery Saver
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 RF signal is the LO RF INJECTION and it is applied to the mixer at
T302 (refer to UHF (403–470 MHz) Receiver Front End Schematic Diagram on page 4-23).
During the transmit condition, when PTT is depressed, five volts is applied to U241 pin 19. This
activates the transmit VCO by enabling the transmit oscillator and the transmit buffer of U241. The RF
signal at U241 pin 10 is injected into the input of the PA module (U101 pin16). This RF signal is the
TX RF INJECTION. Also in transmit mode, the audio signal to be frequency modulated onto the
carrier is received through the U201 pin 41.
When a high impedance is applied to U241 pin19, the VCO is operating in BATTERY SAVER mode.
In this case, both the receive and transmit oscillators as well as the receive transmit and prescaler
buffer are turned off.
2.8

UHF2 Transmitter

From VCO
The UHF transmitter consists of the following basic circuits as shown in
• Power amplifier (PA).
• Antenna switch/harmonic filter.
• Antenna matching network.
• Power control integrated circuit (PCIC).
Table 2-5.
Table 2-5. Level Shifter Logic
AUX 4
Low
High
Low
P C I C
Vcontrol
P A
Driver

Figure 2-8. Transmitter Block Diagram

AUX 3
High (@3.2V) High (@4.8V)
Low Low
Low Hi-Z/Float (@2.5V)
Vcontrol
PA-Final
Stage
TRB
Antenna
Jack
Antenna Switch/
Harmonic Filter/
Matching Network
Figure 2-8.
2-11