
Theory of Operation
3-11
3.8
700 MHz Transmitter
The 700 MHz transmitter consists of the following basic circuits as shown in Figure 3-10.
•
Power amplifier
•
Antenna switch/harmonic filter
•
Antenna matching network
•
Power control integrated circuit (PCIC)
Figure 3-10. 700 MHz Transmitter Block Diagram
3.8.1
Power Amplifier
The 700 MHz PA consists of two LDMOS devices:
•
C65 LDMOS driver IC (U101)
•
MRF1517 LDMOS PA final (Q101)
The C65 LDMOS driver (U101) provides 2-stage amplification using a supply voltage of 7.5V. The
amplifier is capable of supplying an output power of 0.5W (U101 pins 6 & 7) with an input signal of
2mW (+3dBm) at U101 pin 16. The current drain is typically 180mA while operating in the frequency
range of 746-794 MHz.
The LDMOS PA final was designed to supply an output power of 2.75W with an input signal of 0.3W.
The PA current drain is typically 1050 mA while operating in the frequency range of 746-794 MHz.
The PA power output can be changed by varying the C65 LDMOS driver control voltage.
3.8.2
Antenna Switch
The antenna switch circuit consists of two pin diodes (CR101 and CR102), a pi network (C106-7,
C109 and L103), and three current limiting resistors (R101-3). In the transmit mode, pin 32 of PCIC
U102 goes high which applies a B+ source to the antenna switch circuit to bias the diodes "on". The
shunt diode (CR102) shorts out the receiver port and the pi network. This operates as a quarter wave
transmission line to transform the low impedance of the shunt diode to a high impedance at the input
of the harmonic filter. In the receive mode, the diodes are both off, creating a low attenuation path
between the antenna and receiver ports via L103.
PCIC
PA
Driver
Antenna Switch/
Harmonic Filter
PA Final
Stage
Vcontrol
Vcontrol
Antenna
Matching
Network
From VCO
Power Amplifier (PA)
Summary of Contents for HT1250-LS+
Page 10: ...viii ...
Page 12: ...x Product Safety and RF Exposure Compliance ...
Page 16: ...1 4 Introduction ...
Page 58: ...4 22 Maintenance ...
Page 64: ...5 6 Schematic Diagrams Overlays and Parts Lists ...