Audio/Signaling; 200 Mhz Transmitter; Power Amplifier (Pa); Figure 3-5. 200 Mhz Transmitter Block Diagram - Motorola HT1250-LS+ Service Manual

Theory of Operation
3.4.5

Audio/Signaling

The audio/signalling/filter/companding IC (ASFIC) and the audio power amplifier (Figure 3-3) form the
main components of the audio/signalling section of the controller board. Inputs include a 16.8 MHz
clock from the synthesizer, recovered audio and squelch, MCU control signals, and external or
internal microphones. Outputs include a microprocessor clock (CLK), modulator output to the
synthesizer, and amplified audio signals to an internal or external speaker.
An additional opamp gain stage (U2000 in 200 MHz models, or U851 in 700 MHz models, and
associated circuitry) is included to increase the receiver audio level from the IFIC into the ASFIC
audio processing IC. This is necessitated because of the low voice deviation levels encountered in
trunked systems operating at 12.5 kHz channel spacing.
3.5

200 MHz Transmitter

The 200 MHz transmitter consists of the following basic circuits as shown in Figure 3-5.
•

Power amplifier (PA).

• Antenna switch/harmonic filter.
• Antenna matching network.
• Power control integrated circuit (PCIC).
From VCO
3.5.1 Power Amplifier (PA)
The 200 MHz PA consists of two LDMOS devices:
• C65 LDMOS driver IC (U3501)
• MRF1517 LDMOS PA final (Q3501)
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 (U3501 pins 6 & 7) with an input signal of
2mW (+3dBm) at U3501 pin 16. The current drain is typically 180mA while operating in the frequency
range of 216-222 MHz.
The LDMOS PA is capable of supplying an output power of up to 6W with an input signal of 0.5W. The
current drain is typically 1400 mA while operating in the frequency range of 216-222 MHz. The power
output can be varied by changing the bias voltage.
PCIC
Vcontrol
Power Amplifier (PA)
PA
Driver

Figure 3-5. 200 MHz Transmitter Block Diagram

Vcontrol
Antenna Switch/
PA Final
Harmonic Filter
Stage
Antenna
Matching
Network
3-5