Transmit Operation; Radio-Frequency Power Amplifier (Rf Pa) And Output Network (On) - Motorola PM1200 Basic Service Manual

Basic Theory of Operation: Radio-Frequency Power Amplifier (RF PA) and Output Network (ON)
3.5.3

Transmit Operation

Transmitter
Voice audio from the microphone is delivered via the Mic (Jack) Unit to the MAIN Unit, after passing
through amplifier Q3039/Q2108 (NJM2902V), pre-emphasis, limiter (IDC instantaneous deviation
control), and LPF Q2001 (NJM2902V); the signal is adjusted for optimum deviation level and
delivered to the next stage.
Voice inputs from the microphone and CTCSS are frequency-modulated to the VCO of the
synthesizer, while DCS audio is modulated by the reference frequency oscillator of the synthesizer.
At the output of the synthesizer, the signal passes through diode switch D1022 (1SS321) to obtain
the required RF drive prior to delivery to the PA Unit. There the RF signal is amplified by parallel
junction transistors Q6006/Q6007 (SD1405X2) and passes through antenna switching relay RL6001
and a low-pass filter circuit, and ultimately to the antenna connector.
RF output power from the final amplifier is sampled by a CM coupler and is rectified by D6006/
D6007 (MA729X2). The resulting DC is fed through Automatic Power Controller Q1038
(NJM2902V), Q1001 (2SC4154E), Q1004 (2SB1132Q) to control the gain of the transmitter RF
amplifier and thus regulate the power output.
Generation of spurious products by the transmitter is minimized by the fundamental carrier frequency
being equal to the final transmitting frequency, modulated directly in the transmit VCO. Additional
harmonic suppression is provided by a low-pass filter consisting of L6009/L6010/L6011 and C6042/
C6043/C6044/C6047/C6074/C6075 and C6045, resulting in more than 65 dB of harmonic
suppression prior to delivery of the RF energy to the antenna.
3.6 Radio-Frequency Power Amplifier (RF PA) and Output Network
(ON)
The RF PA is a three-stage power amplifier consisting of discrete LDMOS transistors:
• Controlled stage
• Driver stage
• Final stage
The RF PA is followed by the ON section, consisting of discrete circuitry with the following functions:
• Antenna switch
• Harmonic filter
• Power detector
6880309U09-A
3-5
October 26, 2007