Table of Contents
4-ii Theory of Operation
4.3 PTT Sensing and TX Audio Processing .................................................... 11
5.0 Transmit Signalling Circuits
................................................................... 11
5.1 Sub-audible Data (PL/DPL)....................................................................... 12
5.2 High Speed Data ....................................................................................... 12
5.3 Dual Tone Multiple Frequency (DTMF) Data............................................. 12
6.0 Receive Audio Circuits
............................................................................ 13
6.1 Squelch Detect .......................................................................................... 13
6.2 Audio Processing and Digital Volume Control........................................... 14
6.3 Audio Amplification Speaker (+) Speaker (-) ............................................. 14
6.4 Filtered Audio ............................................................................................ 15
7.0 Receive Signalling Circuits
..................................................................... 15
7.1 Sub-audible Data (PL/DPL) and High Speed Data Decoder..................... 16
7.2 Alert Tone Circuits..................................................................................... 16
300MHz SPECIFIC CIRCUIT DESCRIPTION
8.0 Receiver Front-End
.................................................................................. 17
8.1 Front-End Band-Pass Filter & Pre-Amplifier.............................................. 17
8.2 Mixer and Intermediate Frequency (IF) Section ........................................ 17
8.3 IF IC (U5201)............................................................................................. 18
9.0 Transmitter Power Amplifier (PA) 5-25W
............................................... 18
9.1 Power Controlled Stage............................................................................. 18
9.2 PA Stages.................................................................................................. 19
9.3 Directional Coupler.................................................................................... 19
9.4 Antenna Switch.......................................................................................... 19
9.5 Harmonic Filter .......................................................................................... 20
9.6 Power Control............................................................................................ 20
10.0 Frequency Synthesis
............................................................................... 21
10.1 Reference Oscillator.................................................................................. 21
10.2 Fractional-N Synthesizer (U8701) ............................................................. 21
10.3 Voltage Controlled Oscillator (VCO).......................................................... 22
10.4 Synthesizer Operation............................................................................... 22
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