Table of Contents
4-ii Theory of Operation
6.0 Receive Audio Circuits............................................................................ 12
6.1 Squelch Detect .......................................................................................... 12
6.2 Audio Processing and Digital Volume Control........................................... 13
6.3 Audio Amplification Speaker (+) Speaker (-) ............................................. 13
6.4 Handset Audio ........................................................................................... 14
6.5 Filtered Audio ............................................................................................ 14
6.6 Discriminator Audio (Unfiltered)................................................................. 14
6.7 Option Board Audio ................................................................................... 14
7.0 Receive Signalling Circuits..................................................................... 15
7.1 Sub-audible Data Decoder (PL/DPL) ........................................................ 15
7.2 High Speed Data Decoder......................................................................... 15
7.3 Alert Tone Circuits ..................................................................................... 16
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 Power AmpliÞer (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 (U7501) ............................................................. 21
10.3 Voltage Controlled Oscillator (VCO).......................................................... 22
10.4 Synthesizer Operation ............................................................................... 22
11.0 Receiver Front-End.................................................................................. 24
11.1 Front-End Band-Pass Filter and Pre-Amplifier .......................................... 24
11.2 Mixer and Intermediate Frequency (IF) Section ........................................ 24
11.3 IF IC (U5201)............................................................................................. 25
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