Maxon Telecom SM-2000 service manual RF Section SM-2150/VHF

Page 18

MAXON SM-2000 MOBILE

THEORY OF OPERATION

RF SECTION (SM-2150/VHF)

VHF Transmitter

The Transmitter is comprised of:

1.Microphone Audio Circuit

2.Transmitter Stage and Harmonic Filter

3.Automatic Power Control

4.Frequency Synthesizer Circuit

Microphone Audio Circuit

Microphone audio from the digital circuitry is amplified, pre-emphasized, limited, and mixed with Sub-Audible and applied to the VCO at Pin 1, through RV101, to balance VCO and TCXO modulation.

Transmitter Stage and Harmonic Filter

When in the transmit mode of operation the diode D103 is forward biased enabling the RF signal to pass to the pre-driver Q118 by Q116 and Q117. The amplified RF signal is amplified again by Q120. The amplified RF signal passes through the stripline coupler and is fed to the harmonic low pass filter, comprising of L128, L130, L131, and L132, capacitor 271, 273, 275, 277, and 283 and then to the antenna connector (ANT). The stripline coupler provides a sample of the RF signal for the automatic power control. During transmit D107 is forward biased which connects the power amp, D120, to the antenna. D120 is forward biased inhibiting transmit signal power from being fed to the receiver circuitry.

Automatic Power Control

The automatic power control contains the stripline coupler, diode D107 and variable resistor VR104, IC104A, and three transistors Q123, Q124, and Q125. IC104A forms a differential amplifier. The RF signal present in the coupler is rectified by D107, to produce a DC voltage proportional to RF power which is applied to Pin 3 of IC104A. TX 8.5V is also applied to the input of IC104A via the potential divider R168, VR104, and R170 at pin 2. The different signal at output of IC104A is passed to Q124 and Q125 to produce a constant power output to the antenna connector (ANT). VR104 is used to adjust the RF high power level. VR105 is used to set the low power setting.

RX & TX Voltage Controlled Oscillator Circuit

The circuit contains two VCO’s. One for producing carrier frequencies during transmit and one for producing the local oscillator frequency during receive. The circuit also has RX and TX power line filters.

RX & TX Power Line Filters

Transistors Q353 and Q354 are configured as TX5v and RX5v power supply ripple filters respectively.

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December 98

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Contents SM-2000 Page Table of Contents Maxon SM-2000 MobileSchematics and Block Diagrams PCB LayoutsSM-2000 Exploded View Specifications Transceiver Nominal PerformanceGeneral Operational Bandwidth BandTransmitter Specfications TX Tone Modulation Characteristics SUB Audible Tones DCSReceiver Subaudible tones CTCSS/DCS RX Tone Demodulation CharacteristicsAccessories Reliability AnalysisIntroduction Unpacking Information Check the carton carefully for the following itemsGeneral Description Description of Controls Description of ControlsSM-2000 Rear Panel CTCSS/DCS Decode Circuits Theory of OperationDigital Circuitry CTCSS/DCS Encoder CircuitEeprom PLL RF Section SM-2150/VHF RX VCO TX VCOReceiver Out of Lock DetectorPrescaler RF AmplifierAudio Amplifier Receiver Audio CircuitLow Pass Filter Noise Detect Circuit SQ BoardRF Section SM-2450/UHF UHF TransmitterFrequency Synthesizer Circuit Voltage Controlled Oscillator Circuit First MixerSecond Oscillator- Mixer- Limiter and FM Detector Noise Detector Squelch Mute CircuitSquelch Noise Band Pass Filter Speaker Audio AmplifierTest Setup Transmit TestsSM-2150/VHF Alignment Procedure PLL AlignmentDiscriminator Tuning Receiver Alignment RFSquelch Adjustment Automatic Power AdjustmentCTCSS/DCS Deviation Adjustment Modulation Deviation AdjustmentAlignment Points Diagram SM-2150/VHF SM-2450/UHF Alignment Procedure Page-28 December ∙ Note Release the PTT button when changing channels Page-30 December Alignment Points Diagram SM-2450/UHF Component Replacement Surface Mount ComponentsSurface Mount Removal Surface Mount Component Replacement Surface Mounted Integrated Circuit ReplacementParts List Common to SM-2000 REF # DescriptionDigital Parts Common on Both VHF & UHF Digital Section Band Chart REF # Description FRONT-END PCB Assembly Main Board PartsPage-38 December Page-39 December Page-40 December Page-41 December SM-2450A UHF Band SM-2450C UHF Band SM-2450D UHF Band Band Chart SM-2450E UHF BandPage-44 December Page-45 December Page-46 December Page-47 December Component Pinout Page-49 December Page-50 December Voltage Charts SM-2150/VHF IC409Page-52 December Transistor Voltage Chart Voltage Charts SM-2450/UHF IC404 Page-56 December Trouble Shooting Chart SM-2000 Status Indications & Audible TonesPage-58 December Replacement Module Schematics MMBV109MA-4472 Microphone Schematic Page-61- December Page-62- December Page-63- December Page-64- December SW5 SM-2000 Control HeadSM-2150/VHF Main Board Assy Page-66 DecemberSM-2450/UHF Main Board Assy FrontSM-2000 SM-2150/VHF Exploded View Parts List Intentionally Left Blank
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