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

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.

Page-14-

December 98