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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.
MAXON SM-2000 MOBILE
THEORY OF OPERATION
Page-14-
December 98