User's Manual

Maxon SM5102Radio
Power Amplifier and Harmonic Filter
The power amplifier contains transistors Q12, Q13, Q18, Q39, Q28 and associated inductors, capacitors
and resistors. When the radio is in transmit mode the diode D1,D15 is forward biased enabling the
modulated RF signal from the VCO (amplified by the first stage amplifier / buffer Q12 and Q13) to pass to the
pre-driver Q18 via Q39. The output signal is passed from Q39 to Q28 where it is then amplified for
transmission. The amplified RF signal is passed through stripline coupler and is fed to the harmonic low
pass filter, comprising L21, L25, L22,L27,C103,C168,C296, C100, C65, C297,C99 and then to the antenna
connector (ANT). The stripline coupler provides a sample of the RF signal for the automatic power control.
During transmit D1,D15 is forward biased which connects the power amp to the antenna. D2,D15 is forward
biased inhibiting transmit signal power from being fed to the receiver circuitry.
3.2 VHF Receive
The receiver is comprised of:
• RF amplifier
• First mixer and first IF amplifier
• Second mixer, second IF amplifier and FM detector
• Receiver audio circuit
• Mute (Squelch) circuit
RF amplifier
The receiver Front End module contains two stages of filtering and an amplifier, Q601. The module filters
out the unwanted frequencies and provides a gain of typically 12dB for the wanted frequencies. The wanted
RF signal at the operating frequency is passed to the first mixer.
The signal received from the antenna is routed through the 7
th
order Chebyshev low-pass filters contained
C601, C603, C604, CL603, C605,C608 and L601, L602, L604 and passed through Front End Module (RF
amplifier) via pin 1. The front-end module contains D601 to Q601, the front end module is configured to
enable the RF signal at the operating frequency to pass to the first mixer.
First Mixer and First IF Amplifier
The VCO local oscillator signal routed through buffer transistors Q2 is filtered by C14, C15, C16,C17 and
L2and L4. D4 produces a difference frequency IF of 45.1MHz from pin 6 of front end module and the filtered
VCO local oscillator signal at pin 4. The 45.1MHz difference frequency is filtered by the 2-pole crystal filter
FL4,5. The tuned circuit T1 and T2 and associated components provide matching of the crystal filter to
ensure good pass-band response and selectivity. The IF signal is amplified by Q3 and passed to the second
mixer, second IF and FM detector U2.
Second Mixer, Second IF, FM Detector
U2 is a single conversion FM receiver integrated chip and contains the second mixer, second IF amplifier
and FM detector. The second local oscillator frequency is determined by the crystal Y6 connected to pin 1 of
U2. The first IF signal is received at pin 16 of U2 and applied to the mixer. The output of the second mixer,
a frequency of 455kHz, is the difference between the IF signal and the second local oscillator. The 455kHz
passes via pin 5 and is applied to a 455kHz bandpass filter, FL2, (12.5/25kHz channel spacing) or FL3
(12.5kHz channel spacing). The selection of the filters is accomplished by diodes D11 (input) and D12
(output) whose bias is controlled by software and applied to the diodes from pin 98 of the microprocessor
(U18). The output of FL2/FL3 is passed via pin 11 to a high gain amplifier coupled to the adjustable
quadrature detector Y1 (pin 10). Any detected signal is produced at pin 96 of U18 and applied to the
Receiver Audio Circuit and the Mute (Squelch) circuit.