Maxon SL7000, Operational Description

Page: 7 / 62

SL7000SERIES HAND HELD
7
3. THEORY OF OPERATION
INTRODUCTION
SL7000 is 256 channel portable FM transceiver constructed with a microprocessor controlled, temperature compensated
Phase Locked Loop (PLL) frequency synthesizer. The radio features a double conversion receiver and a direct FM
transmitter modulator. A special integrated circuit provides support to sub-audible signaling (CTCSS & DCS) and most of
the receiving parts are switched off periodically in the power saver mode to reduce battery current drain during standby.
The Block Diagram RF and Control Circuit Diagrams for SL7000 shall be used in associate with the following circuit
description.
CIRCUIT DESCRIPTIONS
1) PHASE-LOCK LOOP (PLL) CIRCUIT
* REFERENCE OSCILLATOR
The reference oscillator in the Frequency Synthesizer IC401, uses the TCXO401 as a stabilized source of
15.3MHz. The reference oscillator frequency drives the divider in the Frequency synthesizer to produce a
comparison frequency.
This comparison frequency is selected by decoding the first three bits of the data input from microcomputer.
* PROGRAMMABLE DIVIDER
The programmable divider in IC401divides the VCO output . It consists of a two-modulus pre-scalar with a 7bit
control register followed by a 11-bit internal programmable divider. The overall division ratio is selected by a single
19-bit word located on the serial data bus.
* PHASE COMPARATOR
The digital-type phase comparator in IC401 compares the divided VCO frequency with the comparison frequency.
It generates a correction voltage that is applied to a low-pass filter consisting of R616,R624,R625, C613,C616,
C619. This voltage is applied to the VCO circuit. The phase comparator also provides the lock detect signal .
* VCO CIRCUIT
The VCO circuits contains a separate RX VCO (Q651,D651,D652) and TX VCO (Q661,D661,D662,D663,D664). The oscillated
signal is amplified at the buffer amplifiers (Q686,Q687)and is then applied to the T/R switch (D696,D697). Then thereceive 1st
LO (Rx) signal is applied to the 1st mixer (Q831)and the transmit (Tx) signal to the amplifier circuit (PA MODULE unit; Q701).
A portion of the signal from the buffer amplifier (Q686) is fed back to the PLL IC) via the doubler circuit (Q681)
as the comparison signal.
2) TRANSMITTER
* MIC AMP CIRCUIT
Voice signal from the microphone are applied to microphone amplifier IC355. IC311 is configured as a low-pass
filter that has a 6dB/oct response between 300Hz and 3kHz
and eliminates harmonics above 3 kHz.
and
The
pre-emphasized audio signal.
The filtered AF signals from the IC311 are applied to the FM/PM switch (IC202), and pass
through the low-pass filter (IC201). The filtered signals are applied to the D/A converter (IC203). The output signals from the
D/A converter (IC203) are applied to the modulation circuit (D662).
* VCO AND AMPLIFIER
The VCO signal output is switched by diodes D696 and then amplified by Q701, Q711, Q721 and then fed to
power amplifier Q731.
* POWER AMPLIFIER CIRCUIT
Q731 is provided with approximately 7.4 DC power source.
The power detector circuit (PA MODULE unit; D761,D771) detects the transmit power output level and converts it into DC
voltage.The output voltage is at a minimum level when the antenna impedance is matched with 50
Ω
and is increased when it
is mismatched. The detected voltage is applied to the differential amplifier IC791
, and the “PA_TUNNING_2” signal from the
D/A converter controlled by the MCU is applied to the other input for reference. When antenna impedance is mismatched, the
detected voltage exceeds the power setting voltage. Then the output voltage of the differential amplifier IC791 controls the
input bias voltage of the drive (PA MODULE unit;Q721,Q731)
The signals from Q731 is supplied through a low-pass filter made up of L201, L202, L203 and C201-C206, C214