THEORY OF OPERATION
SST-442
16
INTRODUCTION
The SST-442 is a four-channel handheld transceiver
built on a single multi-layer printed circuit board. Both
sides of the main printed circuit board are populated
with components, with the bottom side containing
only surface mounted components.
The SST-442 is frequency synthesized, with all
functions of the radio controlled by microcontroller.
POWER SUPPLY AND VOLTAGE DISTRIBUTION
The SST-442 is powered by an internal 6-cell
rechargeable battery pack. The battery pack supplies
power to the radio via two contact terminals that are
connected to the PCB through P302. F301 is a 3A
fuse in series with P302 for short circuit protection.
The battery pack may be charged through 2.5 mm
charge jack J301 via CR302 and F301. Zener diode
CR301 prevents the batteries from discharging
through the charger accessory, stops a reverse
voltage from being applied to the battery pack
through J301, and prevents a high voltage from being
applied to the radio circuitry.
SW301 ON/VOL UP switch is pressed to turn on the
SST-442, turning on voltage pass transistor Q304 via
R302 and CR303. Q304 supplies power to IC309, a
+5 VDC regulator used to power microcontroller
IC301. Pin 13 of IC301 turns on Q303, which keeps
Q304 turned on after SW301 has been released.
Power is removed from the SST-442 by pressing the
VOL DN/OFF switch SW302 until a beep is heard
from the speaker, at which time Q304 is turned off.
+5 VDC regulator IC309 supplies power directly to
microcontroller IC301, bilateral audio gates IC304,
reference oscillator Y302, synthesizer IC401, and
PTT detect transistor Q302.
The T/R output at Pin 11 of IC301 lets Q101 switch
the reg5 VDC to the receiver. Pin 11 of
IC301 will also turn on Q202, which turns on voltage
pass transistor Q204. With Q204 emitter connected
to the +VBATT battery supply, the collector output is
connected to the emitter of Q202 through voltage
divider R202/R203, whose values are selected to limit
the TX+V voltage to +6 VDC.
Power Strobe
The SST-442 handheld includes a power strobe
feature, which reduces battery current drain by
periodically removing voltage from part of the radio.
The strobe off cycle time is programmable using the
PC Programming Kit. The +5V SW power strobe
output at Pin 10 of IC301 controls Q306, which
switches the reg5 VDC to the audio
processing circuitry and the synthesizer circuitry.
This includes IC303A 2.5 VDC (Vag) for bias on
audio processing circuitry, IC303C audio high pass
filter, IC308 and IC305A audio low pass filter for sub-
audible frequencies, IC305B audio limiting amplifier,
IC306 digital potentiometers, IC303B audio summing
node amplifier, and IC303D audio low pass filter.
Low Battery Voltage Detection
Battery voltage is measured at A/D input Pin 16 of
IC301 through voltage divider R303/R305. The radio
will emit a periodic beep if low battery voltage is
detected, and will turn the radio off if the battery
voltage drops below +5.2 VDC. This is to protect the
microcontroller and its EE memory from corruption
due to low supply voltage.
REFERENCE OSCILLATOR
Reference oscillator Y302 is a 1.5 ppm temperature
controlled, voltage controlled oscillator (TCVCXO)
operating at 14.4 MHz. The Pin 4 output of the
TCVCXO provides a reference for the frequency
synthesizer IC401 at Pin 20. IC401 is programmed to
provide an output at Pin 1 that is ¼ of the reference
(3.6 MHz), which is applied as a reference oscillator
to Pin 27 of IC301 microcontroller. The Y302 pin 4
output is also multiplied up to 43.2 MHz by Q105 and
its associated circuitry to provide a receiver 2nd local
oscillator signal.
SYNTHESIZER
The SST-442 radio is built around a common phase-
locked loop (PLL) that consists of a voltage-controlled
oscillator (VCO) and a frequency synthesizer. The
PLL generates both the receiver 1st local oscillator
and transmitter carrier signals. Control signals from
microcontroller IC301 and Reference oscillator Y302
are routed to frequency synthesizer IC401 per the
following chart:
Pin Numbers
IC301
Y302
IC401
Clock
12
-
18
Data
5
-
19
Latch
20
-
17
LD
12
-
2
T/R SW
11
-
-
REF IN
-
4
20
REF OUT
27
-
1
+5 SW
10
-
12
+5V
1, 15, 28
2
5, 14
Q404, CR401, C414 and associated components
provide a filtered supply for the VCO oscillator and
buffer amplifiers.
Prescaler Divider / Synthesizer Controller
IC401 contains both a prescaler and synthesizer
controller. The prescaler squares and divides the
VCO output present at pin 11 by either 64 or 65,
determined by a synthesizer controller logic signal.
The exact number of times the prescaler is instructed