User's Manual

THEORY OF OPERATION SST-442
18
1st Mixer
The amplified received input signal is applied to the
gate of FET mixer Q103. The 1st local oscillator
signal from the synthesizer module is applied to the
source of Q103. L106, C117 and C118 tune the drain
output of Q103 to 43.65 MHz and apply it to YF101, a
43.65 MHz two-pole crystal filter. Q104 and
associated components amplify the 43.65 MHz IF
signal and apply it to the input of the 2nd mixer at Pin
16 of IC101.
FM Receiver Subsystem
A multi-function integrated circuit, IC101 and
associated components for the FM-receiver
subsystem. The subsystem performs the following
functions: 1) 2nd mixer, 2) 2nd IF amplifier, 3) FM
detector, and 4) noise amplifier.
The Pin 4 output of 14.4 MHz reference oscillator
Y302 is multiplied up to 43.2 MHz by Q105 and
associated components. This signal is applied to the
2nd local oscillator input at Pin 1 of IC101. The 43.65
MHz signal at Pin 16 and the 2nd local oscillator are
mixed, with the resulting 450 KHz output signal
appearing at Pin 3. This signal is filtered by a 450
KHz 6-pole ceramic filter YF102 and applied to the
input of the limiting IF amplifier at Pin 5. IC101 Pin 6
is the de-coupled input to the IF amplifier, Pin 7 the
limited IF output signal. An internal quadrature
detector, whose center frequency is determined by
450 KHz quadrature coil T101, detects the FM IF
signal. One input of the quadrature detector is
connected internally to the IF signal at Pin 7, while
the other input is the phase-shifted signal from T101
at Pin 8.
Demodulated audio appears at Pin 9, where a low-
pass filter (R114, C130) removes spurious quadrature
output prior to application to the voice/tone
conditioning audio circuitry. Demodulated audio from
Pin 9 is applied to the Pin 10 input of a noise
filter/amplifier consisting of R112, C123, C124, R110,
R111 and R113. The Pin 11 output of the noise
amplifier is applied to a biased noise detector CR103,
with the output connected to an A/D input of IC301 at
Pin 19. The SST-442 is serial programmed to set the
squelch threshold and hysteresis.
Voice / Tone Conditioning in Receive Mode
SST-442 audio conditioning filter circuits are shared
with the transmitter. The same high-pass
filter/amplifier (IC303C and associated components)
used in receive voice band conditioning is used in the
transmit band. Similarly, the low-pass filter (IC308
and associated components) used for selective signal
tone decode filtering is also used for selective signal
encoding. Altering circuit configuration with bilateral
switches IC304A, B and C permits the use of the
same audio filtering system for both transmit and
receive modes.
After R114 and C130 remove the 450 KHz element
from the demodulated audio output at Pin 9 of IC101,
C333 couples the audio signal to bilateral switch
IC304C for subaudible tone detection and connection
to IC304A for voice band audio processing.
Voice Band
Bilateral switch IC304A passes the received audio
signal to the input of IC303C, which along with its
associated components for a high-pass filter/amplifier
circuit that attenuates audio signals below 300 Hz.
The output of IC303C is applied to the input of
IC305B limiting amplifier. Bilateral switch IC304B is
switched in receive to insert C351 into the feedback
circuit of IC305B, providing de-emphasis of the
received audio signal. Bilateral switch IC304B also
removes R335 from the Pin 6 input to reduce the gain
and prevent limiting.
The output from IC305B is applied to the input of
IC303D, which along with its associated components
form an 18dB/octave low-pass filter for frequencies
above 3000 Hz.
The output of IC303D is connected to IC306C volume
control prior to connection to audio amplifier IC307.
The output of IC303D is also connected to IC306F
voice deviation control, which is completely closed in
receive mode to prevent received signals from
modulating the VCO and reference oscillators.
Audio frequency tones from Pin 24 of microcontroller
IC301 are applied to the Pin 6 input of IC305B to
provide the SST-442 alert tones.
Sub-Audible
Bilateral switch IC304C passes the received audio
signal to the input of IC305A, which amplifies the
signal and applies it to the Pin 2 input of IC308, a 5-
pole low-pass filter that attenuates frequencies above
250 Hz.
The output at Pin 5 is applied to an A/D input of
IC301 at Pin 18 for tone decoding. An internal digital
signal processing routine programmed into
microcontroller IC301 is used to decode the correct
selective signaling code. The output at Pin 5 of
IC308 is also connected to tone deviation control
IC306D, which is completely closed in receive mode
to prevent received subaudible tone signals from
modulating the VCO and reference oscillators.
Audio Amplifier
Receive audio from volume control IC306C is applied
to the Pin 2 input IC307, a 1 Watt audio amplifier.
C365 couples the Pin 5 output to the internal 8
Ω
speaker SP301 via audio jack J303.
Microcontroller IC301 Pin 22 switches DC power to
the audio amplifier by turning on Q07, which then
turns on pass transistor Q308 to apply battery voltage
to Pin 6 of the audio amp. CR307 prevents an
inadvertent DC voltage at J303 from damaging the
audio amplifier.