User's Manual

THEORY OF OPERATION SST-442
17
to change divisors is determined by the channel
frequency.
IC401 contains a digital phase detector that works as
follows: when an operating channel is changed or the
receive/transmit mode switched, a new synthesizer
operating frequency is selected. Microcontroller
IC301 clocks new data into IC401 internal buffer in
synchronization with clock pulses. The channel
information is stored in EE memory of IC302 and is
loaded into RAM when the channel is selected.
Once new data is loaded into the buffer, a single
pulse from IC301 appears at IC401 to instruct the
synthesizer controller to latch and execute the new
data. IC401 utilizes internal circuitry to determine
whether the present VCO output frequency is correct
by comparing the phase and frequency of the VCO
signal at Pin 11 to the 14.4 MHz reference oscillator
at Pin 20. IC401 produces an output signal at Pin 6,
a single-ended phase/frequency detector output,
proportional to the phase difference between the two
input signals.
The loop filter C429, C427, R419, R420, R418, and
C426 transform the Pin 6 output signal to a DC
voltage for application to the VCO tuning varactor
CR402. The synthesizer system is “locked” when the
phase and frequency of both the reference and the
divided VCO signal are the same.
The maximum amount of current this output can sink
or source is determined by the value of R421 tied to
Pin 8 of IC401, with the output current programmable
to 25, 50 or 100% of maximum.
VCO / Buffer Amplifiers
Q403, L401, CR402 and associated components
form the VCO (Voltage Controlled Oscillator), a
resonant circuit that oscillates at frequencies from
416 MHz in receive (receive frequency - 43.65 MHz)
to 470 MHz in transmit. Varying the voltage at CR402
changes the varactor capacitance, which in turn
alters the VCO output frequency.
When in transmit mode a +5 VDC T/RSW signal is
applied to Q406, which turns on Q405 to draw current
through pin diode CR404 and L403. With CR404
biased on, L402 is effectively shorted to ground,
shifting the VCO frequency up 43.65 MHz.
Q401 and Q402 are buffer amplifiers, with Q401
feeding in the input of the synthesizer at Pin 11, the
receiver 1st local oscillator and the transmitter pre-
amplifier.
Oscillator Modulation
When the SST-442 is in transmit, modulation balance
control IC306E passes TX audio through to the VCO
modulation input at R416. TX audio is applied to
varactor CR403 to modulate the VCO. TX audio is
also routed to the Pin 1 input of TCVCXO reference
oscillator Y302. Low frequency tones modulate the
reference oscillator because the synthesizer is not
able to track them.
DIGITAL POTENTIOMETERS
IC306 contains 6 digital potentiometers programmed
by IC301, sharing the same clock and data outputs
used by the synthesizer and a separate Digital Pot
Latch signal from Pin 8. The digital potentiometers
are used in conjunction with IC303B, a summing
node amplifier used for modulating the VCO and
reference oscillator. IC306A, B, D, E, and F can only
be changed through serial programming, and can
only be performed by an authorized licensed RF
technician.
IC306A is connected through R352 to the Pin 6 input
of IC303B. IC306A adjusts the DC output of IC303B
to tune the reference oscillator frequency.
IC306B is unused.
Volume control IC306C applies the processed voice
band signals at IC303D to audio amplifier IC307
when in receive mode. Depressing the ON/VOL UP
switch SW301 increases the setting of IC306C while
VOL DN/OFF switch SW302 decreases it.
Transmitter tone deviation control IC306D applies the
output of the selective signaling low-pass filter IC308
to the Pin 6 input of IC303B through R350. IC306D is
completely closed in receive mode.
Transmitter modulation balance control IC306E is
used to apply the Pin 7 output of IC303B to the VCO
modulation input. This will set the ratio of the
modulating signal applied to the VCO and the
reference oscillator. IC306E is completely closed in
receive mode.
Transmitter voice deviation control IC306F applies
the processed voice band signals at IC303D to the
Pin 6 input of IC303B through R349. IC306F is
completely closed in receive mode.
RECEIVER
As mentioned before, Q101 switches the regulated
+5 VDC to the receiver. The +VRX receiver voltage
is switched at the strobe duty cycle if programmed for
power strobe.
RF Amplifier
A received signal from the antenna passes through a
low-pass filter (L207, C216, C217, L206, C101, and
C202) to the receiver headend. L101 and the
associated capacitors form a bandpass filter ahead of
low-noise RF amplifier Q102. L101 and C103 provide
a notch at the image frequency, 87.3 MHz below the
receive frequency. The amplified RF signal is applied
to a 2-pole bandpass filter consisting of L103, L104,
and associated capacitors. This circuit can be tuned
for any 10 MHz band between 450 and 470 MHz.