THEORY OF OPERATION
SST-442
19
ANTENNA SWITCHING / LOW PASS FILTER
A low-pass filter comprised of filter L203, C213,
C214, L204, C217, L207, and C216 removes
harmonics from the transmitter output before applying
the RF signal to the antenna. Received signals pass
through the low-pass filter before entering the
receiver RF amplifier circuitry.
Two PIN diodes (CR201, CR101) and associated
components form the antenna switching circuit. With
the SST-442 in receive mode, no voltage is applied to
the PIN diodes and they do not conduct. This reverse
biases CR201 to prevent the transmitter amplifier
from affecting the receiver tuning and removes
CR101 from the receiver input. Incoming signals
from the antenna pass through the low-pass filter,
then L206 and C102 to the receiver headend.
When the SST-442 is switched into transmit mode,
transmitter supply voltage is switched on by Q201
and Q202 and applied to R207. Current flows
through R207, L205, CR201, L206 and then CR101
to ground, forward biasing the PIN diodes. CR201
passes transmitter RF power to the antenna while
CR101 shunts the receiver RF amplifier input to
ground. L206 provides sufficient impedance to
isolate the transmitter power from the receiver RF
amplifier.
TRANSMITTER
Keying
The SST-442 transmitter is keyed when PTT switch
SW304 is depressed. Electret condenser
microphone M301 is connected in series to ground
with the PTT switch, which when depressed draws
current through M301, SW304, CR304 and R307 to
turn on pass transistor Q302. Q302 then turns on
Q301 to pull the TX Key Pin 2 input of microcontroller
IC301 low. The microcontroller T/R output at Pin 11
then goes high to turn on Q202, which turns on pass
transistor Q201 to apply +6VDC to the transmitter as
described previously. The transmitter can also be
keyed through J301 with an audio accessory that
inserts a microphone and PTT switch in series to
ground, drawing current through CR304.
+VTX Supply
With the transmitter enabled as described above, the
+VTX voltage from Q201 is applied to transmitter RF
amplifiers Q203 and Q204. +VTX supply also
forward biases the PIN switching diodes CR201 and
CR101 as previously described. The transmitter RF
final amplifier Q205 is powered by the battery supply.
The Pin 11 T/R output of microcontroller IC301 can
be PC programmed to hold the transmitter on after
the PTT switch has been released to allow tone
related turn-off codes for squelch tail elimination.
Power Amplifier
Q203, Q204 and associated components amplify the
VCO signal and apply it to the input of RF Final
amplifier transistor Q205. The collector output of
Q205 is matched to the antenna switching circuitry
and applied to the antenna through the low-pass
filter.
Voice / Tone Conditioning in Transmit Mode
SST-442 audio conditioning filter circuits are shared
with the receiver. The same high-pass filter/amplifier
(IC303C and associated components) used in receive
voice band conditioning is used in the transmitter
voice band. Similarly, the low-pass filter (IC308 and
associated components) used for sub-audible tone
decode filtering is also used for tone encode. 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.
Voice Band
M301 microphone audio is passes through CR304,
C334 and is switched by IC304A to the input of
IC303C, which along with its associated components
form 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 switches R335 in parallel with
R336, raising the gain of limiting amplifier IC305B for
full limiting of the transmitter voice audio. The output
of IC305B is applied to the input of IC303D, which
along with its associated components form an 18
dB/octave low-pass filter for frequencies above 3
KHz. The output of IC303D is applied to voice
deviation control IC306F, which sets the level of the
processed transmitter audio applied to summing node
amplifier IC303B used to modulate the VCO and
reference oscillator.
Sub-Audible
Microcontroller IC301 generates the sub-audible
selective signaling encode waveforms at Pin 21 and
applies them to the input of buffer amplifier IC305A.
The output of IC305A is applied to the input of IC308,
a 5-pole low-pass filter that attenuates frequencies
above 250 Hz. C342 and C343 set the corner
frequency of the low-pass filter, with C342 switched in
by Pin 7 of IC301 to lower the corner frequency for
lower tones. The output of IC308 is applied to tone
deviation control IC306D, which sets the level of the
transmitter sub-audible encode tones applied to
summing node amplifier IC303B used to modulate
the VCO and reference oscillator.