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SECTION
5.
CIRCUIT DESCRIPTIOIU
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5.1
BLOCK
DIAGRAM
GENERAL
The block
diagrafl Of TS-820 is shown on page 24.
The transceiver
is composed of the single superheterodyne
receiver and the single conversion type transmitter with a fil-
.
ter type SSB generator. It uses 8.830 kHz IF and its local
oscillator is
equipped with a PLL (phase locked loop) circuit
controlled by VFO. Both the receiver and transmitter use
balanced type mixer circuits with dual gate MOS
FET's. thus
minimfzing spurious during transmission and preventing the
effects of strong signals and suprious signals during
recep~
tton.
Taking full advantage of PLL system. the transceiver offers
IF SHIFT
functlon (electronic system pass band tuning1 and
permit one SSB filter to provide the same effect obtained by
US8 and LSB filters. The transcetver also includes a number
of latest accessory circuits such as the RF speech processor.
the digital display circuit (option), transmit monitor circuit.
etc.
TRANSMITTER SECTION
The microphone signal is fed to the IF unit, amplified by the
microphone amplifier and is then fed to the ring modulator
composed of 4 diodes. The
DSB output from the modulator
passes through buffer amplifier and the crystal filter. The
SSB signal thus obtained is further amplified and fed to the
transmit mixer of the RF unit.
The transmit mixer is a double balanced type mixer
with 2
MOS FETs The local oscillator is VCO type (voltage control-
led oscillator) whose output is controlled by the PLL circuit
to minimize spurious during transmission. The SSB signal
converted into the wanted transmit frequency is amplified by
the transmit driver tube
112BY7A) before being fed to the
two
S2001A power amplifier tubes of the final stage.
They operate in class AB 1 because of low distortion amplifi-
cation fo
SSB signal.
The output signal is
transm~tted through a pi-network to the
antenna.
RF
NFB is applied from the output of final stage to the driver
stage to further reduce the cross modulation distortion.
RECEIVER SECTION
The antenna signal passes through the RF
All switch.
where
it
is attenuated
by
about 20 dB as necessary, and is
then amplified by the RF unit.
The amplified signal is fed to the buffer
amplif~er and mixed
with VCO output in the balanced mixer with 2 MOS FETs to
produce 8.830 kHz IF signal.
This signal
1s fed to the IF unit, passes through the nolse
blanker circudt and the crystal filter. and is finally amplified
by the 3-stage IF amplifier. so that it is converted
Into AF
slgnal by the ring detector.
The AF signal
1s applted lo the AF-AVR unit where it is am-
pl~fied to the level to drive the speaker The frequency cha~
iacteiistic of the AF amplifier is automatically changed to
suit CW or SSB operation by means of the MODE switch.