Yaesu FT-857 series, Technical Supplement

Page: 10 / 82

10
Circuit Description
Transmit Signal Circuitry
Microphone Amplifier Circuit
The audio signal from microphone jack is amplified by
audio amplifier
Q1109 ( 2SC4154E ) on the MAIN Unit,
and then is applied to electronic volume control IC Q1087
( M62364EP ), the level of which is set via the User Menu.
SSB Modulator Circuit
The output (audio signal) from the electronic volume con-
trol IC is passed through audio amplifier
Q1118
(
NJM2902V
) to the balanced modulator IC Q1071
( SA602AD ) which produces a Double Sideband (DSB)
signal by applying the carrier signal from the CAR-DDS
IC
Q1062 ( AD9835BRU ). The DSB modulated signal
(455 kHz) is fed to ceramic filter CF1004 (or the optional
mechanical filter) which strips residual carrier and the
undesired sideband, resulting in a Single Sideband (SSB)
signal.
AM Modulator Circuit
As in the SSB modulator circuit, a carrier signal appro-
priate to the transmitting mode (AM) from the CAR-DDS
Unit and an audio signal from the microphone are applied
to balanced modulator IC
Q1071 ( SA602AD ). The con-
trol signal from Mode Switch IC Q1003 (
BU4094BCFV
)
causes a voltage (“AM 5V”) to be sent from transistor
Q1058 (
2SC4154E
). This voltage is applied to IC Q1071
via D1059 ( BAS316 ), causing the balanced modulator to
lose balance. The restored carrier signal and modulated
signal are then fed to the TX mixer via ceramic filter
CF1004 .
FM Modulator Circuit
The output (audio signal) from the electronic volume con-
trol IC is passed through the pre-emphasis circuit which
consists capacitor C1492 and resistors R1493 and R1477,
a n d I n s t a n t a n e o u s D e v i a t i o n C o n t r o l
Q1119
( NJM2902V ), to the splatter filter which consists Q1119 ,
capacitor C1430, and resistors R1358 and R1384. The fil-
tered audio signal is applied to the FM modulator circuit,
which produces the FM signal. The FM modulator circuit
uses a voltage controlled crystal oscillator (VCXO) which
consists
Q1055 (
2SC4400
), D1046 (
1SV229
), and
X1002 (22.7767 MHz).
1st IF Circuit/1st Mixer Circuit
The modulated SSB/AM signal is applied to the 2nd Mixer
Q1082 (
SA602AD
), which produces the 68.33 MHz 2nd
IF signal utilizing the 2nd local signal (68.875 MHz). The
2nd IF signal is fed through the 2nd
IF filter XF1004 which
strips away unwanted mixer products, then passes through
the 2nd IF amplifier
Q1061 (
BB304CDW
) to the double
balanced mixer D1034 ( HSB88WS ) which produces the
transmit frequency by applying the local signal (68.430-
538.330 MHz) from the PLL Unit. The transmit signal is
passed through a low-pass filter (1.8-29.7 MHz), a high-
pass filter (50-54 MHz), a band-pass filter (144-146 MHz),
or a band-pass filter (430-450 MHz) which consists of
various inductors and capacitors. The filtered transmit sig-
nal is amplified by
Q1017 (
2SC3357
), Q1011
( 2SK2596 ), Q1006 / Q1007 ( 2SK2973 ), and Q1001 /
Q1002 (
2SK2975
), and is applied to the Power Ampli-
fier: Q3022 / Q3023 ( 2SC5125 : HF/50 MHz) or Q3024
(
2SC3102
: 144/430 MHz).
ALC Circuit
The output from the directional coupler is routed from
connector J3001 and applied to the ALC circuit via con-
nector J1001 on the MAIN Unit.
The ALC circuit consists of an op-amp circuit for ampli-
fying the forward and reflected voltage, a time-constant
ALC amplifier, and a transmit signal control circuit on
the MAIN Unit. The forward voltage from connector
J1001 on the MAIN Unit is added with a DC control volt-
age and is then applied to op-amp IC
Q1111 (
NJM2902V
).
The reflected voltage is added with a DC control voltage
and is then applied to op-amp IC
Q1112 (
NJM2904V
).
In the event of high SWR conditions (SWR 3:1 or more),
transmitter output is reduced, thus protecting the PA Unit
from potential damage; a “HI SWR” indication also ap-
pears on the LCD, alerting the user to an antenna prob-
lem.
The ALC amplifier magnifies the forward wave output
via transistor
Q1009 (
2SC4154
). This output then passes
through a fast-attack, slow-delay RC time-constant cir-
cuit, which consists of R1051 and C1051, for input to the
TX signal control circuit on the MAIN Unit. The TX con-
trol circuit adjusts the IF amplifier gain via gate 2 of FET
Q1061 ( BB304CDW ) of the 2nd IF amplifier circuit, to
prevent the TX output from exceeding the preset level.