Yaesu FT-897 Technical Supplement page 9

ALC Circuit
The output from the directional coupler is routed from
connector J3002 and applied to the ALC circuit via con-
nector J1001 on the MAIN Unit.
The ALC circuit consists of an op-amp circuit for am-
plifying 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 out-
put via transistor Q1009 (2SC4154). This output then
passes through a fast-attack, slow-delay RC time-constant
circuit, which consists of R1051 and C1051, for input to
the TX signal control circuit on the MAIN Unit. The TX
control circuit adjusts the IF amplifier gain via gate 2 of
FET Q1061 (BB304CDW) of the 2nd IF amplifier cir-
cuit, to prevent the TX output from exceeding the preset
level.
PLL Frequency Synthesizer
The PLL Frequency Synthesizer consists mainly of the
master reference oscillator circuit, 2nd local oscillator cir-
cuit, PLL IC, and CAR-DDS and REF-DDS units, which
digitally synthesize carrier outputs, plus a PLL circuit
which contains a voltage controlled oscillator (VCO).
Master Reference Oscillator Circuit
The master reference oscillator uses a Crystal Oscilla-
tor (oscillation frequency: 22.625 MHz) composed of
Q5001 (2SC4400-4), X5001, TC5001, C5001, and
R5005. The reference oscillator signal passes through a
buffer amplifier Q5002 (2SC4400-4), and is then fed to
the MAIN Unit via J5002.
CAR-DDS Circuit REF-DDS Circuit
DDS ICs Q1062 (AD9835BRU), and Q2016
(AD9850BAS) each contain a shift register, selector,
phase accumulator, and ROM. The reference oscillation
frequency (22.625 MHz) that is delivered to each of the
DDS Units is applied to each DDS IC after amplification
by transistors Q1043, Q1046, Q1048, and Q1059 (all
2SC4400-3).
Circuit Description
The DDS outputs contain digital amplitude data cor-
responding to serial frequency data from CPU IC Q1049.
The DDS frequency range is 453.5 ~ 466.5 kHz (center
frequency = 455.0 kHz) for the CAR-DDS, and 7.2-8.0
MHz for the REF DDS.
1st Local Oscillator Circuit
VCO output is buffer-amplified by Q2011 (2SC5374)
and Q2022 (UPC2713T), and passes through a low-pass
filter. It is then fed to the TX/RX frequency mixer circuit-
ry of the MAIN Unit.
2nd Local Oscillator Circuit
The 2nd LO circuit is a Hartley-type overtone oscilla-
tor circuit (frequency: 67.875 MHz) composed of Q1052
(2SC4400-3) on the MAIN Unit.
PLL Circuit
The PLL circuit is a frequency mixing type composed
of a VCO, mixer, PLL IC, and loop filter. The VCO con-
sists of five circuits (VCO1, VCO2, VCO3, VCO4 and
VCO5), with a frequency range of 68.430-538.330 MHz
divided into five bands, allocated to the five VCO cir-
cuits. VCO1-VCO5 consist mainly of FETs Q2004,
Q2005, and Q2006 (all 2SK210GR); transistors Q2009
and Q2010 (both 2SC5374); diodes D2001-D2006 (all
HVC362), D2007 (1SV282), D2008 (1SV281), and
D2009 (1SV286); and coils T2001-T2003, L2010, and
L2011.
The VCO switching signal from the connector J2002
is used to drive switching transistors Q2001, Q2002,
Q2003, Q2012, and Q2013 (all DTC124EU) to switch
the source terminal of the oscillator FET.
The 68.430-538.330 MHz VCO signal is buffer-am-
plified by Q2023 (UPC1688G), and fed to PLL IC Q2021
(FQ7925.)
The REF-DDS signal (7.2-8.0 MHz) is fed to PLL IC
Q2021 after it passes through a low-pass filter composed
of C2064, C2067, C2069, C2071, C2075, L2014, L2015,
and L2016, and is fed to PLL IC Q2021 (FQ7925.) The
phase of the reference frequency and that of the signal
input to the PLL IC are compared, and a signal whose
pulse corresponds to the phase difference is produced.
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