Theory Of Operation - Yaesu FT-227RB Instruction Manual

THEORY OF OPERATION
The block diagram, as well as the following circuit
description, will provide you with a better under-
standing of this transceiver. Refer to the schematic
diagram for circuit details.
The FT-227RB transceiver consists of a transmitter
and a double conversion superheterodyne receiver.
A digital phase lock loop synthesizer provides
operation on any of the 800 channels within the
frequency range of 144-148 MHz. Solid state
circuitry is employed throughout, and computer
theory is utilized for frequency selection. The
transceiver is designed for use from a 13.8 volt
DC power source.
TRANSMITTER
The transmitter produces a Frequency Modulated
(FM) signal. The audio signal from the microphone
is set to the proper level
by VR201 ,
and is ampli-
fied by
Q201, Q202 ,
and Q203 (2SC372Y). The
audio output from
Q2 02
is coupled to the Instan-
taneous Deviation Control (IDC) circuit, where
both positive and negative peaks are clipped by
diodes
D2 0 1
and
D202
(1S1555). The output from
the last amplifier
(Q203 )
is fed through a low-pass
filter, which attenuates frequencies above the
speech range caused by clipping at the IDC; the
deviation level is set by VR 202 . This control is
normally set for a deviation of ± 5 kHz. The speech
signal is then applied to the phase modulator
varactor diode,
D401
(1SV50), which varies the
frequency of the 10.7 MHz crystal controlled oscil-
lator,
Q401
(2SC372Y). The frequency modulated
10.7 MHz signal is then amplified by a buffer
amplifier,
Q402
(2SC372Y), and fed to the balanced
mixer,
Q403
and
Q404
(2SK19GR), where the
signal is converted up to 144-148 MHz by mixing
with the 133.3-137.3 MHz signal delivered from
the Voltage Controlled Oscillator (VCO). The out-
put from the balanced mixer is fed through the
tuned circuits, consisting of
T404 —T407 ,
to a
MOSFET amplifier,
Q405
(3 SK51-03).
T404 —T407
are tuned to the transmitting frequency by varactor
diodes
D403 —D406 ,
whose capacitances are varied
in accordance with the DC output voltage from
the PLL unit. The 144-148 MHz signal is then
amplified by amplifier stages
Q204
(2SC535A),
Q206
(2SC2053),
Q207
(2SC730), and
Q208
(MRF212), which deliver 10 watts of RF energy,
through a diode switch and low pass filter, to the
antenna.
Diodes
D205
and
D2 11
(1S188FM) rectify a small
portion of the RF output and apply the resultant
DC voltage to the meter, which indicates relative
transmitter power output.
The DC output from
D20 5
is also delivered to the
lamp driver,
Q209 —Q210
(2SC372Y), turning the
ON AIR lamp on during transmission.
If the transmitter is keyed without an antenna
being connected, or if a high SWR exists in the
antenna system, the reflected power is detected
through
T202
and a diode,
D2 0 8
(1S188FM),
which produces a DC voltage.
Q211
(2SC372Y)
conducts with the application of the DC voltage
through VR205 , causing a decrease in the collector
current of
Q212
(2SC372Y).
Thus, the collector voltage of
Q213
(2SA496)
drops, causing Qi (2SD235) to decrease current
and supply voltage to the PA amplifier, preventing
damage to the transistor. The threshold level is set
by VR2os •
The antenna changeover circuit consists of switch-
ing diodes
D206
and
D207
(MI1301).
The tone burst circuit consists of a timing
generator and a gated multivibrator. With the
BURST switch in the ON position, a DC voltage
is applied to the tone burst current. When the
transmitter is keyed,
Q214
(2SC372Y) conducts,
triggering the one-shot multivibrator,
Q215
(MC14011
B).
The other half of
Q215
generates
a tone signal, which is amplified by buffer
Q216
(2SC372Y), and applied to the microphone circuit
of the transmitter.
The tone frequency is adjusted by VR 20 7 , while
the output level of the burst signal is adjusted by
VR2 09 . The burst duration is set by VR2os .