Yaesu FT-2500M Technical Supplement page 9

Circuit
Descripton
Receioe Signal Path
Incoming
RF
from the antenna jack
is deliv-
ered to the Main Unit and
passed
through
a
low-
pass
filter
and a
I/+-wave antenna switching
network
consisting
of coils
L1003
&
Ll-004,
ca-
pacitors
C1003
&
C1004,
and
diodes D1001
&
D1025. Signals
within
the frequency range of the
transceiver are then passed through
a varactor-
tuned bandpass
filter
consisting of T1001/T1002
before RF amplification by Q1001 (3SK131). The
amplified
RF
is then bandpass filtered again by
varactor-tuned resonators
T1,003/T1004,
to
en-
sure pure in-band
input to
1st
mixer
Q1002/Q1003 (2SK302GR x
2).
Buffered output from the VCO Unit is ampli-
fied by
Q1005
(25C2759) and lowpass filtered by
LL009 and
C1.042/C1043,
to provide a pure
1st
local signal between
LL8.6
and 152.6MH.2 to the
1st mixer. The 21.4-MHz
'l.st
mixer product
is
passed
through dual monolithic crystal filter
XF1001
&
XF1002
(t7.5 kHz
BW) to
strip away all
but
the desired signal,
which
is
then amplified
by
Q1004 (25C2714Y).
The
amplified
1st IF
signal is applied to FM IF
subsystem
IC Q1008 (MC3372ML), which
con-
tains the 2nd mixer, 2nd local oscillator,
limiter
amplifier, noise amplifie1, S-meter amplifier and
squelch gates.
A
2nd local signal is
generated
from
21.855
MHz
crystal XL001,
which produces
the 455
kHz
2nd IF
when mixed with the
1st IF
signal
within
Q1008.
The 2nd IF is
passed
through
ceramic
filter
CF1001
to strip away un-
wanted mixer products, and then applied to the
limiter
amp
in
Q1008,
which removes amplitude
variations
in
the 455
kHz IF before detection of
the speechby ceramic discriminator CD1001.
Detected audio is delivered from
Q1008
to the
CNTL
Unit,
where
it
passes
through buffer am-
plifier Q2018 (2sc2712cR) and
audio gateQ2002
(DTC114EK) before de-emphasis, and high-pass
filtering
by
Q2001-2 (NJM29O2Mj/4). Another
audio
muting gate,
Q2003 (DTC114EK), passes
the signal, which is then combined
with
beeper
audio originating from the
microprocessor
through
Beeper
volume trimrner VR2001. The
level of the combined receiver and beeper audio
on the CNTL
Unit is
set
by
VOL
potentiometer
VR3002 on the DISP
Unit
and is then
retumed the
Main
Unit for
amplification
by
Q1023
(TDA2003H) up to
2
watts for the optional head-
phone jack or 8-ohm loudspeaker.
Squelch
Control
The squelch circuit consists of noise amplifier
Q1007 (2S'C2712GR) and
a highpass
filter
and
squelch trigger
within
Q1008
on the
MAIN Unit,
and control circuitry within
microprocessor
Q2006 (SC418082CFU) on the
CNTL Unit.
When no carrier is received, noise at the out-
put
of the detector stage in
Q1008 is
amplifiedby
Q1007
and highpass filtered
by
the noise
amp
section of Q1008, and then rectified by
D1012
to
provide a DC control voltage for
the
squelch
switching section
within
Q1008.
With no
carrier,
pin
14
of
Q1008 is
high. This signal
is
inverted by
Q1006 (lMHs) and delivered to
BUSY
pin
15
of
main microprocessor
Q2006 on the
CNTLUnit
as
the
Scan
Stop signal,
which
causes the
BUSY
indi-
cation on the display when the squelch is
open.
This signal also
causes
the microprocessor to cut
receiver audio
in two
places
on the CNTL Unit
mentioned already: opening audio mute
gate
Q2002,
and
pulling the audio line to ground
just
before the audio amplifier
at Q2003, thus silenc-
ing the receiver while no signal
is being received,
and
during
transmission.
When a carrier appears at the discriminator,
noise is removed
from
the
output, causing pin
1,4
of
Q1008
to go low. This signals the microproces-
sor
to
activate the
BUSY
indicator through LCD
driver
Q3001 (LC7582E). The
microprocessor
then
checks
for
CTCSS
tone detection
from
the
FTS-17A Tone Squelch
Unit (if installed),
and
for
Digital
Code Squelch
information (if the
FRC-6
Pager
Unit
is installed).
If
not transmitting
and
tone squelch is
not
activated,
or
if
the received
tone
mitches
that programmed, the microproces-
sor switches Q2002 and Q2003 to
allow audio to
pass
to the
amplifier
and loudspeaker.
Center-Stop Scanning
To
ensure that scanning stops at the center of
a
detected signal, discriminator
output from pin
9
of
Q1008
is also delivered to comparator circuit
Q2001 (NJM2902-3/+
&
-a/+)
on the CNTL Unit,
which
compares
the discrirninator output to
a
preset
DC level. The output of the comparator
connects
to
the Scan Stop
line, preventing
the
received signal
from signalling the
microproces-
sor
to
stop scanning
until
the signal
has
been
FT-2500M Technical Supplement
1-7