Yaesu FT-51R, Technical Supplement

Page: 22 / 83

Circuit
CTCSS tone, if present, and adds keypad beeps and
DTMF monitor tones as needed. The resulting audio
is delivered through the respective volume control
and audio selector Q1006
(MB87078PF)
on the CNTL
Unit, which selects VHF and UHF audio on the inter­
nal or external (EAR jack) loudspeakers, separate or
mixed, through Ql018, Q1019
(2SC4617x2)
and
audio amplifier Q1023
(TDA7233D)
on the CNTL
Unit.
UHF Reception
UHF
signals passed by the duplexer are applied to
a varactor-tuned band-pass filter consisting of L3029,
TC3005 and D3021/D3025 before
RF
amplification by
Q3039
(2SC4228).
The amplified RF is then band­
pass filtered again by varactor-tuned resonators
TC3004, L3028, D3020, D3024 and TC3003, L3027,
D3019, D3023, further amplified by Q3038
(2SC4228)
and filtered once more by TC3002, L3026, D3018 and
D3022 to ensure pure in-band input to 1st mixer
Q3035
(2SC4226).
Tuning voltage for the varactors is
provided by UHF PLL Q3009, buffered by Q3015
(2SK880GR).
Buffered output between 371.475
�
391.475-MHz
from the 430-VCO Unit is amplified by Q3003
(2SC4245)
and applied to the 1st mixer. The resulting
58.525-MHz 1st mixer product is passed through
monolithic crystal filter XF3001(± 7.5-kHz BW) to
strip away all but the desired signal, which is then
amplified by Q3034
(2SC4215Y)
before delivery to
FM IF subsystem IC Q3037
(MC3372VM),
which con­
tains the 2nd mixer, 2nd local oscillator, limiter ampli­
fier, noise amplifier, S-meter amplifier and squelch
gates.
A 2nd local signal is generated from 58.07-MHz
crystal X3002, which produces the 455-kHz 2nd IF
when mixed with the 1st IF signal within Q3037. The
2nd IF is passed through ceramic filter CF3001 to strip
away unwanted mixer products, and is then applied
to the limiter amp in Q3037, which removes ampli­
tude variations in the 455-kHz IF before detection of
the speech by ceramic discriminator CD3001.
Detected audio from pin 9 of Q3037 is delivered
via the 430 Mother Unit, CONTACT Unit and CNTL
Unit (for sampling by CTCSS
&
DTMF detectors and
de-emphasis). Receiver audio amplification is as al­
ready described.
UHF Squelch Control
When no carrier is received, noise at the output of
the detector stage in Q3037 is amplified and band­
pass filtered by the noise amp section of Q3037 and
the network between pins 9 and 10, and then rectified
by D3014. The resulting DC squelch control voltage is
passed to pin 98 of main CPU Q1002. While no carrier
is received, pin 55 of Q1002 remains low, signaling pin
7 of sub-CPU Q1001 on the Control Unit to keep the
green (Busy) half of the UHF LED off, and holding
the AF MUTE U line low to block UHF receiver audio
from passing through analog gate Q1015-2 while no
signal is being received, as well as during transmis­
sion.
When a carrier appears at the discriminator, noise
is removed from the output, causing pin 55 of Q1002
to go high and signaling microprocessor Q1001 to
activate the BUSY LED. The microprocessor then
checks CTCSS chip Q1004
(AK2343)
and DTMF de­
coder chip Q1007
(TC35305F)
for CTCSS or DTMF
code squelch information, respectively. If not trans­
mitting and tone squelch is not activated, or if the
received tone matches that programmed, the micro­
processor stops scanning, if active, and allows audio
to pass through mute and amplifier IC Q1023
(TDA7233D)
to the loudspeaker.
UHF Single-Band Dual Receive
When UHF single-band dual receive operation is
active, a portion of the received UHF RF passing
through the antenna switching network (and applied
to the UHF front end) is sampled via diode D2008 on
the 144-Mother Unit and diode D3026 on the 430-
Mother Unit. Here, the signals are band-pass filtered
and amplified by Q2017 and Q2018 (both 2SC4227)
before application to mixer Q2016 (2SC4537). This
mixer also receives the third harmonic of the 144-
VCO Unit. The resulting 45.05-MHz sub-receiver 1st
IF signal is applied to the VHF receiver IF chain just
before monolithic filter XF2001, and is then handled
just as a VHF signal would be in VHF operation.
Transmitter Audio Stages
Speech input from the microphone is delivered to
Q1006
(MB87078PF)
for pre-emphasis then passes
through the splatter filter section within Q1004 and
Q1006
(MB87078PF)
for IDC (instantaneous devia­
tion control) before delivery to the modulator.
If Tone Burst or DTMF is enabled for transmission,
the tone is generated by main CPU Q1002, buffered
by Q1004 and applied to the IDC stage in place of
speech audio. Also, the tone is amplified for monitor­
ing in the loudspeaker, as mentioned before. The sub­
CPU closes Mic Mute gate Q1004 when transmitting
a tone, or when transmission is disabled.
FT-51R Technical Suoolement