Yaesu FT-2800M Technical Supplement Download

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Circuit Description

Reception and transmission are switched by “RX” and
“TX” lines from the microprocessor unit (MPU). The re-
ceiver uses double-conversion superheterodyne circuitry,
with a 21.7 MHz 1st IF and 450 kHz 2nd IF. The 1st lo-
cal, produced by a PLL synthesizer, yields the 21.7 MHz
1st IF.

The 2nd local uses a 21.250 MHz (21.7 MHz - 450 kHz)
signal generated by a crystal oscillator. The 2nd mixer
and other circuits use a custom IC to convert and amplify
the 2nd IF, and detect FM to obtain demodulated signals.

During transmit, the PLL synthesizer oscillates at the de-
sired frequency directly, for amplification to obtain RF
power output. During transmit, voice modulation and
CTCSS (or DCS) modulation are applied to this synthe-
sizer. Transceiver functions, such as TX/RX control, PLL
synthesizer settings, and channel programming, are con-
trolled using the MPU.

Receiver

Incoming RF signals from the antenna connector are de-
livered to the RF Unit, and pass through a lowpass filter
(LPF) consisting of coils L303, L305, and L307, capaci-
tors C303, C309, C310, and C316, and antenna switching
diodes

D305

and

D306

(Both

MA111

) for delivery to the

receiver front end.

Signals within the frequency range of the transceiver are
then passed through a varactor-tuned bandpass filter con-
sisting of diodes

D308

D322

D310

and

D323

(all

HVC3508

), and coils L334, L335, L320 and L322 and

associated circuitry before RF amplification by

Q316

(

3SK240

The amplified RF signal is then bandpass filtered again
by varactor-tuned resonators consisting of diodes

D312

D324

D313

and

D325

(all

HVC3508

), and coils L336,

L337, L325 and L327 and associated circuitry, to ensure
pure in-band input to 1st mixer

Q321

(

3SK240

Buffered output from the VCO Unit is amplified by

Q318

(

2SC5006

) and lowpass-filtered by L339, C414, and

C413, to provide a pure 1st local signal between 112.3
and 152.3 MHz, which is delivered to the 1st mixer.

The 21.7 MHz 1st mixer product then passes through dual
monolithic crystal filter F301 (7.5 kHz BW), and is am-
plified by

Q327

(

2SC4215Y

) and delivered to the input

of the FM IF subsystem IC

Q334

(

TA31136FN

This IC contains the 2nd mixer, 2nd local oscillator, lim-
iter amplifier, FM detector, noise amplifier, S-meter am-
plifier and squelch gates.

The 2nd local in the IF-IC is produced from crystal

X302

(21.250 MHz), and the 1st IF is converted to 450 kHz by
the 2nd mixer and stripped of unwanted components by
ceramic filter

F302

. After passing through a limiter am-

plifier, the signal is demodulated by the FM detector.

Demodulated receive audio from the IF-IC is amplified
by

Q336

(

2SA1588Y

) and

Q338

(

UMX2M

). After vol-

ume adjustment by the AF power amplifier

Q337

(

LA4425A

), the audio signal is passed to the optional

headphone or 8-ohm loudspeaker.

PLL Synthesizer

The 1st local maintains stability from the PLL synthesiz-
er by using a 21.250 MHz reference signal from crystal

X301

. PLL synthesizer IC

Q333

(

LV2105V

) consists of

a prescaler, reference counter, swallow counter, program-
mable counter, a serial data input port to set these counters
based on external data, a phase comparator, and a charge
pump.

The PLL-IC divides the 21.250 MHz reference signal by
4,250 using the reference counter (5.0 kHz comparison
frequency). The VCO output is divided by the prescaler,
swallow counter and programmable counter. These two
signals are compared by the phase comparator and ap-
plied to the charge pump. A voltage proportional to their
phase difference is delivered to the low-pass filter circuit,
then fed back to the VCO as a voltage with phase error,
controlling and stabilizing the oscillating frequency. This
synthesizer also operates as a modulator during transmit.

The RX VCO is comprised of

Q323

(

2SC5006

) and

D314

D326

(Both

1SV282

), and oscillates at 21.7 MHz

during receive.

The TX VCO is comprised of