Yaesu FT-817 Technical Supplement page 10

Circuit Description
AGC Circuit
The AGC circuit consists of D1064 (1SS302), transistor
Q1066 (2SC4154E), and associated parts on the MAIN
Unit. Output from the AGC circuit is fed back to the RF
and IF stages, controlling their gain levels.
FM IF Circuit/FM Demodulator Circuit
The 2nd IF signal is fed to IC Q1069 (BA4116FV) for FM
demodulation.
The FM demodulator IC contains a mixer, limiter ampli-
fier, filter amplifier, squelch trigger, and demodulator. The
IF input signal is band-limited by ceramic filter CD1002
(CDBC455CX24, BW:±4 kHz), amplified by the limiter
amplifier, and demodulated into an audio signal by the
demodulator; it then passes through a filter (R1308 and
C1293). The signal next passes through a de-emphasis cir-
cuit which consists of R1256 and C1286.
The squelch circuit selectively amplifies the noise com-
ponent of the demodulator output using the filter ampli-
fier inside the FM IC and the active band-pass filter con-
sisting of an externally attached resistor and capacitor.
This circuit uses a signal detected by D1065 (DA221).
SSB/CW Demodulator Circuit
The 2nd IF signal is applied to the SSB demodulator Q1055
(SN16913P) on the MAIN Unit, which produces audio
by applying a carrier signal from the CAR-DDS IC Q1031
(AD9835). Similarly, the CW signal is demodulated us-
ing a carrier signal which is offset by the "Pitch" frequency.
The demodulated SSB and CW signals are each stripped
of high-frequency components by an active low-pass fil-
ter which consists of op-amp IC Q1093-1 (NJM2902V).
Then, they enter the VR Unit via J1008.
AM Demodulator Circuit
The 2nd IF signal from the IF Unit is applied to diode
D1060 (BAS316) for AM demodulation.
The output from the detector passes through analog switch
Q1049 (BU4066BF). Then, it enters the VR Unit via J1008.
Audio Amplifier Circuit
The demodulated signal that is selected by one of analog
switches IC Q1057 (BU4066BCF: according to the recep-
tion mode) passes through the audio amplifier IC Q1094
(NJM2902V), volume control VR4901, and IC Q1070
(TDA7233D) to drive the internal or external speaker with
a maximum output of approximately 1.0 Watt.
10
Transmit Signal Circuitry
Microphone Amplifier Circuit
The audio signal from microphone jack J1014 on the
MAIN Unit is amplified by transistor Q1092 (2SC4154E)
on the MAIN Unit, and then is applied to electronic vol-
ume IC Q1071 (M62364FP), which is controlled via the
User Menu (Item #46: SSB MIC).
The output (audio signal) from the electronic volume IC
is amplified by Q1096 (NJM2902V) and fed to balanced
modulator IC Q1087 (SN16913P) through the low-pass
filter IC Q1096 (NJM2902V).
During FM transmission, the audio signal is adjusted via
the User Menu (Item# 29 : FM MIC). The audio signal that
has passed through the pre-emphasis circuit (C2201 and
R2228 on the MAIN Unit) may be mixed with a tone sig-
nal from CPU IC Q4004 (HD6432345A20FA), and is then
amplified and limited by op-amp IC Q1095-4 (NJM2902V)
of the IDC circuit. The audio then passes through the splat-
ter filter (secondary active low-pass filter) formed by op-
amp IC Q1095-1 (NJM2902V), R1321, and R1322, plus
C1344, and is then fed to the frequency-modulator circuit
on the MAIN Unit through R1183 and R1477 for setting
of the frequency deviation.
SSB Modulator Circuit
The carrier signal appropriate to the transmitting mode
(LSB or USB) is applied from the CAR-DDS Unit to bal-
anced modulator IC Q1087 (SN16913P) on the MAIN
Unit, and is modulated by microphone audio.
The balanced modulator produces the upper and lower
side bands and carrier signal. The carrier and audio sig-
nal are suppressed and the carrier balance is adjusted by
VR1001. As a result, the output signal obtained is a DSB
signal with a carrier suppression of 30 dB or more (addi-
tional carrier suppression is supplied by the SSB filter).
The DSB modulated signal (1st IF signal: 455 kHz) then
passes through ceramic filter CF1004 (CFJ455K14) or the
optional mechanical filter U1003 on the MAIN Unit, strip-
ping residual carrier and the undesired sideband; the sig-
nal then passes as an SSB signal through buffer-amplifier
Q1040 (BB301C).