Yaesu FT-7900E, Technical Supplement

Page: 7 / 50

FT-7900R/E Technical Supplement
Circuit Description
VHF Reception
The incoming VHF signal is passed through a low-pass
filter network, antenna switching diodes
D1074 ( RLS135 ),
D1075 ( RLS135 ) and D1015 ( HVC131 ), and another low-
pass filter network to the RF amplifier Q1008 ( 3SK296ZQ ).
The amplified RF signal is passed through a varactor con-
trolled bandpass filter consisting of L1017, L1018, and
D1010 , D1011 , and D1012 (all HVC365 ), then applied to
the first mixer Q1009 ( 3SK296ZQ ) along with the first
local signal from the PLL circuit.
The first local signal is generated between 189.05 MHz
and 191.05 MHz, depending on the receiving frequency,
by the VHF VCO, which consists of
Q1076 ( 2SC5374 ) and
varactor diodes D1049 ( HVC365 ), D1050 ( HVC131 ),
D1051 ( HVC365 ), and D1052 ( HVC362 ).
UHF Reception
The incoming UHF signal is passed through a low-pass
filter network, antenna switching diodes
D1077
( MA4P1250 ) and
D1018
( RLS135 ),
D1019
( RLS135 ), and
D1084 ( HSC277 ), and another low-pass filter network to
the RF amplifier Q1001 ( 3SK296ZQ ). The amplified RF
signal is passed through a varactor-controlled band-pass
filter consisting of L1006 and L1007, and
D1002 , D1003 ,
and D1004 (all HVC350B ), then applied to the first mixer
Q1002
( 3SK296ZQ ) along with the first local signal from
the PLL circuit.
The first local signal is generated between 384.95 MHz
and 394.95 MHz, depending on the receiving frequency,
by the UHF VCO, which consists of
Q1072 ( 2SC5006 )
and varactor diodes
D1046
( HVC375B ) and
D1047
( HVC350B ).
IF and Audio Circuits
The 45.05 MHz first IF signal is applied to the monolithic
crystal filter
XF1001 which strips away unwanted mixer
products, and the IF signal is applied to the first IF ampli-
fier
Q1016 ( 2SC4400 ). The amplified first IF signal is then
delivered to the FM IF subsystem IC Q1023 (
TA31136FN
),
which contains the second mixer, limiter amplifier, noise
amplifier, and FM detector.
The 44.6 MHz second local signal is derived from 11.15
MHz crystal X1002, the frequency of which is multiplied
by four at
Q1022 (
2SC4400
), producing the 450 kHz sec-
ond IF signal when mixed with the first IF signal within
Q1023 ( TA31136FN ).
The 450 kHz second IF signal is applied to the ceramic
filter
CF1001 which strips away all but the desired shig-
nal, and then passes through the limiter amplifier within
Q1023 ( TA31136FN ) to the ceramic discriminator CD1001
which removes any amplitude variations in the 450 kHz
IF signal before detection of speech.
The detected audio passes through the de-emphasis net-
work, a low-pass filter consisting of
Q1028 ( NJM2902V )
and associated circuitry, and a high-pass filter consisting
of
Q1028 ( NJM2902V ) and associated circuitry. The fil-
tered audio signal is passed through the audio volume
control IC
Q1034 ( M51132FP ) which adjusts the audio
sensitivity to compensate for audio level variations, then
delivered to the audio switch
Q1036 ( DTC144EE ).
Squelch Control
When no carrier received, noise at the output of the de-
tector stage in
Q1023 ( TA31136FN ) is amplified and band-
pass filtered by the noise amp section of
Q1023
( TA31136FN ). The resulting DC voltage is applied to pin
2 of main CPU Q1065 ( M3826AEFGP ), which compares
the squelch threshold level to that which set by the front
panel SQL knob.
While no carrier is received, pin 53 of
Q1065
( M3826AEFGP ) remains “low,” to disable audio output
from the speaker.
Transmit Signal Path
The speech signal from the microphone passes through
the MIC jack J2001 to AF amplifier
Q2011 (
NJM2904V
)
on the PANEL unit. The amplified speech signal is sub-
jected to amplitude limiting by
Q1040 (
NJM2902V
) on
the MAIN unit. The speech signal then passes through
low-pass filter network
Q1040
( NJM2902V ) and band
switch Q1039 ( BU4066BCFV ) to the VHF VCO or UHF
VCO.
VHF Transmit Signal Path
The adjusted speech signal from Q1040 ( NJM2902V ) is
delivered to VHF VCO Q1076 ( 2SC5374 ) which frequen-
cy modulates the transmitting VCO made up of D1049
(
HVC365
). The modulated transmit signal passes through
buffer amplifiers Q1077 and Q1078 (both 2SC5374 ). The
amplified transmit signal is then applied to the Pre-Drive
amplifier
Q1084 ( RQA0004PXDQS ) and Driver amplifi-
er Q1086 ( RD07MVS1 ), then finally amplified by Power
amplifier Q1087 ( RD70HVF1 ) up to 50 Watts. This three-
stage power amplifier’s gain is controlled by the APC cir-
cuit. The 50 Watt RF signal passes through high-pass fil-
ter and low-pass filter network, antenna switch
D1072
and
D1073 (both L709CER ), and another low-pass filter net-
work, and then is delivered to the ANT jack.
D-1