Receiver Circuits - Icom IC-R7100 Service Manual

3·1 RECEIVER CIRCUITS
3·1·1 RF CONVERTER CIRCUIT
(CONV, DOUBLER AND HPF UNITS)
The RF conve rter ci rc uit couvert s the 1025-1999.9999
MH z RF siçnats to 25-1024 .9999 MHz RF signais.
The amplified signal is app/ied to IC1 on the CONV UNIT
and is th en mixed with RF sig nals coming trom ICt on the
HPF UNIT. The resulting 25 - 1024.9999 MHz signais
are applied to the RF UNIT through J2.
3·1·2 ATTENUATOR CIRCUIT (RF UNIT)
The h igh-pass filter attenuates the image signals be low
1200 MHz and prevents 1 GHz
(7
dBm) LO signaIs from
ente ring the antenna connector.
(2) 1025.0000-1999.9999 MHz
RF signals trom the antenna connector (J7) are appned
to a high·pass fil te r (st rip fine. L3. Cl - C6) on the HPF
UN IT through the 1 GHz converte r switching relay (RL
t .
RL2).
(1 ) 25.0000-1024.9999 MHz
RF sig nals trom an antenna conn ec tor (J7) pass through
the 1 GHz converter switc hing relay (RL1, RL2) and
bypass Ihe RF converter circ uit. The signals are applied
to the RF UNIT through J2 .
0 2
RF UNIT
"TT CONT
f-
ICS ,0 12
trom
LOOIC UNIT
IC5 ( pin&4)
Tne attenuator ci rcuit attenuales Ihe signal strength 1 0
20 dB to proteet Ihe RF amplifier trom distortion when
excessively strong signals are received .
ICS switc hes Ihe power souree of tne attenuator circ uit
ON or OF F by using an "ATT" signal trom the LOGIC
UNIT. Q21 anc DS2 provide the converted and stabie
voltage to IC5.
When the [ATT] swi tch is pushed, the CPU (ICS. pin
64) on the LOGIC UNIT outputs a " HIGH" signal. The
" HIGH" signaI is app/ied to !C5 (pin 11 ) and activates the
at tenuator c irc uit (R2 - R8).
The fIItered signals are applied la the atlenuator circuit
(R2- R8) through a swi tching diode (D3).
ca
corn-
pensates for attenuation when high frequencies are
received. The resulting signals ar e applied to the RF
c ircuit through a switching diode (D4).
When the CPU (ICS, pin 64) on the LOGIC UNIT out-
puts a " LOW" signal, Q 12 and R10G act as an inverter.
The inverter supp/ies a " THROUGH" signal to ICS (pin 10) ;
then, the f iltered signals pass through a switchlng diode
(D l ) and are appfied to t he RF ci rcuit through another
switching diode (0 2).
The 25- 1024.9999 MHz signals are app/ied to a band·
pass filter (strip fi ne, L3, C l - C6) to suppress out-ot-band
signaIs. The fllt ered signa ls either ente r or bypass an
attenuator ci rcuit.
, ,
I
ICl
I
r-"I-~6""-<'i 'J1
OPF
(X
l,t
Ll---L..J
11024.9999MHz
I
S IQM
!
I
I
, ,
, ,
I
,
I
I
I
I
I
I
CONV UNIT
Fig.
1
500MH'
. 3
O.
166.667MHz
.3
03
r-
~
0 "
1 55.555MHz
0 1 ""'-
02
OU""/ \
AMP
'--r-'
r----------,
L----o........
~JIW
HPF RF AMP
I
_ " IC
I
1025 ......
I
"..·........'1 I
L
HPF UNIT
----""'T-----;
I
DOUBLER UNIT
I
I I
, ' DOOMH' ,
, .. I----f'-----'
, Q 1 ,
I I
L J
\V
A 55.555 MHz reie renee signal is produced by an
osc illato r c ircuit (X l , Ot) and is then muit iplied by three
at L2 and L3. Th e result ing 166 .666 MHz signaI is
amp/ified at Q3 and is then multiplied by three at Q4.
The resulting 500 MHz (0 dBm) signal is doubied to
produc e a 1 GHz (7 dBm) LO signaion the OOUBLER
UN IT.
• RF CONVERTER AND ATIENUATOR CIRCUITS
The filtered signaIs pass thr ough a wide range RF
amplifier (IC1) to provide 20 dB gain over a wide-band
treq uencv range ano are then applled to a mixer circuit
(IC1) on the CONV UNIT. ICl employs a OBM (Double
Balanc ed Mixer). The signals are mixed wi th the 1 GHz
LO signaI comin g trom the OOUBLER UN IT.
3-1