Icom IC-R7100 Service Manual page 11

3·1·8 2nd MIXER CIRCUIT (MIX2 UNIT)
The 2nd mixer circuit converts Ihe 1st IF signal to a 2nd
IF signa I.
A 256 or 768 MHz (0 dBm) 2nd LO signa I is applied
10
a
1 GHz cu tott lew-pass filter (strip line, Cl04, C105, C146)
on the MIX2 UNIT
10
suppress Ihe high harmonie com-
ponents through P3 from the PLL UNIT.
The filtered signal is ampllfied al Ihe 2nd LO amplifiers
(016, Q17). The 2nd LO amplifiers (Q16, 017) employ
wide frequency band amplifiers with approximately 10 JB
gain and amplify Ihe 2nd LO signal to approximately 10
dBm. The amplified signal is applled
10
an attenuator
circuit (R116, R118).
3·1·9 AGC BUFFER AMPLIFIER CIRCUIT
(RF UNIT)
The AGC voltage shifts from approximately 4
V 10
0
V.
The AGC buffer amplifier (020) shifts approximalely 1.5 V
to the minus voltage thaI Ihe RF amplifiers (01-04)
require .
3·1·10 2nd IF CIRCUIT (RF AND MAIN UNITS)
The 10.7 MHz 2nd IF signal is applled
10
a low-pass filter
which cuts off Ihe frequencies around 400 MHz . The
filter consists of a strip line and C142 -C144 and
prevents Ihe high harmonie components from entering
tne 2nd mixer (IC7) on Ihe MIX2 UNIT_
The 266.7 or 778.7 MHz 1sI IF signal is mixed al IC7 wlth
the 2nd LO signaI to produce a 10_7 MHz 2nd IF signal.
The 10.7 MHz 2nd fF signaI is output trom IC? (pin 1) and
is Ihen applled
10
Ihe RF UNIT_ IC7 employs a OBM
(Double Balanced Mixer).
Tabla 3 shows
Ihe relation between reeerve frequency
and 2nd LO Irequency.
The filtered signaI passes Ihrough a 2nd IF amplifier (018)
wilh approximalely 20 dB gain and is Ihen applled to the
MAIN UNIT Ihrough J4.
A portion of Ihe 2nd IF signal output from IC7 passes
through a buffer-amplifier (019) and is then applied to a
20 MHz cutoft low-pass filler (L60, Cl14 -C116). The
fillered signal is applied
10
Ihe [IF OUn jack on the rear
panel.
RECEIVE FREOUENCY [MHz] 2nd LO FREOUENCY [MHz]
25- 89 .9999 768
90 - 249.9999 768
250- 511.9999 768
512 -1024.9999 256
Tabla 3
In WFM mode, lhe 10.7 MHz 2nd IF signal passes Ihrough
Ihe IF amplltiers
(1.1,
055) and is then appli ed to a high·
quality
eerarme
filler (Fll) coverlng the 230 kHz (3 dB)
bandwidth. The filtered signal is amplified al 014 and
is th en re-amplifled at iCl1 . 014 and IC11 provide
approxlmately 40 dB gain in WFM mode. The ampllfied
signal passes through another hiqh-quallty ceramlc filter
(F12) covering the 230 kHz (3 dB) bandwidth and is Ihen
applied
10
Ihe WFM demodulalor circuit.
• IF CIRCUIT
In other modes, the 10.7 MHz 2nd IF signa I is applied
10
a pair of crystal filters (FI3) covering a ±7.5 kHz (3 dB)
bandwidth in order to obtain wide selectIon capability
and to pass only desired
signaIs.
The fil1ered signal is
applied
10
021 .
FILZ UNIT
MAIN UNIT
I.
r----I<f--
WFM
circuit
I.
'----I<f--FM.AM.SS8
c ircuit
I
I
I
I
I
I
I
I
I
I
I
AGC
I
IIM
I
I
l~
I
MAIN UNIT
I
I
I
I
I
'""
LO
IIM
r-;;';;;-;;;;;-l
I
IC7
I
I • 10.7~z
I X •
I I
I
768MHZ
I
I
~
I
I
2!>6MHz
I
I
,.dLO
I
I /
\ ~017 1
L_I-__
...J
Fit /
7787Mz
(RX IntQ : 2S...... S11 .9999MHz)
r-------...,
I
I
I I
I I
r--------...,
I
" .lM"'
I
I
\ F ' /
I
I I
L __
~~~:
__
J
(R X IreQ : S12- 1024.9999M H z)
RF UNIT
,,~
pu.
UNIT
Buller
r:
L......1
V
>-- ---+
0'9
IIF OUT)
JACK
Fig. 5
3-5