DUAL- CONVERSION F
:--AGC
,.
Dot_or
"
Am,
7
"'"
t;)
r'"""='","""'d
Mille.
''''~'
31.1 MHz Measure _ ypiCiI
~
..
_.L_~
Frequency 10.2 MHz
1st
2nd
~1lJ,tor
Oscillator
Inj&Ct
2n<llf
F,equern:y
r
Inject
From
RF
Am,
a..nnel
selector
-----
Fig.
16-Dual-Conversion
IF
stage with
un-<smplifi<U1
l,t
fF frequency.
The operation
or
a dual-<:onvenion
rereiwr
is similar
to
that
of
the
IingJe-<:onversion receiver
with
the
exception
of
an additional
mixer
and
local oscillator.
Most
dual-convelllion
receiven
follow
the
block
dia-
gram shown in Fig.
16
. In this
type
of receiver, there
is no amplification for the
fint
(high)
IF
frequency
other
than
that
gain
the
mixer
stage provides.
In
this
example,
the
lillit
1<;>C8./
oscillator operates
at
aoout
37.7 MHz,
or
10.7
MH'l'
above
the
incoming signal's
frequency.
The
exact
frequency
of
the
local oscillator
is
selected
by
the
channelleleclw
switch.
The
output
of
the first mixer
is
fed
to
a link circuit
made up of two
twled transformers
with
the
second-
ary winding
of
the first translonner
CQnnecWd
to
the
primary
of
the second
tran~fonner.
This circuit
is
tuned
to
10.7 MHz. This eliminates the
other
fre.
quencies
produ~d
by the mixer.
The
output
of
the second link
tmnsfonner
is
fed to
the
input
of
the second mixer, where a frequency of
approximately 10.2
MHz.
is
used
to
CQnvert
the signal
to
41H.
KHz. In this example, the
se<:ond
oscillator
ia
ll1l1O
cry~tal
controlled and adjusted by the channel
selector switch. In many
dual~onversion
re.:eivers,
however, this second Oscillator operates
on
a fixed
frequency,
with
the channel selection being made by
varying the frequency
of
the
first oscillator.
The
output
of
the se.:ond mixer
is
amplified
in
several
tuned
amplifier
s~
and detected the same as
in
D.
$ingle~onversion
receiver.
,.
o.too:lOr
J
AGC
Inject
"
Monitor
Voltage
I
-
Measure
-~i~~~
F
"'llU"""
V
'"
O$cillIolOr
Inject
RF
TYPICAL
1
....
~-;7.6MH'r'~":-
,"
Mix
....
Fig.
17-D=I.C<>nuer"lion
IF
Itage with IImplifUid
111
IF
frequency.
22
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