LM1865N National Semiconductor, LM1865N Datasheet - Page 12

• Manufacturer Part Number: LM1865N
• Description: ADVANCED FM IF SYSTEM
• Manufacturer: National Semiconductor

Specifications of LM1865N

• Function: FM IF System
• Rf Type: FM
• Package / Case: 20-DIP (0.300", 7.62mm)
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *LM1865N; LM1865

Application Notes
WIDE BAND AGC THRESHOLD ADJUSTMENT
There are a number of criteria that determine where the
wide band threshold should be set If the threshold is set too
high protection against IM
set too low the front end under certain input conditions
may be needlessly gain-reduced sacrificing signal-to-noise
performance Ideally the wide band threshold should be set
to a level that will insure AGC operation whenever there are
out-of-band signals strong enough to generate an IM
uct of sufficient magnitude to exceed the narrow band
threshold Ideally this level should be high enough to allow
for a single in-band desired station to AGC the tuner only
after the maximum signal-to-noise has been achieved
In order to insure that the wide band loop is activated when-
ever the IM
termine the minimum signal levels for two out-of-band sig-
nals necessary to produce an IM
range for the wide band loop to be activated whenever the
tuner output exceeds the rms sum of these signals There
are many combinations of two out-of-band signals that will
produce an IM
combination whose rms sum is a minimum at the tuner out-
put IM
equation
where a
V
UD1
IM
3
3
e
e
e
at the tuner output is given according to the
aV
constant dependent on the tuner
out-of-band signal 400 kHz from center frequen-
cy applied to tuner input
3
exceeds the narrow band threshold V
UD1 2
3
Define A
of a given level However there is only one
V
A1
A2
UD2
e
e
e
(assuming no gain reduction)
tuner gain at center frequency
tuner gain at
tuner gain at
3
will be lost If the threshold is
(Continued)
3
equal to V
f
f
o
o
a
a
400 kHz
800 kHz
NB
Then ar-
NB
3
prod-
de-
(1)
FIGURE 12
12
V
WB s
In general due to tuned circuits within the tuner the tuner
gain is not constant with frequency Thus if the tuner is kept
fixed at one frequency while the input frequency is changed
the output level will not remain constant Figure 12 illus-
trates this
It can be shown that for a given IM
V
tuner output is given by the equations
Therefore in order to guarantee that the AGC will be keyed
for an IM
The right hand term of equation (4) defines an upper limit for
V
at the tuner output for two out-of-band signals V
V
tuner input
UD1
WB
UD2
V
V
V
UD1
UD2
V 2 NB
UD2
called V
and V
as expressed in equations (2) and (3) applied to the
a
e
e
e
3
1 12
0 794
e
(A1) (1 12)
out-of-band signal 800 kHz from center frequen-
cy and 400 kHz away from V
er input
UD2
WBUL
V
NB
that produces the smallest rms sum at the
A2
A1
A1
A2
we need only satisfy the condition
V
2
2
IM
A2
A1
WBUL
a
IM
3
V
a
NB
a
3
is the rms sum of all the signals
2
a
A2 (0 794)
3
UD1
the combination of
applied to tun-
TL H 7509 – 14
A1
A2
2
2
V
UD1
NB
a
and
2
(2)
(3)
(4)