LM1865N National Semiconductor, LM1865N Datasheet - Page 11

• 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

the tuner is not desensitized by the AGC to weak desired
stations below the narrow band threshold
Application Notes
voltage while V
until it reaches the level of the wide band threshold at the
tuner output When this occurs V
keep V
drop as the amount of AGC increases while V
stant by the AGC
When compared to the simple case of a single in-band sig-
nal we see that because of the presence of a strong out-of-
band signal AGC action has occurred earlier For the simple
case AGC started when V
above AGC started when V
achieves an early AGC when there are strong adjacent
channels that might cause IM
signals aren’t present
For the range of signal levels that the tuner was gain-re-
duced and V
the recovered audio as compared to the case where there
was no gain reduction in this interval Note however that
o l
V
D k
WB
D
as V
V
is allowed to increase V
WB
D
there was loss in signal-to-noise in
takes over the job Thus V
D t
3
D t
(Continued)
V
FIGURE 11 Affect of Meter Load on Narrow Band Threshold
and a later AGC when these
WB
UD
V
NB
For the two signal case
is no longer needed to
Thus the LM1865
FIGURE 10 Narrow Band Threshold Circuit
D
D
will increase
is held con-
UD
will
11
10
NARROW BAND AGC THRESHOLD ADJUSTMENT
Both the narrow band and wide band AGC thresholds are
user adjustable This allows the user to optimize the AGC
response to a given tuner Referring to Figure 5 when the
meter output exceeds 1V a comparator closes SW1 A sim-
plified circuit diagram of this comparator is shown in Figure
The 1K resistor in series with pin 8 allows for an upward
adjustment of the narrow band threshold This is accom-
plished by externally loading pin 8 with a resistor Figure 11
illustrates how this adjustment takes place
From Figure 11 it is apparent that loading the meter output
not only moves the narrow band threshold but also de-
creases the meter output for a given input
In general one chooses the narrow band threshold
based on what signal-to-noise compromise is considered
acceptable
TL H 7509 – 13
TL H 7509– 12