LM1865N National Semiconductor, LM1865N Datasheet - Page 7

• 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
ADJUSTABLE MUTE STOP THRESHOLD
The threshold adjustments for the mute and stop functions
are controlled by the same pins Thus the term mute stop
will be used to designate either function
The adjustable mute stop threshold in the LM1865 allows
for user programming of the signal level at which muting or
stop indication takes place The adjustment can be made in
two mutually exclusive ways The first way is to take a volt-
age divider from the meter output (pin 8) to the off channel
mute input (pin 13) When the voltage at pin 13 falls below
0 22V an internal comparator is tripped causing muted or
causing the stop output to go low Adjustment of the voltage
divider ratio changes the signal level at which this happens
The second method of mute stop detection as a function of
signal level is to use the presence of ultrasonic noise in the
recovered audio to trip the internal comparator As the sig-
nal level at the antenna of the radio drops the amount of
noise in the recovered audio both audible and ultrasonic
increases
The recovered audio is internally coupled through a high
pass filter to pin 13 which is internally biased above the
comparator trip point Large negative-going noise spikes will
drive pin 13 below the comparator trip point and cause
mute stop action A simplified circuit is shown in Figure 4
Since the input to the comparator is noise the output of the
comparator is noise Consequently a mute stop filter on pin
12 is required to convert output noise spikes to an average
DC value This filter is not necessary if pin 13 is driven from
the meter
Adjustment of the mute stop threshold in the noise mode is
accomplished by adjusting the pole of the high pass filter
coupled to the comparator input This is done with a series
capacitor resistor combination R9 C11 from pin 13 to
ground As the pole is moved higher in frequency (i e R9
gets smaller) more ultrasonic noise is required in the recov-
ered audio in order to initiate mute stop action This corre-
FIGURE 4 Simplified Level Mute Stop Circuit
7
sponds to a weaker signal at the antenna of the radio In
choosing the correct value for R9 it is important to make
sure that recovered audio below 75 kHz is not sufficient to
cause mute stop action This is because stereo and SCA
information are contained in the audio signal up to 75 kHz
Also note that the ultrasonic mute stop circuit will not oper-
ate properly unless a tuner is connected to the IF This is
because at low signal levels the noise at the tuner output
dominates any noise sources in the IC Consequently driv-
ing the IC directly with a 50
than driving the IC with a tuner and therefore not realistic
The RC filter on pin 12 not only filters out noise from the
comparator output but controls the ‘‘feel’’ when manually
tuning For example a very long time constant will cause
the mute to remain active if you rapidly tune through valid
strong stations and will only release the mute if you slowly
tune to a valid station Conversely a short time constant will
allow the mute to kick in and out as one tunes rapidly
through valid stations
The advantage in using the noise mute stop approach ver-
sus the meter driven approach is that the point at which
mute stop action occurs is directly related to the signal-to-
noise ratio in the recovered audio Furthermore the mute
stop threshold is not subject to production and temperature
variations in the meter output voltage at low signal levels
and thus might be able to be set without a production ad-
justment of the radio The noise mute stop threshold is very
insensitive to temperature and gain variations Proper oper-
ation of this circuit requires that the signal level mute stop
threshold be set at a signal level that achieves 45 dB S N
(
the signal level stop threshold can be set to a much larger
level by gain reducing the tuner (ie pulling the AGC line) in
scan mode and then releasing the AGC once the radio
stops on a station In an environment where temperature
variations are minimal and manual adjustment of the signal
level mute stop threshold is desired then the meter driven
approach is the best alternative
g
3 dB) in mono in a radio In an electronically tuned radio
TL H 7509 – 8
generator is much less noisy