LM1865 datasheet(7/16 Pages) NSC

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Application Notes

ADJUSTABLE MUTESTOP THRESHOLD

The threshold adjustments for the mute and stop functions

are controlled by the same pins Thus the term mutestop

will be used to designate either function

The adjustable mutestop 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

022V 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 mutestop 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

mutestop 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 mutestop 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 mutestop 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

capacitorresistor combination R9 C11 from pin 13 to

ground As the pole is moved higher in frequency (ie R9

gets smaller) more ultrasonic noise is required in the recov-

ered audio in order to initiate mutestop action This corre-

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 mutestop action This is because stereo and SCA

information are contained in the audio signal up to 75 kHz

Also note that the ultrasonic mutestop 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 50X generator is much less noisy

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 mutestop approach ver-

sus the meter driven approach is that the point at which

mutestop 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 mutestop threshold is very

insensitive to temperature and gain variations Proper oper-

ation of this circuit requires that the signal level mutestop

threshold be set at a signal level that achieves 45 dB SN

(g3 dB) in mono in a radio In an electronically tuned radio

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 mutestop threshold is desired then the meter driven

approach is the best alternative

TLH7509 – 8

FIGURE 4 Simplified Level MuteStop Circuit

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