データシートサーチシステム |
|
LM1865M データシート(PDF) 7 Page - National Semiconductor (TI) |
|
|
LM1865M データシート(HTML) 7 Page - National Semiconductor (TI) |
7 / 16 page 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 7 |
同様の部品番号 - LM1865M |
|
同様の説明 - LM1865M |
|
|
リンク URL |
プライバシーポリシー |
ALLDATASHEET.JP |
ALLDATASHEETはお客様のビジネスに役立ちますか? [ DONATE ] |
Alldatasheetは | 広告 | お問い合わせ | プライバシーポリシー | リンク交換 | メーカーリスト All Rights Reserved©Alldatasheet.com |
Russian : Alldatasheetru.com | Korean : Alldatasheet.co.kr | Spanish : Alldatasheet.es | French : Alldatasheet.fr | Italian : Alldatasheetit.com Portuguese : Alldatasheetpt.com | Polish : Alldatasheet.pl | Vietnamese : Alldatasheet.vn Indian : Alldatasheet.in | Mexican : Alldatasheet.com.mx | British : Alldatasheet.co.uk | New Zealand : Alldatasheet.co.nz |
Family Site : ic2ic.com |
icmetro.com |