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LM1865M Folha de dados(PDF) 8 Page - National Semiconductor (TI) |
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LM1865M Folha de dados(HTML) 8 Page - National Semiconductor (TI) |
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8 / 16 page  Application Notes (Continued) STOP TIME An electronically tuned radio (ETR) pauses at fixed intervals across the FM band and awaits the stop indication from the LM1865 If within a predetermined period of time no stop indication is forthcoming the controller circuit concludes that there is no valid station at that frequency and will tune to the next interval There are several time constants that can affect the amount of time it takes the LM1865 to output a valid stop indication on pin 16 In this section each time constant will be discussed Deviation Stop Time Constant An offset voltage is generated by the AFT if the LM1865 is tuned to either side of a station Since deviation stop detec- tion in the LM1865 is detected by the voltage at pin 14 it is important that this voltage move fast enough to make the deviation stop decision within the time allowed by the con- troller The speed at which the voltage at pin 14 moves is governed by the RC time constant R5 C9 This time con- stant must be chosen long enough to remove recovered audio from pin 14 and short enough to allow for reasonable stop detection time Signal Level Stop Using Ultrasonic Noise Detection As previously mentioned the R6 C8 time constant on pin 12 is necessary to filter the noise spikes on the output of the internal comparator in the LM1865 This time constant also determines the level stop time When the voltage at pin 12 is above a threshold voltage of about 06V the stop output is low The maximum voltage at pin 12 is about 08V The level stop time is dominated by the amount of time it takes the voltage at pin 12 to fall from 08V to 06V The voltage at pin 12 follows an exponential decay with RC time constant given by R6 C8 For example if R6 e 25k and C8 e 22 mF the stop time is given by t eb(24k) (22 mF) fin 0608J which yields t e 15 ms It should be noted that the 06V threshold at pin 12 has a high temperature dependence and can move as much as 100 mV in either direction Signal Level Stop Using the Meter Output Pin 8 As mentioned previously R6 C8 is not necessary when the meter output is used to drive pin 13 Consequently this time constant is not a factor in determining the stop time Howev- er the speed at which the meter voltage can move may become important in this regard This speed is a function of the resistive load on pin 8 and filter capacitance C5 AGC Time Constant In tuning from a strong station to a weaker station above the level stop threshold the AGC voltage will move in order to try to maintain a constant tuner output The AGC voltage must move sufficiently fast so that the tuner is gain in- creased to the point that the level stop indicates a valid station This time constant is controlled by R11 and C13 DISTORTION COMPENSATION CIRCUIT The quadrature detector of the LM1865 has been designed with a special circuit that compensates for distortion gener- ated by the non-linear phase characteristic of the quadra- ture coil This circuit not only has the effect of reducing dis- tortion but also desensitizes the distortion as a function of tuning characteristic As a result low distortion is achieved with a single tuned quad coil without the need for a double tuned coil which is costly and difficult to adjust on a produc- tion basis The lower distortion has been achieved without any degradation of the noise floor of the audio output Futh- ermore the compensation circuit first-order cancels the ef- fect of quadrature coil Q on distortion When measuring the total harmonic distortion (THD) of the LM1865 it is imperative that a low distortion RF generator be used In the past it has been possible to cancel out dis- tortion in the generator by adjustment of the quadrature coil This is because centering the quadrature coil at other than the point of inflection on the S-curve introduces 2nd har- monic distortion which can cancel 2nd harmonic distortion in the generator Thus low THD numbers may have been obtained wrongly Large AFT offsets asymmetrical off tuning characteristic and less than minimum THD will be observed if alignment of the quadrature coil is done with a high distor- tion RF generator Care must also be taken in choosing ceramic filters for the LM1865 It is important to use filters with good group delay characteristics and wide enough bandwidth to pass enough FM sidebands to achieve low distortion 8 |
Nº de peça semelhante - LM1865M |
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Descrição semelhante - LM1865M |
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