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LM1865N Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LM1865N Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 16 page Application Notes (Continued) The LM1865 has been carefully designed to insure low AFT offset current at the point of minimum THD AFT offset cur- rent will cause a non-symmetric deviation mutestop win- dow about the point of minimum THD No external AFT off- set adjustment should be necessary with the LM1865 The amount of resistance in series with the 18 mH quadrature coil drive inductor L1 has a significant effect on the mini- mum THD This series resistance is contributed not only by R13 but also by the Q of L1 The Q of L1 should be as high as possible (ie Ql50) in order to avoid production prob- lems with the Q variation of L1 Once R13 has been opti- mized for minimum THD adjustment on a radio by radio basis should be un-necessary DUAL THRESHOLD AGC (AUTOMATIC LOCALDISTANCE SWITCH) There is a well recognized need in the field for gain reducing (AGCing) the front end (tuner) of an FM receiver This gain reduction is important in preventing overload of the front end which might occur for large signal inputs Overloading the front end with two out-of-band signals one channel spacing apart and one channel spacing from center fre- quency or two channel spacings apart and two channel spacings from center frequency will produce a third order intermodulation product (IM3) which falls inband This IM3 product can completely block out a weaker desired station The AGC in the LM1865 has been specially designed to deal with the problem of IM3 With the LM1865 system a low AGC threshold is achieved whenever there are strong out-of-band signals that might generate an interfering IM3 product and a high AGC thresh- old is achieved if there are no strong out-of-band signals The high AGC threshold allows the receiver to obtain its best signal-to-noise performance when there is no possibili- ty of an IM3 product The low AGC threshold allows for weaker desired stations to be received without gain-reduc- ing the tuner It should be noted that when the AGC thresh- old is set low there will be a signal-to-noise compromise but is assumed that it is more desirable to listen to a slightly noisy station than to listen to an undesired IM3 product The simplified circuit diagram ( Figure 5 ) of the AGC system shows how the dual AGC thresholds are achieved Vm e 1V corresponds to a fixed in-band signal level (de- fined as VNB) at the tuner output VNB will be referred to as the ‘‘narrow band threshold’’ VWB also corresponds to a fixed tuner output which can either be an in-band or out-of- band signal This fixed tuner output will be called the ‘‘wide band threshold’’ Always VWB l VNB R11 and C13 define the AGC time constant A reverse AGC system is shown This means that VAGC decreases to gain-reduce the tuner The LM1865 AGC output is an open-collector current source capable of sinking at least 1 mA TLH7509 – 9 FIGURE 5 Dual Threshold AGC I1 e GM1 Vm only if Vm l 1V otherwise I1 e 0 Gm1 VWB e constants IAGC e Gm2 Vo where Gm2 e I1 26 mV and Vo l VWB otherwise IAGC e 0 9 |
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