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

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

product can completely block out a weaker desired station

The AGC in the LM1865 has been specially designed to

With the LM1865 system a low AGC threshold is achieved

whenever there are strong out-of-band signals that might

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-

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

simplified circuit diagram (

Figure 5 ) of the AGC system

shows how the dual AGC thresholds are achieved

fixed tuner output which can either be an in-band or out-of-

band signal This fixed tuner output will be called the ‘‘wide

the AGC time constant A reverse AGC system is shown

The LM1865 AGC output is an open-collector current

source capable of sinking at least 1 mA

TLH7509 – 9

FIGURE 5 Dual Threshold AGC

9