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LM1865M データシート(PDF) 9 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
部品番号 LM1865M
部品情報  Advanced FM IF System
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メーカー  NSC [National Semiconductor (TI)]
ホームページ  http://www.national.com
Logo NSC - National Semiconductor (TI)

LM1865M データシート(HTML) 9 Page - National Semiconductor (TI)

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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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