LA1136N Datasheet, PDF (20/28 Pages) Sanyo Semicon Device – AM Tuners for Car Radios and Home Stereos

LA1136N, 1136NM, 1137N, 1137NM
(2) Methods for improving tweet characteristics
1 A twin varactor diode can be used, as is used for FM detection. However, the chip size is four times that of a standard
varactor diode having the same capacitance, resulting in increased cost and a narrower capacitance range.
2 The RF AGC ON level can be reduced to prevent the varactor diodes from distorting the signal. However, the
signal-to-noise ratio is limited to approximately 50 dB or lower.
3 The varactor diodes can be replaced by an up-converter. However, this increases the susceptibility to interference.
(3) Countermeasures for LA1136N, LA1137N
Although this explanation is for the LA1137N, it is identical for the LA1136N, except that the pin numbers are different.
Output
Tweet prevention
active
Maximum beat noise with
tweet prevention Inactive
10 dB Improvement
Maximum beat noise with
tweet prevention active
Zero beat noise
The AGC ON level for fr = 90 kHz is reduced
by 10 dB to reduce the signal level on the
varactor diodes. The microcontroller determines
when the tuner is receiving at 900 kHz and
forces the HIGH-level signal present on pin 18
and SD V_DD to LOW, reducing the ON level by
10 dB. This gives the signal-to-noise
characteristics shown below.
RF AGC on level
AGC variable
gain
Stop detector
cutoff circuit
Stop detector
circuit
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