(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

Maximum beat noise with

tweet prevention Inactive

10 dB Improvement

Maximum beat noise with

tweet prevention active

Zero beat noise

Tweet prevention

active

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

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

LA1136N, 1136NM, 1137N, 1137NM

No. 3507-20/28