LA1136N Datasheet, PDF (19/28 Pages) Sanyo Semicon Device – AM Tuners for Car Radios and Home Stereos

LA1136N, 1136NM, 1137N, 1137NM
(2) SD sensitivity adjustment method
The SD sensitivity is determined by the voltage level of the two inputs to the comparator. Because the absolute value is
determined by the resistor as described above, no matter which level, the sensitivity can be adjusted through both levels.
However, from the standpoint of the stability of the system as a whole and given the possibility that the S-meter output will
be used for other purposes, it is best to make adjustments by adjusting the value on pin 9.
Because the S-meter I/O characteristics during a seek operation or a receive operation are basically as depicted in the diagram
below, so allowing for potential differences, the setting level for the adjustable range should be 25 to 60 dBµ.
Receive mode
Station seek mode
Antenna input level – dBµ
IF count control voltage (STRQ)
IF buffer signal
Ω
(3) Support for the LM7000 (IF count method)
Implemented as shown in the figure at right.
15. Tweet Prevention Circuit
(1) Cause and countermeasures for Tweet distortion
The beat noise frequency characteristics for LA1137N is fr = 900 kHz and the corresponding circuit where it is generated are
shown below. The minimum signal-to-noise ratio for tuners using a reference input signal is 30 dB.
Tweet input
Signal Characteristics
Complete alignment (Maximum
beat noise at 900 kHz)
Complete alignment
(zero beat noise)
Antenna input – dBµ
Mechanism behind the deterioration of the characteristics
The 900 kHz RF input signal from the antenna is amplified by the FET RF amplifier before passing through the RF
alignment stage to the mixer. If this signal is large, the varactor diodes can cause second-harmonic distortion of 1.8 MHz.
The 900 kHz and 1.8 MHz components each generate an IF signal; (450 + α) kHz for the 900 kHz signal, and (450 – α) kHz
for the 1.8 kHz signal. These two signals interact to produce a beat frequency at 2 × α kHz.
No. 3507-19/28