LA1136N Datasheet, PDF (21/28 Pages) Sanyo Semicon Device – AM Tuners for Car Radios and Home Stereos

LA1136N, 1136NM, 1137N, 1137NM
(4) The accurate voltages for each pin
During seek:
When the voltage of pin 11 is HIGH 5 V, transistor N is ON and transistors N3, N4, N6, N7, N8 and N9, OFF.
Pin 18 is connected to the collector of N10 and through a pull-up resistor to V_DD.
Pin 18 is the stop detector output when a carrier is present. When N10 is ON, pin 18 is LOW, V18 6 ON, and when N10 is
OFF, pin 18 is HIGH, V18 6 V_DD.
During stop:
When the voltage of pin 11 is LOW and the voltage of pin 18 is HIGH, transistor N5 is OFF, and transistors N3, N4, N7,
N8, N9, P1 and P2, ON. The SD cut-off circuit for N8 and N9 are ON. Since N9 is ON, no current flows through the stop
detector circuit. Therefore the voltage for pin 18 = V_DD.
When the microcontroller pulls pin 18 to ground, N5 and N7 turn OFF, and N3, N4, N6, N8, N9, P1 and P2, ON. As a
result, N2 is also turned OFF and the amplifier load for fr = 900 kHz is R1. The amplifier load for fr = 900 kHz is R1 // R2.
The AGC ON level is 10 dB lower for R1, R1//R2 than for other frequencies.
16. IF block AGC time constant switching circuit
These tuners incorporate various AGC time constant switching circuits that serve to prevent incorrect stops during seek operations.
A summary of these circuits is provided below.
(1) Problems with IF AGC response, such as incorrect stops during seek operations
1 Incorrect stops (or overshoots) due to S-meter oscillation caused by transient response in IF AGC (secondary filter).
2 Overshoots of adjacent stations or weak signal stations due to the time required to discharge the electric charge that has
built up in the IF AGC capacitor when a seek is initiated while the device is receiving a strong signal
(2) Countermeasures
Although this explanation is for the LA1137N, it is identical for the LA1136N, except that the pin numbers are different.
Countermeasure for problem 1:
When a high signal on pin 11 is detected during a seek operation, change the time constants as indicated below.
First-order filter
t = 1 kΩ × 4.7 µF
Second-order filter
During receive
During seek
Unit (resistance: Ω, capacitance: F)
Countermeasure for problem 2:
AGC capacitor discharge times can be reduced by tracking the S-meter output voltage and rapidly discharging the AGC
capacitor connected to pin 14 as shown below. This circuit is interlocked with the MUTE signal on pin 11 so that it operates
only during seek.
It also reduces the AGC time constant during seek to one-eighth of its normal value.
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