LA1136N Datasheet, PDF (23/28 Pages) Sanyo Semicon Device – AM Tuners for Car Radios and Home Stereos

LA1136N, 1136NM, 1137N, 1137NM
18. Gain Adjustment Method
(1) AGC Operations
The AGC circuits for LA1136N and LA1137N are configured based on the AGC circuit for LA1135.
Although this explanation is for the LA1137N, it is identical for the LA1136N, except that the pin numbers are different.
1 IF stage
IF
amplifer
I
IF
amplifier
II
Detector
Figure 1 AGC gain and distortion characteristics
500 kΩ VR
HW-60303
IF tank II
gain
attenuator
Set value Maximum sensitivity, MS – dBµ
The IF stage block diagram is shown in the figure above. The output current from pin 12 is converted to a voltage and
detected by pin 14 of the IF AGC circuit. IF Amplifier I prevents the input signal from exceeding the dynamic range of IF
Amplifier II. The relationship between the gain and distortion of the IF stage is shown in Figure 1.
Note that when the gain is decreased, the AGC ON level is increased causing distortion at the IF Amplifier II input (At
maximum sensitivity, the antenna input is increased at output –25 dBµ.) This limits the effectiveness of using the IF tank
II for decreasing gain.
2 RF stage
The AGC for signal 1 of LA1137N operates as shown below.
Figure 2 Gain adjustment
Double-
tuned
circuit
Mixer coil, no attenuation
Mixer coil, with
attenuation
Ceramic
filter
mismatch
caused by
resistor R
Distortion
Mixer input – dBµ
The RF AGC circuit reduces the voltage on pin 1 (6 V when no input) by detecting and amplifying the mixer input signal.
When operating, the voltage drops to 3.5 V, approximately 6 mA of antenna dumping current is driven from Pin 3 to hold the
antenna input signal constant. When current flows from Pin 3, the voltage on Pin 1 drops to 0.6 V min. to control the
drain-to-source voltage of the input FET. Because the RF AGC operates independently of the gain of IF tank I, the RF AGC
gain can be reduced without increasing distortion (See Figure 2).
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