LA1136N Datasheet, PDF (22/28 Pages) Sanyo Semicon Device – AM Tuners for Car Radios and Home Stereos

LA1136N, 1136NM, 1137N, 1137NM
17. Preventing Incorrect RF AGC time-constant switching, Seek Operation
These tuners incorporate various circuits that serve to prevent incorrect stops during seek operation. A summary of the RF
amplifiers is provided below.
(1) Incorrect stops during RF AGC, and other problems
1 The RF AGC response time can cause incorrect seek and stop operation when moving from a weak-signal station to a
strong-signal station. When seek operation starts from a strong station, the tuner can overshoot an adjacent weak station if
the AGC time constant is too high.
2 If the wideband AGC ON level is too high, the varactor diodes can generate cross-modulation distortion causing incorrect
seek operation.
3 Input FETs with high I_DSS, required for correct seek and stop operation, are difficult to drive.
(2) Countermeasures
Countermeasure for problem 1:
When the input signal goes from a strong to a weak station, the detector generates a pulse as shown in the figure below. The
AGC capacitor C1 is charged rapidly for the duration of this pulse. It also reduces the AGC time constant during charging to
one-third of its normal value.
Charging
circuit
Ω
Countermeasure for problem 2:
The wideband AGC ON level should be raised as far as practicable within cross-modulation disrtortion and sensitivity
constraints. If a pin-diode is used, the sensitivity is not suppressed even if the AGC ON level is increased considerably.
However, if the varactor diodes in the RF input circuit are exposed to strong interference signals of any frequency, the
resulting non-lincarity can cause seek and stop operation errors.
Previous devices used the same S-meter linearity for both seek and stop operations, where the linearity was extended to
100 dBµ by the IF AGC and RF AGC. When the wideband RF AGC operated, the S-meter signal was amplified causing
incorrect stop operation. For example, modulation sidebands could stop the tuner one station too early.
The S-meter linearity has now been restricted to approximately 60 dBµ minimum during the seek and stop operations for
LA1136N and LA1137N. When the RF AGC operates during seek operation, the S-meter signal amplifier is disabled. This
reduces the potential for tuning error when using the stop detector method (such as LM7001, etc.).
Countermeasure for problem 3:
For LA1137N, a transistor and base current supply circuit (200 µA maximum) have been incorporated into a cascade AGC
configuration to simplify the use of high I_DSS FETs. An external resistor is required.
Note that V_AGC–0 = 6.1 V for no input signal. Also, V_CC should be chosen carefully since it affects both the dynamic range
and the sensitivity.
Ω
Large I_DSS
Charging
circuit
Resistor added
Ω
No. 3507-22/28