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

(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.

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

and the sensitivity.

Charging

circuit

Ω

Charging

circuit

Resistor added

Ω

Ω

LA1136N, 1136NM, 1137N, 1137NM

No. 3507-22/28