SM9501A/B

NIPPON PRECISION CIRCUITS INC.—8

FUNCTIONAL DESCRIPTION

Antenna Input and Tuning Capacitor Switching Function

There are three antenna inputs: IN1, IN2, and IN3. When FCN is open (or HIGH), the internal analog switch is

OFF and IN1–IN2 are the antenna inputs (60kHz mode). When FCN is LOW, the analog switch is ON, con-

necting IN3 and IN2. C2 is then connected in parallel to C1 in the tuning circuit, reducing the resonant fre-

quency (40kHz mode).

FCN should be left open if not using the tuning capacitor switching function, and IN2 should be connected to

IN3 externally.

AGC Amplifier and Gain Hold Function

The input voltage from the antenna is amplified by the AGC amplifier. The gain can be monitored by the volt-

age on pin CP, and can be changed by varying the CP voltage. An external capacitor Cp can be connected to CP

to stabilize the voltage, but the gain tracking time is dependent on the capacitance. When HLDN is open (or

HIGH), the gain automatically adjusts to follow the post-amplifier detector signal. When HLDN is LOW, the

immediately preceding gain is held for an interval determined by the Cp capacitance.

FCN

Analog switch

Antenna input

Tuning capacitor

Receiver frequency

Open or HIGH

OFF

Between IN1 and IN2

C1

60kHz

LOW

ON

Between IN1 and IN2, IN3

C1 + C2 parallel

40kHz

IN1

IN3

IN2

C2

C1

L1

FCN

AGC

HLDN

Gain tracking

Open or HIGH

Auto tracking

LOW

Gain held fixed

AGC Amp

Peak Hold Detector

Bottom Hold Detector

HLDN

CP

CB

Cb

Cp