SM9503AV datasheet(7/9 Pages) NPC

7 / 9 page

SM9503A

NIPPON PRECISION CIRCUITS INC.—7

FUNCTIONAL DESCRIPTION

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.

Crystal Filter Circuit

External crystals are used as ﬁlters. The center frequency and bandwidth of the ﬁlters is determined by the

crystal characteristics. If the center frequency is lower than the target frequency, it is necessary to add CL

capacitor for the adjustment frequency. If Q of the crystal ﬁlter is higher and the output delay is larger, it is nec-

essary to add RL to adjust it. Adding a compensation capacitor CC, it is possible to select built-in or external,

cancels the high-frequency components pass through the crystal parallel capacitance. Compensation capacitors

are built in, and wiring them inside makes possible to select versions of the required capacitances from 0.5pF

to 2.0pF.

Detector Circuit

The ampliﬁed signal is full-wave rectiﬁed and passed through a lowpass ﬁlter detector. The detector output is

input to peak hold (pin CP) and bottom hold (pin CB) circuits to form the decoder reference potentials and

peak hold potential for AGC control.

The case of using the external compensation

capacitor

XO1

XO2

AGC Amp

Post Amp

The case of using built-in compensation capacitor

XO1

XO2

AGC Amp

Post Amp

Amplifier

Rectifier

LPF

Peak/

Bottom

Hold

Peak hold

Bottom hold

VDD potential

SM9503AV Datasheet(PDF) 7 Page - Nippon Precision Circuits Inc