LA72914V

No.A1859-7/9

Continued from preceding page.

Pin No.

Pin Name

Description

Equivalent circuit

12

AGC monitor

FM AGC monitor pin.

12

13

GND

GND pin.

14

VCO CTL

Modulation frequency fine control pin.

Adjustment frequency is, about

±220kHz at 0.5V to 4.5V

DC.

14

C1

VR1

2.4V

15

ALC CTL

Audio ALC filter pin.

Please connect 0.47

μF capacitor.

15

C1

16

TX Audio IN

(TX-OFF_CTL)

Audio signal input pin.

Internal bias is 2.4V DC. Please use capacitor coupling. If

DC voltage set to under 0.4V DC, then modulator circuit

stops. Demodulation circuit is work. This is receiving only

mode. (TX-OFF: 0 to 0.4V)

16

C1

2.4V

Signal processing outline

[Modulation block]

• Audio signal is input from pin16. (Internal bias is DC 2.4V.) Go through AMP (6dB)/ALC/LPF (10 kHz) and emphasis circuit,

then input FM-modulation block. After FM-modulation block, go to output driver block then FM-audio signal output from pin1.

• ALC: ALC circuit work at pin16 input level over than 0.5Vp-p.

• LPF: LPF gain is -3dB at 10 kHz and -6dB/oct. Keep over modulation from emphasis characteristic.

• Emphasis circuit: The pin3 owns an Emphasis characteristic which connected capacitor. Recommended value = 6800pF.

• FM modulation: Carrier frequency is choosing from pin6 voltage. When pin16 input level is 0.5Vp-p, then FM deviation set to ±25

kHz. Internal LPF reduce harmonic spectrum.

• Output Driver: Output AC voltage is about 2.0Vp-p. (Load resistance: 1kΩ)