BA7042 Просмотр технического описания (PDF) - ROHM Semiconductor

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BA7042

VCO with sensitivity adjustment function

ROHM Semiconductor 

Optical disc ICs

BA7042

FCircuit operation

(1) Circuit configuration

The control voltage, input through pin 1, is amplified by

the control amplifier, then output to pin 7 and input to the

VCO control circuit. The oscillation frequency of the VCO

is determined by the attached capacitor and controlled

according to the control voltage. Highly modulated FM

signals can be produced by superimposing the control

voltage with AC signals.

(2) Block description

(1) Control amplifier (pin 1, pin 7, pin 8)

The differential amplifier’s positive input is pin 1, its nega-

tive input pin 8, and its output pin 7. Set the input bias volt-

age so that the pin 7 voltage is VCC / 2. The amplification

degree is determined by the external constant.

(2) VCO (pin 2, pin 3, pin 5)

The oscillator’s center frequency is set with an external

capacitor. Use the following equation to calculate oscilla-

tion frequency :

f

1

C (pF)

(1.3 V7)0.15) 102 [MHz]

where C = C2)C3, V7 = pin 7 voltage

The oscillation frequency set by the capacitor is the cen-

ter frequency in the VCO’s frequency variation range.

The VCO can be given a roughly linear oscillation fre-

quency by changing its control voltage.

FOperation notes

(1) Temperature drift of the oscillator circuit can be re-

duced by configuring with a capacitor having a tempera-

ture coefficient. Temperature drift with a capacitor having

a temperature coefficient of zero is *30kHz / _C when

C = 25pF, fO = 14.3MHz, VCTL = 2.5V.

(2) The control amplifier’s frequency characteristics

depend on NF. When used as a reversing amplifier, cutoff

frequency is roughly 100kHz with a gain of 20dB. When

configuring PLL circuits, remember that loop stability is

affected by the amplifier’s frequency characteristics.

111

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