– 10 –

CXA2647N

•EQ

fc Control

RFC

LPF2

LPF1

HPF1

26

VFC

Control Bias

VC – 1V to VC + 1V

Control Bias

VC – 1V to VC + 1V

33k

Ω to 100kΩ

In

Out

25

BST

Boost

Control

24

Amp

The boost gain can be adjusted by adjusting the HPF1

gain.

The cut-off frequency is adjusted by the RFC external

resistance value and the VFC control voltage value.

RFC resistance value: The cut-off frequency fc of each

filter is adjusted by the Pin 26

external resistance value.

The VFC voltage can be varied

using this fc as the reference.

VFC voltage: fc can be adjusted by the voltage applied

to Pin 25. The cut-off frequency control

characteristics are shown in the graph

below.

The boost gain can be adjusted by the BST (Pin 24)

control voltage.

The control characteristics are shown in the graph

below.

The boost gain stands for the increased gain from the

low frequency gain in the fc frequency.

VC – 1.0

VC

BST voltage [V]

VC + 1.0

Freq.

Boost

gain

Gain

fc

8dB

–1dB

5dB

VC – 1.0

VC

VFC voltage [V]

VC + 1.0

1.5fc

fc

0.5fc

RFC pin external resistor value 100k

Ω: fc = 1.6MHz

33k

Ω: fc = 4.8MHz

• APC (Automatic Power Control)

When the laser diode is driven by a constant current, the optical power output has extremely large negative

temperature characteristics. Therefore, the current must be controlled to maintain the monitor photodiode

output at a constant level. This control is performed by the APC function.

2

PD

1 LD

10k

55k

10k

1.25V

56k

56k

10k

1k

The EQ internal block diagram is shown below. The EQ is configured with the filter of the Bessel function of

order 4.

LPF1 =

HPF1 =

LPF2 =