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TZA3001BHL_C1

SDH/SONET STM4/OC12 laser drivers

Philips Electronics 

Philips Semiconductors

SDH/SONET STM4/OC12 laser drivers

Product specification

TZA3001AHL; TZA3001BHL;

TZA3001U

In this example, the resulting bandwidth for the optical 1

modulation current control loop, without an external

capacitor, is:

BONE = -2---π--3---0×-----×4---0--1---0×----–-1-3--0--×--–--1-5--2-0---×0-----8×---0--1---×-0---–-1--3-0----3- ≈ 750 Hz

The resulting bandwidth for the optical 0 bias current

control loop, without an external capacitor, is:

BZERO = 0---2-.--5-π---×-×----3-4---0-0---×-×----1-1---0-0--–-–--3-1--2-×---×-5----05---00-----××-----11---00----–3--3- ≈ 600 Hz

It is not necessary to add additional capacitance with this

type of laser.

Control loop data pattern and bit rate dependency

The constants in equations (1) and (3) are valid when the

data pattern frequently contains a sufficient number of

‘constant zeroes’ and ‘constant ones’. A single control loop

time period (τONE and τZERO) must contain ones and zeros

for at least approximately 6 ns (as provided, for example,

by the A1/A2 frame alignment bytes for STM4/OC12).

In practice, the optical extinction ratio increases if the bit

rate increases. Therefore, it is important to use the actual

data patterns and bit rate of the final application circuit for

adjusting the optical levels.

The laser driver peak detectors are able to track MPD

output current overshoot and undershoot conditions.

Unfortunately, these conditions affect the ability of the IC

to correctly interpret the high and low level MPD current.

In particular, the occurrence of undershoot can have a

markedly adverse effect on the interpretation of the low

level MPD current.

Additional bias by modulation ‘off’ current

Although during operation, the full modulation current

switches between outputs LA and LAQ, a small amount of

modulation current continues to flow through the inactive

pin.

For example, when the laser, whose cathode is connected

to LA, is in the ‘dark’ part of its operating cycle (logic 0),

some of the modulation ‘off’ current flows through LA while

most of the current flows through LAQ. This value

Io(mod)(off) is effectively added to the bias current and is

subtracted from the modulation current. Fortunately, the

value correlates closely with the magnitude of the

modulation current. Therefore, applications requiring low

bias and low modulation are less affected. Figure 9 shows

the modulation ‘off’ current as a function of the modulation

‘on’ current.

handbook, h3alfpage

I o (mod)(off)

(mA)

2

MGS902

(1)

1

(2)

0

0

20

40

60

Io(mod)(on) (mA)

(1) Worst case operation (Tj = 125 °C, VCC = 5.5 V

and worst case parameter processes).

(2) Typical operation.

Fig.9 Io(mod)(off) as a function of Io(mod)(on).

Monitoring the bias and modulation current

Although not recommended, the bias and modulation

currents generated by the laser driver can be monitored by

measuring the voltages on pins TZERO and TONE,

respectively (see Fig.10). The relationship between these

voltages and the corresponding currents are given as

transconductance values and are specified in

Chapter “Characteristics”. The voltages on pins TZERO

and TONE range from 1.4 to 3.4 V. Any connection to

these pins should have a very high impedance value. It is

mandatory to use a CMOS buffer or an amplifier with an

input impedance higher than 100 GΩ and with an

extremely low input leakage current (pA).

2002 Aug 13

9

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