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hfan2_v8.doc 09/01/00

Application Note:

HFAN-2.0Rev 0; 5/00

Interfacing Maxim Laser Drivers with Laser Diodes

[An abridged version of this application note first appeared in theAugust, 2000 issue of Lightwave magazine.]

MAXIM High-Frequency/Fiber Communications Group

Customer Applications Maxim IntegratedApplication Note HFAN-2.0 (Rev. 0, 5/00)

Table of Contents

I. Overview.................................................................................................................................................................1II. Laser Diode Characteristics ...................................................................................................................................1III. Laser Driver Output Structure ..............................................................................................................................2IV. The PC Board Interface........................................................................................................................................3

A. Headroom Issues with DC-Coupling.................................................................................................................3B. AC-Coupling......................................................................................................................................................4C. Interfacing a Laser to a Driver ...........................................................................................................................5D. Additional Board Design Considerations .........................................................................................................7

V. Troubleshooting.....................................................................................................................................................7A. Waveform Compression (Figure 12) .........................................................................................................8B. Relaxation Oscillation (Figure 13).............................................................................................................8C. Overshoot (Figure 14)................................................................................................................................9D. Undershoot (Figure 15) .............................................................................................................................9E. Ringing (Figure 16)..................................................................................................................................10F. Reflections (Figure 17).............................................................................................................................10G. Double-Line Pattern on Eye Diagram (Pattern-Dependent Jitter) (Figure 18)........................................11H. Asymmetrical Eye Diagram (Figure 21) ................................................................................................12

Customer Applications Maxim IntegratedApplication Note HFAN-2.0 (Rev. 0, 5/00) Page 1

Interfacing Maxim Laser Drivers with Laser Diodes

I. Overview

Interfacing laser driver circuits with commerciallyavailable laser diodes at high data rates can be acomplicated and frustrating task. This application noteis intended to briefly address this topic with the goalof providing a useful reference for optical systemdesigners that will simplify this process as much aspossible.

The three major pieces of the laser interface puzzleinclude (1) the output circuit of the laser driver, (2) theelectrical characteristics of the laser diode, and (3) theinterface between them (which is usually implementedusing a printed circuit board). In this application note,the characteristics of the laser diode and laser driverwill first be discussed individually, and then they willbe brought together in a discussion of the printedcircuit board interface. An optimization section is alsoincluded that illustrates possible solutions to a numberof common interface problems.

Although this application note is intended to begeneral in nature, specific examples will focus onMaxim’s 2.5 Gbps telecommunication laser driverssuch as the MAX3867 and the MAX3869.

II. Laser Diode Characteristics

In general, coherent optical output can only begenerated and maintained in a semiconductor laserdiode when the laser current is above its thresholdvalue. For fast switching operation, it is a commonpractice to bias the laser diode slightly above thethreshold to avoid turn-on and turn-off delay. Thelaser optical output depends on the driving currentamplitude and the current-to-light conversionefficiency or slope efficiency of the laser diode. Boththe threshold current and the slope efficiency arestrongly related to laser structure, fabrication process,fabrication materials, and operating temperature.

Figure 1 represents the voltage-current characteristicand the optical output-drive current relationship of atypical laser diode.

As the temperature increases, the threshold current(Ith) goes up in an amount exponentially proportionalto the working temperature (T), which can beestimated by

IT

T

Ith eKITI ⋅+= 0)( (1)

where I0, KI, and TI are laser-specific constants.Example constants for a DFB laser are I0 = 1.8mA, KI

= 3.85mA, and TI = 40°C.

The laser slope efficiency (S) is the ratio of the opticaloutput power (in milliwatts) to the input current (inmilliamperes). Increases in temperature cause theslope efficiency to decrease. The following equationprovides a good estimation of the slope efficiency as afunction of temperature:

ST

T

S eKSTS ⋅−= 0)( (2)

For the same DFB example laser as above, thecharacteristic temperature, TS, is close to 40°C and the

Figure 1. Laser current, forwardvoltage, and optical output

IIth

PO V

V

T1 T2

T1 < T2

Opt

ical

out

put

Laser current

For

war

d vo

ltag

e


other two laser constants are S0 = 0.485mW/mA andKS = 0.033mW/mA.

The relationship between the laser operating voltage(forward voltage, V) and the laser current (I) can bemodeled by the diode voltage-current characteristic

TV

V

S eII ⋅⋅≈ η (3)

where IS is the diode saturation current, VT is thethermal voltage, and η is a fabrication constant. Whenthe laser diode is driven close to and above threshold,the voltage-current relationship is approximatelylinear, as shown in Figure 1.

A simplified model of a laser diode is shown in Figure2. In this figure, the DC-offset voltage, VBG, isassociated with the bandgap voltage of the laser diodeand RL represents the dynamic resistance of the diode.When driving the laser above threshold, the opticaloutput of the laser diode, P0 (shown in Figure 2), canbe estimated by

)(0 thIISP −⋅= (4)

It is important to note that parasitic elements (i.e.,bonding wire inductance) should be considered whenmodeling a packaged laser.

III. Laser Driver Output Structure

The primary function of a laser driver is to provideappropriate currents for bias and modulation of thelaser diode (Figure 3). The bias is a constant current

that pushes the laser diode operating range beyond itsthreshold value and into the linear region. Modulationis an alternating current that is switched on and off insynchronization with the input voltage waveform.Ideally, the bias current should track the changes inthreshold current and the modulation current shouldtrack the changes in slope efficiency.

Figure 2. Simplified laser diode equivalent circuit

RL

VBG

I+

V

−

PO = S·(I-Ith)

(a) Functional diagram

*Figure illustrates DC-coupled bias current.

(b) Input-output characteristics

Figure 3. Laser driver functionality

VCC

I MO

D

I BIA

S*

RM

OD

RB

IAS

IIth

Opt

ical

out

put

PO

Laser current

IMODIBIAS*

*Figure illustrates DC-coupled bias current.

Customer ApplicationsApplication Note HFAN-2.0 (Rev. 0, 5/00)

Laser drivers such as the MAX3867 and theMAX3869 are designed to drive common-anode laserdiodes. The bias current can be set between aminimum value (typically 1-5mA) and the maximumvalue (typically 60-100mA) by using an externalresistor.

It is important to maintain a constant impedance at thecathode of the laser diode such that the load on thehigh-speed output circuit versus frequency will remainstable. An unstable load on the output circuit cancause reflections, ringing, etc., that will degrade thequality of the optical waveform. The shuntcapacitance associated with the biasing current sourceresults in an impedance (ZBIAS) that is a function offrequency. In order to minimize the effect of thisimpedance variation, an external isolation inductor (orferrite bead) is generally connected between thecathode of the laser diode and the bias circuit. Thisinductor has no effect on the DC bias current butappears as a high impedance to the modulationcurrent.

The magnitude of the modulation current isdetermined by external resistor RMOD (Figure 4). Thisresistor controls a current source associated with thedifferential output stage. The laser driver outputs areconnected to the collectors of the output stagetransistors. In most cases, pullup components usedwith the open-collector outputs (resistors or inductors)are external to the laser driver (pullup considerationsare discussed in the PC Board Interface section).

IV. The PC Board Interface

Today’s optical communication systems requireimprovement in operating speed, transmissiondistance, and power consumption. Implementation ofthese improvements calls for faster edge speeds,increased modulation current, and lower supplyvoltages. Therefore, the design of the laser driver andthe corresponding interface to the laser diode areimportant issues for high-speed fiber opticcommunications design.

A. Headroom Issues with DC-Coupling

DC-coupling between the driver and the laser providesa simple and straightforward interface solution, asillustrated in Figure 5. But when the power-supplyvoltage is decreased to +3.3V, the headroom for thedriver may not be enough to enable fast switching.(“Headroom” refers to the difference between the VCC

supply voltage and the sum of the individual voltagedrops along a single circuit path.) Headroomcalculations for circuits that contain a laser diode mustinclude voltage drops due to the laser itself, as well asthe transient voltage drop resulting from the parasiticinductance of the laser package and the voltage dropacross the damping resistor, RD.

Typicdiodes1.2V sum oacrossdiode of this

Figure 4. Typical Maxim laser drivermodulation output structure

OUT+

OUT-Laser driver

RM

OD

OUT+

BIAS

+3.3V

OUT-

RF

CF

RD

Laser driverILIMOD

IBIAS

LB

IMOD IBIAS

Figure 5. DC-coupled interface circuit

Maxim IntegratedPage 3

al long-wavelength Fabry-Perot-style laser require forward-bias voltages on the order ofto 1.8V. This forward-bias requirement is thef the bandgap voltage and the voltage drop the equivalent series resistance of the laser(see Figure 2). The equivalent series resistance type of laser is typically 4 ohms to 6 ohms.


The transient voltage drop is due to fast switchingcurrents across the parasitic inductance associatedwith the laser package. Its magnitude can be looselyapproximated by VL = L ∆i/∆t. If we assume a typicallaser package has a parasitic inductance of about1.5nH, a maximum modulation current of 60mA, anda 20% to 80% rise/fall time of 80ps (for 2.5Gbps),then we can calculate an approximate value for VL.(Note that ∆i during the 20% to 80% rise time isapproximately 60% of the total modulation current or0.6×60mA = 36mA.) Using the above assumedvalues, VL ≈ (1.5nH)(36mA/80ps) = 0.68V.

For an example headroom calculation (for the DC-coupled interface of Figure 5), consider a packagedlaser diode with a maximum forward voltage VF (=VBG + IRL in Figure 2) of 1.6V. We will also assume apackage parasitic inductance of 1.5nH, and a 60mAmodulation current with 20% to 80% edge-speed of80ps, resulting in VL = 0.68V (see precedingparagraph). Finally, we must include the voltage dropacross the series damping resistor RD, which is IMODRD

= 1.2V (assuming RD = 20Ω). The resulting voltage atthe driver output pin can be as low as VLOW = VCC –1.2V – 0.68V – 1.6V = VCC – 3.48V, making 3.3Voperation very difficult.

B. AC-Coupling

The headroom problem described above can beimproved by AC-coupling the driver to the laserdiode. This is accomplished by adding a seriescapacitor, CD, and pullup inductors, LP, as shown inFigure 6.

AC-coupling voltage drops are as follows: (1) The ACvoltage drop across the laser diode is only a functionof the voltage drop across its equivalent seriesresistance (not the bandgap), which is equal to themodulation current times the equivalent seriesresistance; (2) the transient voltage drop due toparasitic inductance remains unchanged from the DCexample; and (3) the voltage drop across the seriesdamping resistor, RD, is equal to one-half themodulation current times RD.

The last point in the preceding paragraph (voltagedrop across RD) can be understood by considering thecurrent through the AC-coupling capacitor, CD, andthe currents at the circuit node that includes thecathode of the laser diode (see Figures 6 and 7). Thecurrent through CD must have an average value of zeroand a total peak-to-peak current swing of IMOD. Tosatisfy these conditions, one-half of IMOD must flowinto CD (from the laser) during an optical high outputand one-half of IMOD must flow out of CD (toward thelaser) during an optical low output. The currentthrough the laser, IL, is then equal to the sum of thecurrents flowing out of the circuit node at the cathodeof the laser diode. This means that during an opticalhigh output IL = IBIAS + IMOD/2 and during an opticallow output IL = IBIAS – IMOD/2. The difference betweenIL during an optical high output and IL during anoptical low output is then (IBIAS + IMOD/2) – (IBIAS –IMOD/2) = IMOD.

The exampcoupled intecoupled chassume the 5Ω, and the

OUT+

BIAS

+3.3V

OUT-

RF

CF

RDCD

Laser driverILIMOD

Figure 6. AC-coupled interface circuit

IBIAS

I BIA

S

I MO

D

LPLP

LB

DC-C AC-C

VF

VLOW

IBIAS + IMOD

IBIAS

Figure 7. O(IL) for D

I MO

D

oupled

VOUT+

lerfaaneqre

L

MO

D

)

L

uC

I

headroom calculatce can now be modi

ges. For the AC-cuivalent series resistasulting headroom cal

I MO

D

tput voltage (VOUT+)- and AC-coupled in

VCC

IBIAS

V

VMOD = IMOD(RD + RL

Max

ion fiedoupncecula

andterf

oupled

VOUT+

i

ule ot

a

I

m IntegratedPage 4

for the DC-sing the AC-d case, wef the laser is

ion is VLOW =

laser currentce circuits


VCC – (60mA)(5Ω) – 0.68V – (60mA/2)(20Ω) = VCC –1.58V. For VCC = 3.3V, this leaves 1.72V ofheadroom for the driver, which enables fast currentswitching in the driver output stage.

The disadvantage of the AC-coupling approach is thatadditional discrete components are required. Theseadditional components include a coupling capacitorand pullup inductors or resistors for driver transistorbiasing (as shown in Figure 6). Because thesecomponents are in the high–speed signal path, theycan cause signal distortion. For this reason, the use ofgood high-frequency PC board layout techniques iscritical (this subject is addressed in more detail later).

The AC-coupling capacitor will introduce a low-frequency cutoff, which can impact the pattern-dependent jitter performance of the system. To reducethe pattern-dependent jitter caused by long strings ofconsecutive identical bits, the value of the AC-coupling capacitor should be as large as possible.Designs that include Maxim’s 2.5Gbps laser driversgenerally use an AC-coupling capacitance valuebetween 0.056µF and 0.1µF.

The pullup inductor or resistor is required in AC-coupled laser interfaces in order to keep the outputdriver transistor biased properly. (Small ferrite beadsare generally used for the inductive pullup.) Thedisadvantages of using a resistor (versus an inductor)in this application are as follows: (1) A resistivepullup (Rpullup) creates a current divider with the rest ofthe laser circuit, taking part of the modulation currentfrom the laser. This does not happen with an inductor.(2) Inductive pullups increase the headroom byallowing the average voltage at the output to be VCC

(as in Figure 7) instead of VCC – (IMOD/2)Rpullup, aswould occur with a resistive pullup.

C. Interfacing a Laser to a Driver

The specifics of the interconnection between thedriver and the laser diode depend on the separationdistance. If this distance is smaller than a couple ofmillimeters (for 2.5Gbps), transmission lines areunnecessary and the priority is to reduce the parasiticelements as much as possible. The inductive loadcaused by the lead and bond wire inductance of thelaser package may need a compensating RC shuntnetwork (see Figure 6), consisting of a resistor (RF)and a capacitor (CF). The purpose of the RC shunt

network is to cancel out the parasitic inductance,thereby maintaining a constant load impedance, whichresults in a reduction of overshoot and ringing. Theseries damping resistor (RD) serves the dual purposeof damping reflections (that cause output distortion)and creating a stable load. Load stability is improvedbecause the load presented by the laser can vary by±20% or more from the nominal value ofapproximately 5Ω (±1Ω/5Ω ≈ ±20%), whereas thecombined load presented by the laser and RD varies byonly ±4% (±1Ω/25Ω ≈ ±4%). For the packagedMAX3867 and MAX3869, initial values for thesecomponents, assuming a coaxial-style packaged laser,are RD = 20Ω, RF = 75Ω, and CF = 3pF. Becausepackage inductance varies for different lasers, thecomponent values for the shunt network may need tobe adjusted for optimum operation. Also, it isimportant to note that the bias inductor is connecteddirectly to the cathode of the laser (rather than theother end of RD) so that RD will not cause headroomproblems in the bias stage of the driver.

When the interconnection distance becomes greaterthan a couple of millimeters, an impedance-controlledtransmission line is required to interface the laser tothe driver (Figure 9). Figure 8 represents the crosssection of a microstrip line, which is commonly usedin printed circuit boards. The characteristicimpedance, Z0, is estimated by

+⋅⋅⋅

+≈

TW

HZ

r8.0

98.5ln

41.1

870 ε

(5)

where W is the width of the top conductor, T is thethickness of the top conductor, H is the thickness ofthe dielectric, and εr is the dielectric constant. For a25Ω microstrip line built on conventional FR4material (εr = 4.7) with a height (H) of 356µm, a linewidth of approximately 1.3mm results.

Figure 8. Cross section of microstrip line

W

H

T

Dielectric Material (εr)


Ground plane

OUT+

OUT-

Driverinterface

25Ω

RD = 20 ΩTop Conductor

A BC

DE

F

G

VCC

VCC

H

Figure 10. High-frequency signal path between a driver and a laser diode

OUT+

BIAS

Vcc

OUT- CD

Driver

25Ω ZL

Vcc

LaserModule

Figure 9. Interfacing a driver to a laser module with transmission line

RD


D. Additional Board Design Considerations

For high-speed differential drivers, it is important tomaintain a balance between the loads on the twooutputs. The loads must be balanced in both themagnitude and the phase of the load impedances(Figure 9). To maintain balanced load impedance,each output is set up to drive an equivalent 25Ω load;the positive output drives a transmission lineterminated on the load end to VCC through a matchingresistor/laser diode (a 25Ω combined load), and thenegative output of the driver is connected to VCC

through a 25Ω resistor. Decoupling capacitors provideAC shorts from the laser anode to ground and fromthe 25Ω resistor on the negative output to ground.

The high-frequency path can be divided into severalportions (refer to Figure 10): Portion A is from thedriver positive output through the top conductor of thetransmission line to the termination resistor; portion Bis the termination resistor and the laser diode; portionC is the decoupling capacitor to ground; portion D isfrom the decoupling capacitor ground contact to thetransmission line image; portion E is the transmissionline image on the ground plane; portion F is from thetransmission line image to the ground contact of theDC-coupling capacitor at the negative output; portionG is the decoupling capacitor at the negative output;and portion H is the resistor at the negative output.

Because the high-frequency return path includes theground plane, it is important to provide gooddecoupling from VCC to ground at both the positiveoutput and the negative output. The decoupling at thepositive output (C) ensures a return current (along thetransmission line image) for proper transmission lineoperation, and the decoupling at the negative output(G) enables a current return to the negative output. Itis always a good practice to use physically smallcapacitors for high-frequency performance in thesedecoupling elements. It is also important to maintainan uninterrupted ground connection between thesetwo capacitors (D, E, F).

It is desirable to keep portions B, D, F, and H (whichare not transmission lines) as short as possible toreduce the propagation delay between the two driveroutputs. This is because the propagation delay can betranslated into unbalanced phase termination betweenthe outputs of the differential pair. One way toimprove this situation is to select physically smalltermination resistors in order to keep the ground

contact of the decoupling capacitors as close to thetransmission line as possible. It is also necessary tominimize the length of the nontransmission lineportion of the printed circuit board at the driver outputpins.

V. Troubleshooting

Experience has shown that, despite the most carefuldesign of the interface between the laser driver and thelaser diode, the optical output is generally less thanoptimum when the system is powered up initially.This leads to the need for troubleshooting (or at leastminor adjustments) of the interface in almost all cases.This section is intended to give the engineer quicksolutions to the most common interface problems.

The following optimization guidelines are based onthe visual output of an oscilloscope connected via anoptical-to-electrical (O/E) converter to the output ofthe laser diode. The input to the laser driver isassumed to be a pseudo-random bit stream (PRBS).The oscilloscope output can be displayed as awaveform (when the oscilloscope is triggered by apattern clock) or as an eye diagram (when theoscilloscope is triggered by the bit clock).

The following are descriptions of eight common laserinterface problems, along with waveform illustrations,a list of possible causes, and suggestions for possiblesolutions. Refer to the interface circuit shown inFigure 11 for all troubleshooting examples.

OUT+

VCC

RF

CF

RDCD

Las

er d

rive

r

RPLP1

LP2 VCC

Figure 11. Interface circuit



A. Waveform Compression (Figure 12)

Problem Description: No clearly distinguishable eyediagram. Dark horizontal line at the lower extreme ofdisplayed eye diagram. Decreasing the bias currentcan compress the top of the displayed waveform tomove down on the oscilloscope display, but thebottom of the waveform is stationary.

Possible Causes: Bias current is set too low. Thedigital zero level is below the threshold of the laser.

Potential Solutions: Increase the laser bias currentuntil the bottom of the waveform begins to move upon the oscilloscope display (indicating that the digitalzero level has been raised above the laser threshold).The eye diagram should become clearlydistinguishable as the bias current is increased.

B. Relaxation Oscillation (Figure 13)

Problem Description: Large overshoot evident indisplayed waveform. Dark horizontal line at the lowerextreme of displayed eye diagram. Decreasing thebias current can cause the digital one level to movedown on the display, while the overshoot remainsconstant or even increases in amplitude. The bottomof the waveform (the digital zero level) remainsstationary as the bias current is decreased.

Possible Causes: Bias current is set too low. Thedigital zero level is below the threshold of the laser.Extra time is required to switch the laser from belowthreshold to a high level, causing a delayed risingedge. The delay in switching results in an increasedbuildup in potential that causes the laser to overshootthe digital one level once the threshold is overcome(known as “relaxation oscillation”).

Potential Solutions: Increase the laser bias currentuntil the bottom of the waveform begins to move upon the oscilloscope display (indicating that the digital

100 ps/div

100

µW/d

iv

Figure 12. Eye diagram showing waveform compression

100 ps/div(a)

1 ns/div(b)

50 µ

W/d

iv50

µW

/div

Figure 13. Relaxation oscillation(a) Eye diagram, (b) Waveform


zero level has been raised above the laser threshold).The overshoot should decrease significantly as thedigital zero level is increased past the threshold value.

C. Overshoot (Figure 14)

Problem Description: The rising edge of thewaveform overshoots the digital one level. Therelative amplitude of the overshoot remains almostconstant as the bias and modulation currents areadjusted. No noticeable ringing is evident.

Possible Causes: (a) Rising edges too fast; (b) ferritebeads (used as pullups) have excessively high Qfactor.

Potential Solutions: (a) Insert a low-pass filter with afrequency cutoff at 75% of the data rate. This willslow the rising and falling edges and decrease theovershoot. (b) Decrease the value of the resistor inparallel with the ferrite bead (RP in Figure 11) to dampout the Q. (c) Adjust the value of the series dampingresistor (RD in Figure 11).

D. Undershoot (Figure 15)

Problem Description: Rising and/or falling edges donot reach high or low level within the first half of theunit interval.

Possible Causes: Overdamped output circuit. InFigure 15, the undershoot was caused by a 0.5pFcapacitor that was placed between OUT+ and OUT-(this was done to damp out some ringing).

Potential Solutions: (a) Reduce capacitance betweenOUT+ and OUT-, if possible. (b) Reduce loadcapacitance on OUT+. (c) Reduce the value of theseries damping resistor (RD in Figure 11).

Overshoot

100

µW/d

iv

Figure 14. Eye diagram showing overshoot

100ps/div

Undershoot

200

µW/d

iv

Figure 15. Eye diagram showing undershoot

50 ps/div


E. Ringing (Figure 16)

Problem Description: Rising and/or falling edgesexhibit ringing relative to the correct levels in adamped oscillation pattern.

Possible Causes: Impedance discontinuities,excessive inductance in the circuit, resonance effectsof circuit components. In Figure 16, the ringing wascaused by removing the resistor in parallel with theferrite bead (RP in Figure 11).

Potential Solutions: (a) Eliminate impedancediscontinuities as much as possible. (b) Reduceparasitic inductance by decreasing the lead length onthe laser diode as much as possible. (c) Decrease thevalue of the resistor in parallel with the ferrite bead(RP in Figure 11). (d) The values of RF and CF (seeFigure 11) can be adjusted to compensate forparasitics of the laser package.

F. Reflections (Figure 17)

Problem Descriptiotransmission-line imappear as overshoot, distortions to the eye dthe problem is the resthe bit rate in order tooscilloscope, as in Figu

Possible Causes: Iminterface circuit.

Potential Solutions: between the laser driveas possible. (b) Ensutechniques are used itime-domain reflectomlocations of impedancthe PC board layocompensation networFigure 11) in order to oat the load end of the t

50 µ

W/d

iv 5

0 µW

/div

1 ns/div(a)

2 ns/div(b)

100

µW/d

iv10

0 µW

/div

Figure 16. Ringing on (a) Eye diagram,(b) Conventional waveform

100 ps/div (a)

200 ps/div (b)

Figure 17. Eye diagrams showing reflections at(a) 2.5 Gbps, (b) 800Mbps


n: Reflections due topedance discontinuities canundershoot, ringing, or otheriagram. One way to verify thatult of reflections is to decrease expand the time scale on there 17.

pedance discontinuities in the

(a) Make sure that the distancer and the laser diode is as shortre proper controlled-impedancen PC board layout. (c) Use a

eter (TDR) to help identifye discontinuities; then improveut. (d) Adjust values ofk components (RF and CF inbtain a better impedance match

ransmission line.

Customer Applications: 1-800-998-9872 Maxim Integrated Application Note HF-2 (Rev. 0, 12/99) Page 11

G. Double Line Pattern on Eye Diagram (Pattern Dependent Jitter) (Figure 18)

Problem Description: Portions of the eye diagramseparate into two distinct lines. The double line effectmay vary when the input data pattern is changed.

Possible Causes: Pattern Dependent Jitter (PDJ)results from wide variations in the number ofconsecutive bits contained in NRZ data streamsworking against the available bandwidth. There are anumber of conditions that can cause this effect.

The distortions in Figure 18 were caused byincreasing the modulation current until the outputtransistor began to saturate, limiting its high-speedswitching capabilities. The decreased switching speedlimits the bandwidth during the rising edge.

The location of the lower -3dB cutoff frequency isimportant, and must be set to pass the low frequenciesassociated with long consecutive bit streams (Figure19) in order to eliminate PDJ.

PDJ can also be present due to insufficient high-frequency bandwidth (Figure 20). If the amplifiers arenot fast enough to allow for complete transitionsduring single-bit patterns, or if the amplifier does notallow adequate settling time, high-frequency PDJ canresult.

Potential Solutions: (a) Increase the value of the ACcoupling capacitor (CD in Figure 11). (b) Increase thevalue of the series damping resistor (RD in Figure 11).(c) Increase VCC. (d) Decrease the modulationcurrent.

50 ps/div

100

µW/d

iv

Figure 18. Eye Diagram Showing Double LinePattern Figure 19. Pattern-Dependent Jitter Due to

Low-Frequency Cutoff

Figure 20. Pattern-Dependent Jitter Due toHigh-Frequency Rolloff

Customer Applications: 1-800-998-9872 Maxim Integrated Application Note HF-2 (Rev. 0, 12/99) Page 12

H. Asymmetrical Eye Diagram (Figure 21)

Problem Description: Rise time may be significantlydifferent from fall time (as in Figure 21) and/or thezero crossings of the eye diagram may be shiftedabove or below the midpoint (pulse-width distortionor PWD).

Possible Causes: Asymmetrical rise and fall timesmay be caused by differences in the current pathsduring the rise and fall times. (This happens becauseeach path may present unique charging/dischargingcharacteristics.) PWD results when the midpointcrossing of a 0–1 transition and a 1–0 transition do notoccur at the same level due to DC offsets (Figure 22).Unequal rising and falling edge speeds and DC offsetsboth contribute to asymmetrical eye diagrams.

Potential Solutions: (a) Eliminate distortionsinherent in input datastream (one way to do this is toclock and latch input data), (b) Use a laser with equalrise and fall times, (c) Adjust values of dampingresistors (RP and RD in Figure 11), (d) Slow the fasteredge down by using a low-pass filter with frequencycutoff at 75% of the data rate.

100 ps/div

100

µW/d

iv

Figure 21. Asymmetrical Eye DiagramFigure 22. Pulse-Width Distortion

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