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application note 1 of 11 www.xicor.com october, 2000 an 140 1 gb/s fiber optic transmitter design using xicor digitally controlled potentiometers (xdcps?). joe ciancio, product development engineer, xicor inc. rex niven, design engineer, forty trout electronics pty. ltd. abstract laser diode (ld) controller/driver ic's at gigabit data- rates typically use specially designed chipsets. in many cases numerous control parameters are required to be set, using resistors which de?ne reference or control currents used by the ld driver circuit. precise and dependable settings are essential in order to achieve a maximized extinction ratio and minimize jitter. xicor digitally controlled potentiometers (xdcps) allow automated setting of these resistor values in a manner which is rapid, stable, repeatable and precise. in addition, xicor dcps offer many other integrated features such as eeprom and trip alarms. introduction optical ?bre communication systems often use semicon- ductor light sources such as a laser diode (ld) or light emitting diode (led). both have the advantage that the digital modulation at logic circuit voltages can be directly applied to the operating bias current of the diode, which in turn varies the output power. laser diodes have advantages for communication systems, since they provide high coupled output power and routinely have high cutoff frequencies which allow for data rates in excess of 10gb/s. such high rates, call for purpose-designed circuits to control laser diode currents. a challenge for control circuits is the knee or threshold effect, below which optical output is minimal as shown in figure 1. many such chipsets for laser diode control exist, most sharing a similar architecture. this comprises a tran- sistor which sets a bias current, another transistor providing extra current to modulate the laser diode into the 1 (maximum power) condition, and a common- emitter differential transistor pair switch. the function of this switch is to divert the modulation current away from the laser diode when a 0 is required. the differential figure 1. idealized light output vs control current light output modulation current design value of light output for a 1 design value of light output for a 0 threshold (i th ) i (ma) l (dbm)
2 of 11 an 140 application note www.xicor.com october, 2000 pair can be directly controlled from an ecl or pecl input (figure 2). the 1 level is often set near the maximum safe level of laser diode power output, while the 0 is set close to the threshold current (i th ), since this allows higher data rates. in practice a small amount light is expected at the 0 level since the "knee" at the threshold of the light versus current (l vs. i) characteristic curve is somewhat rounded. laser diodes exhibit a very wide range of tolerances of their various parameters, and this requires a number of reference values to be adapted to suit the individual component. in addition these values vary signi?cantly with temperature. the bias current (i b ) is controlled by a feedback loop from the laser diodes monitor diode (refer to figure 3). the monitor photo-diode and feedback loop cannot "track" the optical signal at full data rate. therefore, the bias feedback loop is designed such that it maintains an average optical power, which is in?uenced by the rela- tive number of data 1s and 0s. the number of data 1s and 0s may be made consistent by employing a line code such as simple manchester coding, or more ef?cient, omit "8b/10b" coding - as used in gbic (?ber channel) transceiver modules. as previously mentioned, at high data rates the modula- tion current (i mod ) cannot have feedback control, and must be set at a constant level. this therefore imposes the requirement that the modulation current must be desensitized omit, to the effects of temperature ?uctua- tions and device ageing. in the schematic shown in figure 3, the modulation current is set by choosing the appropriate value of r modset . an example of the effect of temperature variation, is to reduce the gradient of the light versus current (l vs. i) characteristic curve of the laser diode. this is reffered to as reduced slope ef?ciency . the lower slope ef?ciency has the potential effect of making the laser diode in 0 condition transmit signi?cant light power, while also reducing the power of the optical 1 level (figure 4). figure 2. laser diode driver block diagram i mod i bias laser diode complementary ecl compatible logic lines data source optical fiber figure 3. sy88922 laser diode driver and sy88905 laser diode controller block diagram i mod i bias laser diode complementary logic lines data source optical fiber r modset i biasset r biasset monitor photo-diode r pinset external voltage reference i md i pinset current comparator error amplifier (current error x10) i biasfb vcc gnd modulation current mirror bias current mirror modulation current switch amplification factor = x40 amplification factor = x40 external voltage reference external voltage reference
3 of 11 an 140 application note www.xicor.com october, 2000 this approaching of the two levels reduces the extinction ratio (the 0 optical power level as a fraction of the 1 optical power level), which has a detremental effect on the theoretical bit error rate (ber) by reducing the size of the "eye digram" opening (figure 5) [1}. to achieve a ber smaller than 1 x 10 -10 , requires the 1 signal level to be at least 12.5 s (standard deviations) greater than the received noise. should the 0 level rise to approach one standard noise deviation, the ber will deteriorate markedly. therefore, the extinction ratio must be maximized for optimum results, although this must be balanced against the need for the zero level to be as far above threshold as possible to avoid bit-pattern dependent jitter [2]. this is achieved by reliable, and precise setting of the various control circuit parameters. these control parameters are usually currents, which are set / programmed by adjustable resistors. xicor digitally controlled potentiometers in laser diode driver control ideally the resistors used to program / set the laser diode driver circuit parameters, should have the properties of : ? have high resolution ? allow adjustment over a wide range of resistance ? be ready immediately after power-up ? have low temperature coef?cient ? be immune to vibration and shocks ? introduce minimal noise, both internally generated and coupled from the exterior ? have low "wiper" resistance ? be very stable with age and other environmental conditions ? be tamper proof ? allow readjustment many times (at circuit set up time, for maintenance, or regular checkups) ? permit a given setting to be reproduced with high accuracy ? have a small footprint for use in small form-factor tranceiver modules figure 4. effect of reduced slope ef?ciency due to laser diode temperature current modulation current mean value of monitor photo- diode output. at 0 ?c current increased modulation current at 65 ?c lower slope efficiency results in smaller extinction ration (0 and 1 levels to approach each other - exaggerated). light output i (ma) l (dbm) l (dbm) i (ma) light output figure 5. bit error rate as a function of signal to noise 10 -5 10 -6 10 -7 10 -8 10 -9 10 -10 2 3 4 5 6 7 8 n n = ratio of (mean ?1? e mean ?0?) signal to (one standard deviation of noise ?0? + one standard of deviation of noise ?1?). ber 0.5 1 erf n () C 1.414 ------------------------ - = ber
4 of 11 an 140 application note www.xicor.com october, 2000 ? avoid out-of-range values which may overdrive (and possibly damage) critical components such as the laser diode. ? readily allow automatic adjustment under digital / computer control ? allow multiple units in one package ? be integrated with other ancillary functions ? require no additional power supplies other than those available for other system functions ? be non-volatile (does not require power to retain resistor settings) ? allow a link to documentation of settings, with time/ date stamp, serial number and operator data the xicor digitally controlled potentiometer (xdcp) products offer all these features, and most will be shown in the example design below. laser diode driver circuit design in this circuit design example, the laser diode is controlled by the micrel-synergy chipset sy88922 and sy88905. this chipset uses pecl compatible data signals, and reference voltages (less control circuit input bias) of 1.0v. the laser diode used is a vertical cavity surface emitting laser (vcsel) with monitor diode, type honeywell hfe4380-321. the complete schematic for this laser diode driver circuit is shown in the appendix. the vcsel has some favorable characteristics compared to the more traditional plane cavity (edge- emitter) type laser diodes. for example, a vcsel has the properties of ? lower threshold current ? less in?uence of temperature on threshold current, with a minimum near room temperature [3] ? critically-damped relaxation oscillation ? only one longitudinal mode, giving narrow spectral width [4] some important characteristics of the hfe4380-321 are summarized in table 1 and table 2. some precautions to be observed with vcsels are: ? the zero point should be as far as possible above threshold to prevent very long tails of the falling edges and bit-pattern dependent jitter [5]. ? the ?bre coupling and ?bre should not favour one polarization or transverse mode as this may result in pulse distortion such as large overshoot or undershoot at the rising edge and zero bounce after the falling edge [6]. ? optical re?ections from the ?bre to the laser should be minimized (as for edge emitters). selecting resistor values in setting the various control circuit parameters, allow- ance must be made for maximum and minimum values, table 1: key parameters of vcsel hfe4380-321 min. typ. max. units optical power output 0.2 0.35 0.8 mw threshold - 3.5 6 ma slope ef?ciency 0.02 0.04 0.1 mw/ma monitor current (at 0.35mwoptical) 0.07 0.275 ma series resistance 15 25 50 w forward voltage 1.6 1.8 2.2 v table 2: temp. dependency of vcsel hfe4380-321 min. typ. max units threshold [1] 1. the threshold varies parabolically with temperature having a minimum at mid range of temperature (around +30c). -1 +1 ma (0 to 70c) slope ef?ciency -0.4 %/c monitor current +0.2 %/c
5 of 11 an 140 application note www.xicor.com october, 2000 as shown in table 3. the calculated resistor values are shown in table 4. an initial design decision is that the laser diode will be operated at the nominal value of 0.35mw. also, opera- tion is assumed to be at 25c. to calculate the range of resistor values required, some basic design equations are required: (we assume zero error current in nominal conditions with normal data signal being transmitted) with the laser diode giving the optical power output "p 1 " at a 1 level, and with an equal number of 1s and 0s, the monitor current will be 50% of the full-power value. v inm = input voltage of current mirror ? bias = bias current mirror gain p 1 = nominal optical output power of a 1 h = slope ef?ciency ? mod = modulation current mirror gain each resistor (r6, r7, r8) depends upon three or more parameters, some of which can have tolerances as high as (+100% / -50%). for example, a simple calculation of minimum and maximum values for r modset reveals a potential range of values of 10:1. however, common-sense engineering would dictate that absolute minimum or maximum possible values repre- sent the combination of the "three-sigma" (3 s ) limit of every component - an extremely unlikely scenario, or gross overdesign! an approach which acknowledges the statistical nature of product parameter distributions would use a root-mean-square method, where the square root of the sum of the squares of the possible variation (in relative terms) is calculated. even the three sigma limit could be considered overkill, since a two-sigma threshold still gives 95% of products within the design range. this approach assumes a normal or gaussian distribution for each parameter. for components which are selected from a population with a much wider distribution, this may not be valid. for parameters which vary over a 4:1 range, this method is obviously only approximate. for the laser diode selected however, the distribution of threshold is indeed close to gaussian form [7]. temperature effects the potential temperature variation of the circuit must be known. using the known laser diode and control circuit temperature sensitivity (see table 2), the range of the error ampli?er gain (used to correct for changes in necessary bias current) should be checked. table 3: parameters of sy88922 & sy88905 chipset min. typ. max. units d v pinset 0.95 1.13 1.35 v d v biasset 0.8 1.0 1.2 v ? bias 28 37 44 a/a ? mod 30 38 44 a/a table 4: range of calculated resistor values min. -3 s typ. -3 s max. units r pinset (r7) 6.9 7.2 14.4 29 38.6 k w r biasset (r8) 4.5 6.9 10.6 16 21.1 k w r modset (r6) 1.4 2.0 4.3 9.5 15.1 k w 1 i pin d v pinset r pinset ------------------------ - ? ?? = 2 d v pinset v pinset v inm C = v inm input voltage of current mirror = i pin monitor photo-diode current = 3 4 i bias b bias d v biasset r biasset ------------------------------------------ - ? ?? i th = d v biasset v biasset v inm C = 5 p 1 hb mod ------------------ d v modset r modset --------------------------- - ? ?? =
6 of 11 an 140 application note www.xicor.com october, 2000 a rise in temperature causes the slope ef?ciency to drop (-0.4%/c), normally causing the 0 level to rise. however, the increase in monitor current (+0.2%/c) will partly compensate for this effect. other resistors a resistor (r5) in series with the complementary tran- sistor in the current switch is used to approximately match the conduction conditions of the other transistor. it is also useful since it allows an oscilloscope to check the drive current waveform which should be similar to that of the laser diode. applying xicor digitally controlled potentiometers given the wide range of variables, it is desired to set the three resistor values (r6, r7, r8) using a dynamic "test- in-circuit" technique. to allow the digital automation of this process and ensure that the resistor settings remain stable, the design uses xicor digitally controlled potentiometers (xdcps) as variable resistors. these devices are ideally suited to the "test-in-circuit" technique of setup, for ?ber optic transmitter designs. the x9258 from xicor contains four dcps, each with 256 taps (steps) of resolution. this device is available with end-to-end resistances of 100 k w or 50 k w. the block diagram in figure 6 shows the main features of the dcp. the wiper / tap position is changed using a "make-before-break" scheme in which the new tap posi- tion switch closes before the previous opens. this method avoids open-loop behaviour of the bias control circuit, in which the bias could be driven to dangerous levels. an industry standard 2- wire serial data bus is used to set the "wiper position" of the dcp. each internal potenti- ometer has a digital "address" to allow it to be individually commanded, and several devices may also be individually addressed on one common 2-wire serial bus. the write protect feature of the device provides improved integrity to the settings, and also prevents tampering. system design considerations as we can see, the xicor dcps are ideally suited to the task of setting the control parameters: r pinset , r modset, and r biasset of the laser diode control chipset. some designers may opt to use a ?xed resistor (with a value slightly less than the minimum calculated in table 4) in series with the dcp. this removes the potentially disastrous consequence of a programming error causing a high value of current to be selected. the resistor varies linearly with "tap position", and the steps of current for a single tap change is much greater when the tap number (and the resistance) is low (refer to figure 7). when the tap number changes from 1 to 0, the series ?xed resistor is all that limits the current. allow- ance must also be made for the wiper resistance, which decoder resistor array r h fet switches r l r w 0 1 2 n wiper register counter non memory volatile wiper 2-wire serial interface 2 figure 6. block diagram of digitally controlled potentiometer
7 of 11 an 140 application note www.xicor.com october, 2000 effectively adds to the digitally-controlled resistance value. the wiper resistance is typically 150 w . to achieve the high resolution of set current, two potentiometers may be used in series. in the design example shown in the appendix, the two ends of the potentiometer have been looped back to give a parabolic curve of resistance versus tap position. near the center tap position, a very ?ne resolution may be achieved (figure 8). the importance of maximizing resolution is that it allows the power levels to be set as closely as possible to the design values. consider for example a system with a range of twelve equal resistor choices. in this case the bias for the 0 level can be set only in increments of 1ma, from 2.5 ma to 7.5 ma. in a worst case scenario, the 0 level could be set nearly 1 ma above the correct value, which could give a 0 level of 0.1 mw. this reduces the extinction ratio to less than 5 db. the use of a resistor with at least 128 taps gives a resolution of 0.1 ma, giving much ?ner increments of optical power, and consequently greater extinction ratio. in the above example two potentiometers are used in series. unlike ?xed resistors, only select resistance values are available for dcps. if the values available prove to be unworkable, the circuit in figure 9 may be used. in this case, the dcp is used in true potentiometer mode (voltage divider), with an analog voltage buffer and a ?xed resistor. in our design example using the sy88922 however, this arrangement would reduce the effect of the temperature compensation built into the reference voltage. it should be noted that attempting to increase the optical power to near maximum (say 800 m w) will require around 10 ma of modulation current. at this level the feedback loop has only limited range (+/- 2ma) to control the output power. transmitter setup procedure now that we have examined how xicor dcps may be used to set the various control parameters of laser diode driver, we shall now examine the setup procedure for the completed circuit (see appendix). the basic algorithm for setting up the circuit is as follows: figure 7. resistor current as a function of tap position resistor current with constant voltage 0 max tap position 50k w 100k w note : series fixed resistor may be used to limit current. i (ma) figure 8. resistance vs. tap position for bias circuit resistance as a percentage of total resistor value 0 max tap position 25% r (%) r l r w non volatile memory wiper counter register figure 9. block diagram of dcp with buffer gnd voltage reference - + to current input 2-wire serial interface rw r h r l
8 of 11 an 140 application note www.xicor.com october, 2000 ? the three resistors r6, r7 and r8 (corresponding to the parameters r biasset, r modset and r pinset ) should be set to their maximum possible values calculated above. ? the feedback from the error ampli?er should be opened, and an optical power meter connected to the optical ?bre output. a more detailed description of the set-up procedure is described below: r biasset with a zero on the data input lines, the value of r biasset should be reduced from its maximum value of 17 k w until the desired optical power value of a 0 is achieved (around 0.035mw in this example). r modset the data is changed to a 1, and the value of r modset adjusted until the desired optical power value of a 1 is achieved (around 0.35mw in this example). r pinset a bit pattern with the same line code as the application is applied to the data inputs, with a data rate of at least 25mb/s [8]. the feedback control loop can now be closed [j2]. the mean optical power should fall to a much lower level as the feedback loop tries to match the optical power to the low i pinset value. the value of r pinset is then adjusted until the optical power meter con?rms the correct value is reached. with a line code yielding a 1:1 ratio (1 vs. 0), this will be 0.18mw. in the design example an independent op-amp 18 on 7301 also monitors the monitor photo-diode current, and gives a voltage at the test connector for future reference. finally, a test at the design data rate should be made to optimize the value of r biasset (i.e. the "0" level) for jitter and extinction ratio. the example "eye-diagrams" in figure 10 demonstrate these points. circuit test results using the circuit shown in the appendix and following the set-up procedure detailed above, the oscilloscope trace in figure 11 shows the actual received signal. figure 10. effect on eye diagram of bias ideal bias point : bias point too low : jitter caused due to bias point being set too low. the ?nal "1" in the pattern "101" rises more quickly than "1001" or "10001".the slow falling edge reduces extinction ratio. figure 11. oscilloscope trace of received signal at 1gb/s
9 of 11 an 140 application note www.xicor.com october, 2000 the test conditions / test set-up used for obtaining this trace were as follows: ? 62.5 m m optic ?ber, 3m in length. ? newport d70xr optical detector, with a rise time of 70ps. ? 1gb/s data rate with a "101010" pattern. no low-pass optical ?lter (after the detector) was used. xicor development tools xicor provides a complete programming development tool (xicor part number: xlabview01 ) based on the labview? platform. this development package combines the required soft- ware graphical user interface (gui) and computer interface hardware, and enables the designer to quickly and easily control any of the devices in the xicor digi- tally controlled potentiometer (xdcp) family. furthermore, the development tool provides the low level labview? software driver required to manipulate all xicor dcp products. in the case that a designer utilizes labview in their existing manufacturing envi- ronment, these low level drivers may be freely incorporated into "set-on-test" programs. the xicor dcp labview? development tool software may be downloaded fee of charge, from the xicor web site at the address http://www.xicor.com .
10 of 11 an 140 application note www.xicor.com october, 2000 references [1] saleh and teich, "fundamentals of photonics", wiley 1991, pp 905-906 [2] honeywell optoelectronics application sheet, "modulating vcsels", jim tatum and jim guenter, p.11 [3] honeywell optoelectronics application sheet, "modulating vcsels", jim tatum and jim guenter, ?gure 2 [4] honeywell optoelectronics application sheet, "vcsel optical characteristics", p1. [5] honeywell optoelectronics application sheet, "modulating vcsels", jim tatum and jim guenter , ?gure 9 [6] honeywell optoelectronics application sheet, "high speed characteristics of vcsels", jim tatum et al, ?gures 8, 9 &10 [7] honeywell optoelectronics, honeywell vcsel manufacturability", 8 aug 2000, p.1 [8] honeywell optoelectronics application sheet, "modulating vcsels", jim tatum and jim guenter , ?gure 6 bibliography ? xicor x9258 data sheet ? xicor an137 application note ? micrel-synergy sy88922 data sheet ? micrel-synergy sy88905 data sheet sites of interest ? xicor inc. (http://www.xicor.com) ? micrel-synergy (http://www.micrel.com) xicor, inc., the xicor logo, e2pot, xdcp, xbga, autostore, direct write cell, concurrent read-write, pass, mps, pushpot, block lock, identiprom, e2key, x24c16, secureflash, and serialflash are all trademarks or registered trademarks of xicor, inc. all other brand and product names menti oned herein are used for identi?cation purposes only,
11 of 11 an 140 application note www.xicor.com october, 2000 appendix 1 - schematic diagram of laser diode driver 1 2 3 4 5 6 78 a b c d 8 7 6 5 4 3 2 1 d c b a title number revision size a3 date: 31-jan-2001 sheet of file: c:\rexpcbs\xicor\xicorld4.sch drawn by: xicor gbic terminal din 2 en 1 out 8 din 3 din din en out 9 vcc 10 gnd 6 rset 5 vref 4 gnd 7 sy88902 x40 u5 sy88902 ak 2 k 1 a 3 vcsel monitor diode ld1 hfe4380-321 r5c 47r r7 r5b 47r r5a 47r c17 0.1uf r8 c20 1nf c21 0.1uf r6 c22 1nf c10 0.1uf c12 0.1uf c13 1nf c14a 1nf r1 82r r2 82r r3 130r r4 130r vcc vcc vin 1 gnd 2 vout 3 u3 7805 + c6 4.7uf tant c9 0.1uf vdd c15 1nf c16 0.1uf vcc vcc vcc high speed differential signal vin 2 gnd 1 vout 3 u1 7905 + c4 4.7uf tant c7 0.1uf + c3 1000uf/16v + c1 1000uf/16v vee vee vee vin 1 gnd 2 vout 3 u2 7805 + c5 4.7uf tant c8 0.1uf + c2 1000uf/16v vcc r10 500r r14 10k r12 560k r11 10k monitor r13 560k monitor 2 1 j3 coax 2 1 j2 coax r16 47k r17 47k r18 47k r19 47k vcc losout 2 ibias 4 vcc 5 ipinset 7 gnd 3 sy88905 losin 1 ibias set 9 ibias fb 8 vref 10 md 6 u4 sy88905 1 2 3 4 5 6 7 8 9 10 j1 con10 r20 47r r21 47r r22 47r r23 47r pecl pecl pecl high vcc-0.880 to vcc-1.165 = 4.00v low vcc-1.810 to vcc-1.475 = 3.35v pecl r28 91r r29 390r r30 390r c18 100nf c19 100nf vcc 7 a1 14 a2 11 a3 24 sda 13 scl 23 a0 2 wp 12 vl0 8 vh0 9 vl1 15 vw1 17 vh1 16 vl2 22 vw2 20 vh2 21 vw0 10 vl3 5 vh3 4 vw3 3 gnd 18 nc 1 100k 100k 100k 100k vpp 19 resistors all 100k with 256 taps u7 x9258 vcc 7 a1 14 a2 11 a3 24 sda 13 scl 23 a0 2 wp 12 vl0 8 vh0 9 vl1 15 vw1 17 vh1 16 vl2 22 vw2 20 vh2 21 vw0 10 vl3 5 vh3 4 vw3 3 gnd 18 nc 1 100k 100k 100k 100k vpp 19 resistors all 100k with 256 taps u6 x9258_soic 1 1 3 3 5 5 2 2 4 4 vcc ve e u8 7301 vcc vee 1 2 j2 con2 vcc mount at ld anode directly to groundplane c14b 10nf

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