Download 10G Linear TIA in Long-reach Multi-mode Applications

10G Linear TIA in Long-reach Multi-mode Applications
By: Senior Application Engineer Lian Zhao, Validation Test Engineer Ariel Nachum, and
Vice President of Technology Loi Nguyen; Inphi Corporation
In enterprise networks, short haul multi-mode fibers (MMF) suffer from multi-path modal dispersion
where different modes of light arrive at the receiver at different times. Increasing capacity requirements and the
demand for high performance have required nearly 99% of the market to upgrade from legacy MMF running at
1 Gb/s to serial 10 Gb/s 220m. Since replacing the legacy OM1 fiber is too expensive, the installed base of
legacy MMFs is severely bandwidth limited due to modal dispersion making it extremely difficult to achieve
220m at 10 Gb/s.
While LX4 initially was deployed using 4x3.125Gb/s, today long-range multi-mode (LRM) at 10 Gb/s
is used for installed legacy fibers, requiring less than half the LX4 solution and significantly lower power
consumption. LRM uses a complex technology – Electrical Dispersion Compensation (EDC) – to compensate
for optical dispersion in the electrical domain. For EDC to compensate effectively for dispersion caused by the
MMF, a linear transimpedence amplifier (TIA) is required. This guarantees that the distorted optical signal is
amplified linearly instead of becoming saturated prior to reaching the EDC.
For LRM applications, the performance of the EDC is critical and for a reliable optical-link
performance with feed-forward equalizer/decision feedback equalizer (FFE/DFE), EDC is the most common
architecture. Adaptive analog technology and Digital Signal Processors are two methodologies that are
commonly used to enable FFE/DFE. Equally important for LRM, is the performance of the TIA. A linear TIA
not only has all the normal limiting TIA performance, like small sensitivity and overload, but it includes unique
linear functions, like Total Harmonic Distortion (THD), Waveform Distortion Penalty (WDP) and Relative
Noise (RN).
Linear TIA unique parameters
Before linear TIAs, transimpedence amplifiers were limited both in linearity and range. The linear
range of a TIA depends on its transimpedence (Zt). The upper 3 dB bandwidth (BW) measurement, for
example, falls within the linear range. A measurement of -20 dBm, available on the latest TIAs, with the Zt of
10k, is considered small signal BW.
For linear TIAs, using an automatic gain control (AGC) circuit to make the linear range from
sensitivity range to overload range is preferred. AGC is an adaptive system where the average output signal
level is fed back to adjust the gain to an appropriate level for a range of input signal levels. Linear TIAs should
remain linear from small signals (sensitivity level) to large signals (overload level). Figures 1 and 2 show the
BW data using the same TIA at different input optical power levels.
Figure 1: 1348TA BW with –20 dBm
Figure 2: 1348TA BW with –6 dBm
These figures show that the BW differs using different input power even when the BW is measured
with the same device. Both the AGC circuit time constant vs. network analyzer sweep time set and the AGC
circuit frequency dependency can cause these significant peaks.
For some TIAs, the AGC time constant can be adjusted using external capacitance. With the
capacitance range of 270 pF to 1 nF, the AGC time constant can be measured from 1-3 μs. The network
analyzer sweep time can be adjusted to be as slow as possible. In this example, the sweep time was 1 second
allowing the AGC time to be neglected. From the initial test results, no difference in the BW measurement is
evident as the sweep time changes. Clearly, the AGC time does not cause the large BW signal peaking.
With frequency dependency, the test results are better when the TIA die uses an electrical signal v of
an optical signal. For an optical signal, the optical sine wave performance must be checked and adjusted using
laser bias current and modulation current to guarantee the laser works in the linear range. For the electrical
signal, the sine wave signal can be added directly to the TIA input using bias T. The results show that in the
overload range, the output amplitude remains constant when the input signal is 1GHz and 2GHz sine wave. The
output amplitude increases as the input signal changes to 4G and 5G at the overload current. These results
indicate that the AGC circuit is frequency dependent, and match the BW curve in Figure 2, which starts to ramp
after 2G.
The following setup (Figure 3) is used to measure the linear TIA BW with AGC circuit on. Two
wavelength signals are used; the 1310 nm at 100M sine wave frequency to turn on the AGC circuit and the
1550nm optical signal to measure the linear TIA BW using the sweep process.
To verify that the set up provides for the BW measurement with AGC turned on, it is important to first
verify the 1310 nm signal at 100M changes the TIA gain. With AGC circuit on, the BW is measured with the –
20 dBm 1550 nm optical signal from the HP8703.
Bias and
Temp control
100M signal
as modulation
Agilent 8703
AC mod
DC bias
T
1310nm DML
1550nm high loss
1310 less loss
ATT
10/90 optical
spliter
ROSA
1550nm -20dBm
1310nm adjustable
Figure 3: ROSA BW measurement with AGC circuit on.
Output vs. input at 100M sine w ave
op t i c a l i n pu t po we r
Figure 4: 1348TA output vs. input at 100M sine wave
First the 1550 nm laser source is blocked, the TIA input is changed to get different TIA output, and the
TIA AGC performance is verified. Figure 4 shows the 10G linear TIA working properly with the 100M sine
wave under different inputs. Figures 5 and 6 show the BW measurement using the above methodology in the
same TIA at different input powers.
Figure 5: BW with input –20dBm
Figure 6: BW with input –6dBm
The methodology for the large signal BW is useful when checking the linear TIA BW with AGC
circuit on. With the Inphi ROSA test, the BW for both the large and small signals is consistent. In the
following BW refers to the small signal BW.
Total Harmonic Dispersion (THD)
To measure the linear TIA THD, a linear 1310 nm laser source is used modulated with a 1 GHz sine
wave. The laser bias and modulation can be adjusted to obtain appropriate signals. The following (Figure 7) is
used to test the laser linearity. An EDFA may be needed to provide enough peak-to-peak overload input optical
power, and the spectrum analyzer must be monitored to guarantee that only a 1 GHz sine wave signal is present
without additional harmonics of the fundamental frequency. The procedure is the same if 2 GHz and 3 GHz
sine wave signals are needed to modulate the laser.
Signal
generator
modulation
Laser
source
Optical
amplif er
orEDFA
New
Focus PD
Spectrum
Analyzer
bias andT EC
TEC and
DC BIAS
Figure 7: Laser linearity Check Setup
After selecting the proper bias current and modulation current, the NEW FOCUS PD is replaced with
the ROSA under test. From the spectrum analyzer, the fundamental signal (1 GHz), 2nd order harmonic (2 GHz)
and 3rd order harmonic (3 GHz) signal power as P1, P2 and P3 are read (Table1). THD is calculated using the
following: THD=sqrt(P2+P3)/sqrt(P1).
Table 1: Typical THD data for 1348TA
Ipp(mA) THD
P1(dBm) P2(dBm) P3(dBm) denominator nominator
0.55 2.86E+00
-11.5
-45.83
0.44 2.59E+00
-11.17
-46
0.28 2.32E+00
-11.5
-46.5
-45 0.008413951
-45.83
0.000240301
0.00873977
0.000226364
-48 0.008413951
0.000195541
Waveform Distortion Penalty (WDP) and Relative Noise (RN)
IEEE 802.3aq defined three different stressed optical signals as precursor pulse, post cursor pulse, and
split symmetric pulse. These parameters and related PIE-D values (a metric used to assess EDC capability to
equalize the link) are used to test the link performance using the linear TIA.
WDP is a deterministic dispersion penalty due to a particular transmitter with reference emulated
multimode fibers and receivers. WDPi is the dispersion penalty of the TP3 test signal, and WDPo is the
dispersion penalty measured at the ROSA output. The distortion contributed by the ROSA is determined by the
following: dWDP= WDPo-WDPi Circadiant Hydra test system is a popular stressor generator used by LRM IC,
module and system vendors, and ensures that difficult stressors are consistent in the semiconductor food chain.
Figure 8 is a typical WDP measurement set up.
According to IEEE802.3aq the simulated fiber stressor are set to 4.1 dB, 3.9 dB and 4.2 dB for precursor, symmetric and post-cursor, respectively. It is better to measure the optical source WDPi for different
stressors separately for improved accuracy.
optical
Circadiant Hydra
TestEquity
Temp
Chamber
current
meter
ROSA
Synchronization
trigger in
ICS
Tech
electrical
Agilent DCAj
Scope
GBIP
Figure 8: Circadiant Hydra WDP set up
VPD
power
WDP for a linear TIA is similar to TWDP for the transmitter. By setting the test signal pattern PRBS9,
the optical power to –6.5 dBm, and the extinction ratio to 3.5 dB, the Hydra optical source WDPi can be
calibrated using different stressors, and the ROSA WDPo can be measured (Table 2).
Table 2: 1348TA symmetric dWDP over temp and power supply
-40C
WDP0
dWDP
2.97
4.108
3.3
3.63
25C
WDP0
dWDP
0.343
4.135
0.37
3.93
0.165
4.045
3.791
0.026
3.968
85C WDP0
dWDP
WDPi
4.015
0.25
3.765
0.28
4.091
0.326
3.765
0.203
4.04
0.275
3.765
RN is the reciprocal of SNR for a signal. The test set up is similar to WDP, when the Hydra test
system is being used (Figure 8), although it can be tested using a scope following the instructions given in
SFF8431 for manual RN calculations. Where RNi is the relative noise of the test signal characterized using O/E
curve, the relative noise contribution of the module is calculated using the following: dRN=sqrt(RNo^2-RNi^2).
Table 3: 1348TA dRN over temp and power supply
temp
-5C
25C
85C
Vcc
2.97
3.3
3.63
2.97
3.3
3.63
2.97
3.3
3.63
RN0
0.024
0.025
0.027
0.024
0.025
0.026
0.024
0.024
0.026
RNi
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
0.020
dRN
0.014
0.015
0.017
0.013
0.015
0.016
0.013
0.014
0.016
Linear TIA 1348TA BW relation with dWDP and dRN
Noise performance and bandwidth are affected by PD capacitance and bond wire inductance on TIA
input node. Therefore, the same TIA assembled by different firms using different photo detectors can differ in
performance. Selecting two different ROSAs, ROSA1 and ROSA2 results in different bandwidths (Figures 9
and 10).
Figure 9: ROSA1 with high BW
Figure 10: ROSA2 with low BW
The different ROSA BWs affect the ROSA’s dWDP and dRN data. Table 4 shows the trend with
using Inphi linear TIA 1348TA.
Table 4: 1348TA ROSA BW, dWDP and dRN
BW
dWDP
dRN
ROSA1
8.2
0.4
0.03
ROSA2
7.3
0.7
0.02
In this section, each of these parameters is unique for the linear TIA 1348TA, not for any TIA+EDC
combined performance.
Linear TIAs with different vendor EDCs
The most important test in LRM applications is the optical comprehensive stressed test. Optical
comprehensive stressed receiver sensitivity and overload can be done using a Circadiant Hydra where an
electrical signal is created using a specified pattern and impaired by:
1.
Gaussian low pass filter
2.
Gaussian white noise source
3.
ISI (pre-cursor, post-cursor and split symmetric-cursor)
The resulting electrical signal is converted to an optical signal using a linear electrical/optical
converter; the optical attenuator is connected before testing the linear ROSA and EDC for comprehensive
stressed receiver sensitivity and overload.
Recently, FFE/DFE (feed-forward equalizer/decision-feedback equalizer)-based EDC techniques for
ISI (Intersymbol Interference) mitigation in optical networks have been commercialized. Today, SiGe process
based EDC is in production stage while power consumption and price motivated IC companies to design CMOS
process based EDC. EDC integrated with CDR and EDC integrated Serdes target LRM markets with different
packages, like SFP+ and next generation X2.
ROSA1 and ROSA2 samples were tested using four different EDC available from different vendors.
The most difficult split symmetric stressor is used for comparison. Figure 11 shows the typical stressor test set
up.
optical
Circadiant Hydra
TestEquity
Temp
Chamber
current
meter
VPD
power
ROSA
electrical
Reference Clock
electrical
EDC+CDR
Figure 11: LRM stressor test set up
The block diagram (Figure 11) EDC+CDR is very general; some EDC and CDR are separate
components, some are EDC+CDR integrated, and some are EDC+Serdes integrated with only a loop back
circuit used. Table 5 shows the results of these tests at nominal conditions with 3.3V power supply for TIA and
PD at room temperature.
Table 5: Test results at nominal conditions
A
B
C
ROSA1
-10.8
-10.2
ROSA2
-10.7 error floor
D
-9.5
-6.5
-10.5
-11
Figures 12 and 13 show the typical stressed test of 1348TA ROSA2 with vendor A symmetric stressed
sensitivity and overload results.
Figure 12: 1348TA ROSA2+EDCA sym sensitivity
Figure 13: 1348TA ROSA2+EDCA sym overload
Table 5 shows that different BW ROSAs work differently with diverse EDC vendors even though the same
TIA is used internally. For split symmetric stressed tests, EDC vendor A and C work well with low BW ROSA and
high BW. Vendor B works better with high BW ROSA, while vendor D is partial to low BW ROSA.
The methodology used to implement these FFE/DFE-based equalizers can be divided into analog and
digital. Theoretically EDC by using adaptive analog methods can tolerate more ROSA noise and higher
bandwidth ROSA. EDC using the DSP method prefers ROSA low noise and needs low bandwidth. With more
solid EDC design using both analog and digital, the comprehensive stressed test results should be very similar
when using different BW ROSAs. The Table 6 shows the updated EDCs with different BW of 1348TA
ROSAs.
Table 6: Tests at nominal condition for 1348TA and PD with updated EDC.
A
B
C
ROSA1
-10.1
-9.5
-10.2
ROSA2
-10.0
-9.3
-10.0
Conclusion
Newer linear TIAs deliver the same or better performance as CMOS based EDCs. Linear TIA +EDC
can be worked not only in LRM multimode fiber applications, but also in single mode fiber applications like
metro and long haul.