VII. Transceiver Design
A successful transceiver design implies a well-designed printed circuit board
(PCB) layout, which is derived from a carefully designed circuit schematic. However,
once a basic circuit schematic is established, many critical portions of PCB design
emerge that are independent of the schematic, such as layout symmetry and minimized
trace length. Therefore, to optimize the design cycle for the first development run of the
transceiver, the schematic and printed circuit board designs were developed with a
parallel-dependant approach, allowing the independent portions to be developed
simultaneously and the dependant portions to be matched and verified incrementally.
Thus, the PCB design progressed and critical portions were optimized even as portions of
the schematic design remained in flux. Then, with the critical portions of the PCB
established, certain changes to the schematic resulted only in minor changes to the PCB
layout. The following sections outline in detail the progressive development of the
schematics and PCB layouts for the first transceiver design run.
i) Schematic Design
As noted above, basic circuit designs were first established for the transmitter
(TX) and receiver (RX) components. The TX component centers on the MAX3287 laser
driver, and the RX centers on the MAX3266 TIA and MAX3264 LA. These designs
were fashioned independently after the learning receiver board schematic and basic
transmitter schematic, respectively, provided in the class lecture slides. Initially, the
input SMA connector on the learning RX board was replaced with the photodiode (PD) in
the RX schematic, and the VCSEL feedback network, regarded as over-design, was
removed for simplicity from the TX schematic.
Development of these portions continued independently and in conjunction with
the optical link budget. Also, the respective MAXIM datasheets played a crucial role in
verification of the appropriate pin states and approximate biasing networks. Resistor and
potentiometer values for the VCSEL biasing and modulation networks, shown in Figure
6, were determined from the finalized optical link budget and included in the TX
schematic.
Figure 6. VCSEL modulation (left) and biasing (right) networks.
In Figure 6, the “PMOD” and “PBIAS” correspond to the potentiometers chosen
to accommodate the appropriate bias and modulation currents for the VCSEL as
mandated in the four-corner worst-case analysis (see Table 3B) of the optical link budget.
In addition, the “RBIAS” and “RMOD” represent the corresponding minimum resistance
values. This design also included discrete power supply filters consisting of a ferrite
bead and two capacitors, depicted in Figure 7, to ensure each of the three chips remained
isolated from high frequency noise on the power supply rail. The complete schematic for
the transceiver is shown in Figure 8.
Figure 7. The schematics of the power supply filter [12].
Figure 8. The finalized revision of the transceiver schematic.
ii) PCB Layout Design
The PCB layout was accomplished utilizing the ExpressPCB layout design
software because ExpressPCB was the chosen vendor to fabricate the printed circuit
boards for the transceiver design. The critical transmitter components, the VSCEL and
appropriate biasing and modulation networks, are shown in Figure 11. To ensure a
balanced differential signal at the VCSEL, the ground separation distance between the
VCSEL and its paired resistor was made as small as possible, as shown in Figure 11B.
Also, all three MAXIM chips on the transceiver layout were connected to a single power
supply and filtered as designed in the schematic. A closer look at an example of the
layout of one of the three necessary power supply filters is shown in the group of
components in the center of Figure 11A.
(A)
(B)
Figure 11. Power supply filter, biasing & modulation networks (A), and the laser diff.
lines (B)
Since the board was large enough to accommodate two transceiver layouts, two
designs were fashioned, as shown in Figure 12. To preserve a high-quality signal, both
designs were carefully inspected to ensure minimized length and maximized symmetry of
high-frequency trances. Also, to further reduce the possibility of degradation and noise
on the high-frequency signal paths, all corresponding wire traces were examined to
ensure joint angles were fashioned as near to 450 as could be determined in order to
prevent high-frequency signal reflection and subsequent degradation. Aside from a
single inductor to filter high-frequency noise, the ground planes for the transmitter and
receiver modules were separated and prevent a possible avenue for cross-talk.
Figure 12. Finalized PCB layout, with non-aggressive (left) and aggressive (right)
designs.
The most notable difference between the more and less aggressive designs is seen
in the spacing of the SMA connector pair, as depicted in Figure 13A, the less aggressive
design provided generous spacing between each connector pair while the more aggressive
design pulled them much more tightly together. In addition to this difference on the
electrical side of the transceiver module, the reason for pursuing the more aggressive
design was to satisfy the design constraint of separating the VCSEL and PD by no more
than 7/8 of an inch, as presented in Figure 13B. Moreover, in accordance with the design
requirements, the VCSEL and PD were placed at the edge of the board for both the
aggressive and non-aggressive designs.
(A)
(B)
Figure 13. Close-ups of the electrical non-aggressive (A-left) and aggressive (A-right), and the
optical non-aggressive (B-left) and aggressive (B-right) ends of the transceiver module.
The primary tradeoff posed by the aggressive design was the risk that the SMA
connector pairs would be placed too close together to be able to attach the corresponding
electrical cables. However, fabrication, construction and testing revealed that this was
not the case. Another drawback of moving the transmitter and receiver components
closer together was the slightly increased possibility of a cross-talk between the RX and
TX sides during simultaneous operation. However, this change also facilitated the
shortening of the wire trace lengths at the SMA connectors, as is apparent in Figure 13A.
This adjustment, theoretically, would more effectively preserve the state of the highfrequency signals along this path. Regardless of the tradeoffs, the opportunity to
incorporate both designs onto a single board served to mitigate the risks posed by the
more aggressive design.
Unfortunately, as shown in Figure 13B, the pin-out nature of selected photodiode
required that the sensitive high-frequency traces from its anode and cathode be drawn
across the PD footprint to properly insert the device into the board. It would have been
preferable if these traces could have been significantly shorted to prevent the possibility
that they would pick up RF from the transmitter, or elsewhere. Even so, no major issues
arose in the formal review of this finalized PCB design, and the layout was subsequently
submitted to ExpressPCB to fabricate the boards for the transceiver design run.
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“Fixes”
Receiver sensitivity
The group focused its efforts in resolving the attenuation-related issues (discussed
in the previous section) exclusively in the lab. Possible heat damage to the TIA chip,
which could lower the sensitivity of the receiver, was initially suspected be partially to
blame for the observed poor eye patterns of signals attenuated at 5dB. To verify this
hypothesis, a second board was constructed using the same PCB layout, and the
photodiode was removed from the original board and placed on new one. Also, the
responsivity of the Hamamatzu unconnectorized PD (0.47) was suspected to be causing
the poor output signal from the receiver. Therefore, the unconnectorized PD, with its
superior responsivity of (0.55??) was mounted on the original board to be tested with
attenuation. The two boards are shown in Figure A.
Figure A.
As shown in Figure B(a), initial attenuation tests using the GDS transmitter to the
board receiver had similar results with Board 2 as seen previously with Board 1, thus
showing that a damaged TIA was not the likely cause of the observed problems.
However, the follow-up test, which ensured that the fiber was firmly pushed into the
GDS transmitter, provided much more encouraging results with pattern PRBS7 at 5dB
attenuation, as shown in Figure B(b). This result gave rise to the suspicion that the
(a)
Figure B. Initial (a) and follow-up (b) RX tests with 5dB attenuation of PRBS7 on Board 2.
original attenuation test results might have been due a combination of bad connection at
the fiber and low output power from the GDS transmitter. To check this, the calibrated
PD was connected to the to the optical output of the GDS. The results of this
measurement are shown in Table A. Thus, it was concluded that with the optical fiber
Table A.
connection attenuation voltage reading optical power
loose
0dB
firm
5dB
0dB
5dB
116.17mV
0.364mW
2.17mV 0.00677mW
19.4mV 0.06053mW
firm connected to the GDS transmitter, an adequate signal could be obtained from the
constructed receiver at 5, or even 10dB, as shown below in Figure C. These attenuation
tests were performed with the transmitter disconnected from the electrical input to verify
that the results obtained were not modified by crosstalk signals induced from transmitter
activity.
Figure C. Receiver output of signal D21.5 at 10dB attenuation.
Crosstalk
When it was observed in the attenuated loopback test that the receiver produced
an eye even with the optical link disconnected, it was determined that significant crosstalk
existed between the transmitter and receiver modules. The initial efforts to reduce this
crosstalk involved shielding between the transmitter and receiver modules. The third,
blank printed circuit board was used as a makeshift shield, as shown in Figure D. Tests
Figure D.
were repeated with a 5dB attenuated signal from the GDS transmitter to the receiver, but
this time the transmitter on the PCB was electrically connected and operating to show
crosstalk effects. The output from the receiver, displayed in Figure E, shows a significant
difference between the cases where shielding was (b, d) and was not (a, c) for both
(a)
(b)
(c)
(d)
Figure E.
PRBS7 and D21.5. Thus, with a simple shielding technique, most of the crosstalk effects
were prevented. It was noted that non-shielded crosstalk was observed to be much
greater for the D21.5 pattern than they were for that of PRBS7. It was presumed that this
effect is due to the regularity of the D21.5 pattern and subsequent RF effects.
When the calibrated PD became available, the optical power output from the
designed transmitter was also measured. From this measurement it was determined that
transmitter was biased to drive a signal with an optical power about twice that of the
eyesafe design level. Thus, with the calibrated PD, we biased the VCSEL to produced an
output signal at approximately 1mW, as designed. Unfortunately, the results from the
loopback tests that followed were mixed. As shown in Figure F, at this optical power, the
5dB attenuated results were actually degraded further when the shield was introduced,
implying the expected tradeoff as reducing signal power to the PD makes it more
(a)
(b)
Figure F.
sensitive to other sources of noise and instability. However, at 10dB and 15dB
attenuation, considerable improvement was noted when shielding was used, though
neither shielded signal produces an adequate eye.
(a)
Figure G.
(b)
(a)
Figure H.
(b)