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Figure 1. Block diagram of hybrid beamforming transmitter.
Figure 2. 1080 glass weave and a 3.5 mil wire
These changes have exposed a number
of microdefects in the signal path that
were of little consequence in previous
products running at lower data rates.
These microdefects include:
1. The parasitic capacitance of the
plated through holes required to
mount the connectors can introduce
substantial bandwidth degradation.
2. Crosstalk between transmit and
receive signals can be severe because
those signals that tolerate 38 dB of
loss at the receivers are far more
susceptible to interference from a
signal leaving a transmitter at full
amplitude.
3. The difference in travel time of the
two sides of a differential pair (skew)
induced by the irregularities in the
weave of the glass cloth required to
provide mechanical strength in the
PCB can cause a signal path to fail.
4. Signal loss along the data paths
is still an issue but, in most cases,
can be handled with the materials
currently available used to fabricate
PCBs and backplanes. However, as the
shift is made to 56 Gb/s and higher,
loss in the data path comes back into
the equation as a major issue.
Solving the first three
problems has met with varied
success.
The first problem (excess capacitance
in the plated through holes) has
been dealt with by using a technique
called back-drilling to remove the
excess capacitance of the connector
plated through holes that extend
below the layer in which the signal
traces are routed. In thick backplanes
this has resulted in very complex
manufacturing procedures to ensure
enough of the plated through hole
copper has been removed to achieve
proper performance while at the same
avoiding drilling so deep that the
connection is severed.
Figure 1 is a loss vs. frequency plot of
two 8” (20 cm) traces. The red trace
is routed near the bottom of the PCB
so that the signal travels nearly the
full length of the vias used to connect
to the trace. The blue trace is routed
near the top of the PCB so that the
signal travels only part of the way
along the vias leaving small parasitic
capacitors attached to each end of
the trace. Clearly, the bandwidth
of the blue trace has been severely
affected by these parasitic capacitors.
Back drilling removes this unwanted
parasitic with the risks noted earlier.
The second problem (excess crosstalk)
has been dealt with by routing the
signals farther and farther apart
from each other so this problem is
New-Tech Magazine Europe l 41