Evaluation Engineering - 23

Now, we come to the latest generation
data communication solutions. Electrical
signaling is now up to 32 Gbps on a single
lane. Optical is being used widely-not
just for long distance communication,
but also for very short distances. On top
of this, new wireless standards such as
802.11ay are being created that provide
performance in the 20 Gbps to 100 Gbps
range. These are part of the 5G landscape
but are also being used to replace existing
WiFi infrastructure.
The ever-growing demand for moving
more data in shorter periods of time is
pushing developers to use higher data
rates and more sophisticated modulation

The Tektronix
MSO 73304DX Mixed
Signal Oscilloscope

schemes such as PAM, OFDM, QAM (e.g.
16 QAM, 64 QAM, 256 QAM), and combinations of these modulation schemes to
pack the data into the shortest possible
time to transmit. This drives demand
onto the oscilloscope for demodulation
capability for electrical, optical, and RF
signals.
With these very high bandwidth communication channels, the designers have
moved to dynamic tuning of the transmitter and receiver equalization. This

happens through a defined link training
process that might last on the order of
500 ms during system power-up. In this
process, hundreds of thousands-or
perhaps millions-of small packets are
transmitted back and forth between the
transmitter and receiver, as the two attempt to arrive at a "most optimal" channel equalization result.
Debugging problems in this process
can be very challenging, as the process
happens once at startup and then is over.
In addition, it is difficult to inspect the
million or so link training packets to
identify a problem. This has driven scope
vendors to provide automated tools that
can decode the link training protocol, remove the redundant data from the analysis cycle, and flag things that might be
indicating problems.
Today, the bleeding edge of data communication is the Tbps research work
that is being done primarily around coherent optical methods, which routinely
uses scope bandwidths in the range of 70
GHz. The developers need to demonstrate
methods that produce reliable communication at these data rates and verify that
BER and EVM demonstrate the targeted
quality of communication. These users
need coherent optical receivers with very
high bandwidth and low noise, as well as
a very high-bandwidth scope. The scope
becomes a proxy receiver for what will one
day be the final channel receiver.
There are many other applications that
are not specifically related to data communication, but require high-bandwidth
scopes. Some examples include automotive radar (60 GHz to 77 GHz), testing of
other types of radar systems, SatCom
testing and more. Historically, these have
often been tested using spectrum analyzers. However, some of these applications
now require an instantaneous acquisition
of a very wide band signal, and spectral
analysis software running on the scope
can perform the needed acquisition and
analysis.

What are users looking for in
high-end oscilloscopes?
In general, most users today want excellent signal fidelity. As the margins for
tests become smaller and smaller, the

user wants to know the scope is not consuming a large amount of that margin
which could result in a false failure of the
test on their system. They need enough
bandwidth to be able to faithfully represent the signal being tested, but not more
bandwidth than the signal contains as
unused bandwidth just means more
noise in the measurement. As data rates
increase, especially on multi-lane communication, the possibility of crosstalk
effects between lanes becomes higher.
As such, it is important to provide tools
that help identify the presence of crosstalk such as bounded uncorrelated jitter
measurements.
The workhorse of the "high-end"
scopes has historically been 8-bit digitizers. Higher resolution is generally a
good thing, but it is not achieved by just
changing the digitizer itself. The entire
front-end of the scope must also have the
noise performance, linearity, and dynamic
range to support the higher resolution
A/D. Effective Number of Bits (ENOB) is
the IEEE defined method to demonstrate
overall signal fidelity of an acquisition system (see spec IEEE-1057). Without a frontend that can support higher resolution,
the value of the higher resolution A/D is
lost. Currently, Tektronix is producing 12bit resolution scopes with bandwidth up
to 8 GHz.
Connectivity is always an important
element of working with oscilloscopes.
Conventional probes are available up to 33
GHz today. It is important that the probe
provide a faithful conveyance of the signal without significant degradation. For
example, Tektronix probes used at this
performance level are characterized during production and S-parameter data is
stored in the probe. When the probe is
connected to the scope, the scope applies
this S-parameter data as an automatic
correction to the signal, thereby providing
a calibrated measurement to the probe tip
and the most accurate representation of
the signal being acquired.
Mike Martin is the senior
marketing product manager
at Tektronix. He can be contacted at michael.a.martin@
tektronix.com
JUNE 2019 EVALUATIONENGINEERING.COM

http://www.EVALUATIONENGINEERING.COM

Evaluation Engineering

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