Operating Basics; Differential Measurements For Serial Data Compliance Testing - Tektronix P7313SMA Technical Reference

Operating Basics

This section discusses differential measurements using an SMA input probe for Serial Data compliance testing. It also
provides information on the probe architecture and operational details to aid in its proper application.

Differential Measurements for Serial Data Compliance Testing

Differential Signaling
Gigabit serial data signals are commonly transmitted using differential signaling techniques because of improved signal
fidelity and noise immunity. Although the physical layer specifications differ somewhat between the different gigabit serial
data communication standards, they have some common elements. Most gigabit serial data signals are transmitted over
50 Ω transmission lines, which are terminated at both ends of a point-to-point differential interconnect. The signal transmitter
provides a 50 Ω source impedance from each of its two differential outputs and the signal receiver provides an effective
50 Ω input impedance on each of its two differential inputs.
The two complementary single-ended signals that make up the differential signal are generally offset from ground at a
common-mode voltage level, which allows the use of unipolar transmitters and receivers that are powered from a single
power supply voltage. The transmitted signals are usually encoded using a DC-balanced encoding technique that allows the
signals to be either AC or DC coupled in the transmission path. If DC coupled, the receiver termination must generally be
terminated to the same DC common-mode voltage as the transmitter, to reduce DC loading on the transmitter output. An
example of the single-ended signals transmitted by an InfiniBand standard driver and the resultant differential signal that
would be measured by a differential measurement system is shown in Infiniband. (See Figure 14 on page 24.)
Although the differential response is generally the primary measurement of interest for a differential signal, full
characterization of the signal also requires measurement of the single-ended response of the two complementary signals
including the DC common-mode voltage.
Pseudo-Differential Measurements
A common differential measurement technique uses two single-ended probes or direct connection to two oscilloscope
channels for the differential signal capture. By calculating the difference between the two input signals using waveform math,
the effective differential signal seen by a differential receiver can be displayed for analysis.
This measurement technique, which is commonly refered to as pseudo-differential measurement, has a number of
limitations when compared to the use of a differential probe like the P7313SMA. In addition to the obvious overhead
of two oscilloscope channels for the measurement instead of the single channel needed by a differential probe, there are
a number of additional problems.
Unlike the differential probe, which has been carefully designed with short, matched-input signal paths, a pseudo-differential
measurement uses two oscilloscope channels, which are physically separated and generally not matched as well. Although it
is possible to deskew the timing differences between two high performance oscilloscope channels to improve the accuracy
of a pseudo-differential measurement, deskewing is a relatively involved procedure that may need to be repeated if any
oscilloscope parameter, such as vertical gain, is changed.
The gain match between two different oscilloscope channels is also a potential problem, particularly at higher frequencies
where channel gain mismatch can contribute to significantly reduced CMRR performance. The CMRR performance of a
differential probe, on the other hand, is generally much better controlled, with fully characterized specifications over the
full probe bandwidth.
P7313SMA Technical Reference
Operating Basics
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