Transmission Lines for Oscilloscope Users, Part 2

Figure 1: A transmission line can be seen as
a series of "buckets" of capacitance charged
to a voltage by the signal as it "walks the line."

In Part 1, we experimented with the signal rise time measurement and saw that it appeared to increase substantially as the length of the interconnecting cable was increased. To understand why, we revisited some basic principles of signal integrity:

1. All interconnects are transmission lines. 
2. Signals are dynamic, and once launched, cannot be prevented from propagating down the transmission line.

Be the Signal

To illustrate the dynamic nature of signals, imagine a very simple, 1 ns long, 50 Ω impedance transmission line. As a 1 V signal is launched into the transmission line and propagates, at each step along the way it asks "What's the impedance of the environment?" at its leading edge. That is the instantaneous impedance, notated as Z. Impedance is always defined as the ratio of a voltage to a current. We know the voltage of this signal (1 V), but how do we find the current at the edge? 

Transmission Lines for Oscilloscope Users, Part 1

Figure 1: The rise time of the Cal signal seems to
increase significantly by increasing the length of the
interconnect cable. Is it true? Click image for details.

This post is the first of a series that will discuss what every oscilloscope user needs to know about transmission lines. It is going to introduce you to the absolutely most important signal integrity principles everybody needs to know when using an oscilloscope to measure signals with rise times shorter than 10 nanoseconds. After demonstrating some easily misinterpreted measurements, we’re going to look “under the hood” at what’s really happening to show you how it's all about the principles of transmission lines. Awhile back, Dr. Eric Bogatin offered a condensed version of What Every Oscilloscope User Needs to Know About Transmission Lines that summed up the key takeaways, but by revisiting “Transmission Lines 101” with us here, we’ll hopefully also show you a different way of thinking about your measurements.

9 Quick Fixes to Improve DDR Probing

Figure 1: Reversed Handsfree mounts and chip
clips help relieve strain on fragile solders.

Probing at DRAM pins as required by JEDEC can be challenging. Here are nine, simple ways to improve your DDR probing.

1. Use positioning tools to relieve strain on probe tips

The Handsfree probe holder included as an accessory with several Teledyne LeCroy probes, such as the WaveLink and DH Series probes, was originally designed to put weight on the probe tip to ensure a good contact. However, many DDR probing applications utilize solder-in (SI) tips, where the greater concern is to relieve strain on the tip so as to not disrupt the solder. It turns out that if you use the Handsfree in a “reverse mounted” orientation (Figure 1), it puts the amplifier in a perfect position to help relieve strain on probe tips.

Oscilloscope Basics: Stabilizing Waveform Display, Pt. 2

Figure 1: A 50 kHz low-pass filter eliminates a
93 kHz interfering signal from a 10 kHz signal (top two grids)
and a 50 kHz high-pass filter cleans up a 93 kHz signal
with an additive 10 kHz interfering signal (bottom two grids).
Click image to expand.

In Pt. 1, we discussed the fundamental cause of unstable waveform displays. In this post, we’ll discuss how to use signal conditioners and conditional triggering to help the oscilloscope ignore extraneous samples when determining where the acquisition trigger event actually occurs.

Coupling 

In the Setup section of the Trigger dialog, Trigger input sources can be conditioned using AC or DC coupling, high-pass filters (LFREJ for low-frequency reject) and low-pass filters (HFREJ for high-frequency reject). The frequency selective coupling paths are used to attenuate extraneous signals. The low-frequency reject inserts a 50 kHz high-pass filter in the trigger signal path, which is useful for eliminating low-frequency interference such as 60 Hz power mains signals. This low-frequency noise can cause erroneous triggers, resulting in an unstable display. The high-frequency reject inserts a 50 kHz low-pass filter. This coupling mode finds use in applications such as troubleshooting switch-mode power supplies, where it suppresses signals at the power supply switching frequency. Like any extraneous signal, high frequency pickup can leak into the input signal and cause trigger instability. Figure 1 provides examples of how the HFREJ and LFREJ coupling filters eliminate interfering signals from the trigger source.

Oscilloscope Basics: Stabilizing Waveform Display, Pt. 1

Figure 1: A free running oscilloscope starts each
acquisition at a different point on the waveform,
resulting in an unstable display.  A triggered oscilloscope
starts each acquisition at the same point on the
waveform, resulting in a stable display. 

An unsynchronized, unstable oscilloscope display is useless for making measurements, but proper triggering can synchronize the oscilloscope sample clock to specific waveform events so that the acquired waveforms appear stable on the display.  Let’s look at why signals can appear unstable and what to do about it.  

Oscilloscopes are sampling devices; they sample the incoming signal at a uniform rate.  The timing of a signal applied to the input of an oscilloscope is most probably asynchronous with the oscilloscope’s sampling clock.  If the oscilloscope timebase is allowed to run free—that is, not synchronized to the timing of the input signal—then each oscilloscope acquisition potentially begins at a different point on the input waveform, as shown in Figure 1.