Is It OK to Use an External 50 Ohm Terminator?

Recently, a reader posed the question in the Comment field on Dr. Eric Bogatin's blog post, How to Choose Between the Oscilloscope's 50 Ohm Input and 1 MOhm Input:  "Is there any difference between using an external 50 Ohm terminator instead of the internal 50 Ohm termination on the oscilloscope--for example, using a RG58/RG174 cable?"

Eric answered:

"In principle, you can use the oscilloscope input set for 1 MOhm termination, then add an external 50 Ohm termination resistor on a BNC Tee connector, for example. This has the advantage that you can actually use any resistor for a load, or terminate signals with an RMS voltage larger than 5 V.

However, there are two problems with using this approach for high-speed signals with rise times shorter than 1 nsec, which require an oscilloscope with bandwidth larger than 1 GHz.

Oscilloscope Basics: Cal Out and Aux Out

Fig. 1: Cal Out and Aux Out 
provide many useful outputs.

Oscilloscopes are generally thought of in terms of the signals that are input to them, but even oscilloscopes that are not equipped with function/signal generators can usually output some useful test signals.

Nearly all oscilloscopes have a Cal Out (calibration output) terminal on the front. Most Teledyne LeCroy oscilloscopes also have an Aux Out (auxiliary output) connector on either the front or back, depending on  model. Both outputs provide configurable signals that can assist you to compensate probes and attenuators, test frequency response, trigger waveform acquisition and coordinate multiple test instruments.

What Happens When You Connect a USB-C Cable

The USB Type-C® connector is designed to be very simple for the user to use: you insert it in either orientation, and a multitude of services just “work”. Though simple to use, it is a complicated connector to program and test, with a very complex system of protocols behind it. There is USB power delivery (USB-PD) and multiple rates of USB data delivery from USB 2.0 through USB4®, specified by the USB Implementers Forum (USB-IF®). There are protocols other than USB, such as DisplayPort™, High-Definition Multimedia Interface (HDMI™), Peripheral Component Interconnect Express (PCIe®), Base-T Ethernet and Thunderbolt™. 

So, what actually happens when you connect a USB-C cable? To understand that, first let’s take a look at the signals and pin assignments in the USB-C connector receptacle (Figure 1).

Figure 1: The USB-C receptacle pin assignments showing the key signals used for device-to-device communications. Related pins have matching color overlays.

Four Essential Oscilloscope Network Security Practices

 Currently manufactured Teledyne LeCroy oscilloscopes utilize either the 64-bit Microsoft® Windows® 10 Professional or 32-bit Microsoft CE platforms to support the oscilloscope application. From a networking perspective, they are for all intents and purposes Personal Computers (PCs).

Using a commonly available computer operating system such as Microsoft Windows on a Teledyne LeCroy oscilloscope offers a multitude of advantages, such as the ability to link third-party software to oscilloscope operations and to connect to a wide variety of hardware. The downside of using a common operating system is the threat of malware. 

Malware (including but not limited to viruses, Trojan horses, worms, bots, keyloggers and spyware) can infect a PC via many paths. Examples include websites, USB memory sticks, emails and your local area network. Simply connecting an unprotected PC (i.e., unprotected Windows-based oscilloscope) to a “compromised” network is enough to infect the PC within seconds. Likewise, a compromised oscilloscope can infect an entire network.

Below, we list four practices Teledyne LeCroy strongly encourages all users to follow to minimize the risks that malware presents. Remember, the time you spend attending to oscilloscope network security is minimal compared to the cost of having to clean up an infected instrument. . .or network!

Oscilloscope Basics: External, Line and Fast Edge "Triggers"

An oscilloscope trigger synchronizes the oscilloscope timebase to the input signal so that the displayed trace is stable. In digital storage oscilloscopes, while the digitizer runs continuously converting analog voltage/current inputs to digital values, it is the trigger event that defines the “acquisition window,” marking the point where data is stored to acquisition memory, locking the signal data for display, measurement and further processing. 

Figure 1: The trigger setup showing the possible choices for the trigger source.

Triggers are set to fire based on the state of a trigger source waveform. What are commonly known as External, Line and Fast Edge "triggers" are not really different trigger types, per se, but alternative trigger sources.  Figure 1 shows the typical setup options for an Edge trigger, the most commonly used trigger type.  With Edge triggering, the oscilloscope is triggered when the source waveform crosses a user-defined threshold level and slope.  Usually, the source will be analog input channel C1-Cn. However, three other sources can be used to initiate an Edge trigger: an Ext(ernal) input, the Line (mains) power and, on some oscilloscopes, the built-in Fast Edge signal. 

Measuring Clock Jitter Sensitivity to Power Rail Noise, Pt. 2

Figure 1. 400 mVpp oscillation on the power trace
is due to 48 MHz clock noise.

In Part 1, we used a function generator to create a power source with a known perturbation. Seeing that the noise on the power rail and the clock period were synchronous when we observed both traces together using a WavePro HD oscilloscope, we knew that there was a clear relationship between the two to be further investigated. Now, we're ready to examine more closely how the clock jitter responds to voltage variations on the power rail.

Measuring Clock Jitter Sensitivity to Power Rail Noise, Pt. 1

Figure 1. Voltage variations on the power rail
shown in the same grid as the clock period track
(jitter track). These waveforms are the basis of
the clock jitter sensitivity measurement. The
inverse relationship between the jitter track and
the power trace shows that the clock
is sensitive to variations in rail voltage.

In a previous post, we described A Robust Method for Measuring Clock Jitter with Oscilloscopes as variation in a clock signal’s period. Clock jitter is characterized by the standard deviation (sdev) of the clock period measurement. The track function of the clock period sdev shows us the variations in jitter over time, synchronous with the waveform source. 

In this post and the next, we’ll show how to make use of the clock period track function to match jitter variations to possible sources of jitter, in particular to voltage variations on the clock power rail. The offset voltage of a function generator powers a clock signal source. By creating a known variation in the function generator output, we can match that to the resulting clock jitter to calculate the clock jitter sensitivity to rail voltage changes. A known clock jitter sensitivity value can help you predict how a design will respond to rail voltage changes.

Setting Up Your Oscilloscope for EFT Testing

Figure 1: The typical EFT test signal consists of
multiple exponential pulses arranged as pulse bursts.

A third type of electromagnetic compatibility (EMC) testing deals with how devices respond to electrical fast transients (EFT). EFTs are a series of fast, high frequency pulses, often occurring in bursts. These transient events are the result of electrical arcing. EFT pulse bursts occur when a power connection is made or broken, equipment is powered down or circuit breakers are switched. They also occur when inductive loads such as relays, switch contactors or heavy-duty motors produce bursts of narrow high-frequency transients on the power distribution system when de-energized.

The typical compound waveform used for EFT testing is shown in Figure 1.

Figure 2: Acquisition of two EFT bursts at
1.25 GS/s, zoomed to show several timing epochs.

The pulse waveform for EFT testing is defined by a risetime of 5 ns and a pulse width of 50 ns. These pulses are combined into bursts of 5 to 50 pulses spaced at from 10 to 100 µs (10 to 100 kHz). The bursts are typically spaced as much as 300 ms apart. Figure 2 shows an EFT test signal, with two EFT bursts captured by an oscilloscope on channel 2 sampled at 1.25 GS/s.

The two EFT pulse burst are shown in the upper left-hand trace (the two pink blocks). A series of zoom-on-zoom traces are opened to show several timing epochs. Trace Z2 (second from top left) shows a single burst. The zooms keep expanding the horizontal scale until finally at Z8 (bottom right) we see a single EFT pulse.

Following are six, important things to do to make sure you get the best EFT test measurements from your oscilloscope.

Setting Up Your Oscilloscope for Surge Testing

Figure 1: A typical EMC surge test waveform with
1.2 µs rise time and 50 µs half amplitude time.
Zoom trace Z1 shows the details of the rise time.
Click any image to expand it.

Electrical surges result from phenomena like lightning strikes and switching transients.  Electronic devices are subjected to simulated surges to confirm that they continue to operate properly following a surge, just as they are tested against electrostatic discharge. 

Surge pulses are similar to ESD pulses in that they have a very fast rise time, but the fall time is much, much slower. Figure 1 shows a typical surge pulse waveform. Surge testing involves similar measurements to those made for  ESD pulse testing—such as rise time, pulse width and max—but surge testing also requires some additional measurements, such as area under the pulse curve and transmitted charge. 

Figure 2: To verify the surge generator waveform, the
oscilloscope is connected to the generator through an attenuator.

As with ESD pulse tests, oscilloscopes are primarily used to “test the tester,” confirming the output of the surge generator. So, before a surge generator is hooked up to the device under test, it's required to hook it up to an oscilloscope and conduct a series of measurements to make sure the surge pulse is within specification. Figure 2 shows a typical surge test setup.

Following are three, important things to do to make sure you get the best surge measurements from your oscilloscope.

Setting Up Your Oscilloscope for ESD Pulse Testing

Figure 1: An ESD calibration test setup. 
The ESD gun discharges its waveform
into a properly attenuated current target. 

Electrostatic discharge (ESD) pulse tests are a type of conducted immunity testing done to confirm that a device can withstand a sudden transient electrostatic discharge. It is done by using an ESD gun to shoot a pulse of the required voltage at a device while testing that the DUT continues to operate properly. The ESD pulse shape simulates a person, carrying a static charge, touching a device. When their fingertip first touches the device, there is a leading edge with a high peak and fast decay, often visible in the real world as a spark flying from the fingertip. This is followed by a second edge due to the charge in the rest of the human body that propagates toward the fingertip with a time delay.

Oscilloscopes are most often used to “test the tester” in ESD pulse test setups, confirming that the pulse from the ESD gun is the right shape and meets the requirements of the standard to which the device is being tested. A typical calibration test setup is shown in Figure 1. The pulse from the ESD gun is fired directly into a current shunt target connected to the oscilloscope through an attenuator required to keep the signal within the limits of the oscilloscope’s 50 Ω input, which is used for this testing. Then, key parameters of the ESD pulse are measured per one of several standards, such as IEC 61000-4-2.  

ESD standards require a range of measurements. The most common are the initial edge 10% to 90% rise time, peak amplitude, pulse width, amplitude and current levels at specified times from the initial edge (e.g., T1 and T2), and time to half value. 

Following are four, important things to do to make sure you get the best ESD pulse measurements from your oscilloscope.

Oscilloscope Basics: When to Use Trend to Graph Oscilloscope Measurements

Figure 1: Applying the Trend operator to the
same input waveform illustrates how the
Trend is asynchronous to the input waveform.  

In a previous post, we described the characteristics of the Track math function and two key applications of using Tracks to graph oscilloscope measurement data: anomaly detection and waveform demodulation. In this post, we'll discuss the characteristics and uses of the Trend function.

To illustrate an important distinction between Tracks and Trends, the Trend math operator in Figure 1 is now applied to the same signal as was the Track in our previous post without first reacquiring the input waveform. 

Note that unlike a Track, the Trend is not time-synchronized to the input waveform. Only the order of events, and not the timing of events, is retained. The underlying shape of the Track may be displayed in the Trend because the same measurement values from a single acquisition are displayed in the same sequence—however, the timing information of when each of the values has occurred is not retained in the Trend. Therefore, unlike the Track, the Trend does not point to the location of an anomaly. Without time scaling, the Trend does not have the frequency information needed to demodulate an input waveform.

Oscilloscope Basics: When to Use Track to Graph Oscilloscope Measurements

Figure 1: Pulse Width Modulated waveform (yellow)
and Track math operator (blue),
where the X-axis scaling is identical for both.

Modern oscilloscopes contain many tools that can be used for analyzing data, including Track and Trend math functions. Both Tracks and Trends graphically display measurement results and locate anomalies. The main similarity between Tracks and Trends is that the Y-axis of both operators is the measurement parameter itself (for example, Pulse Width, Duty Cycle, Rise Time, Slew Rate, etc.). The main difference between the two math operators is their X-axis, in which the Track uses the identical X-axis and synchronous horizontal scaling as the input waveform, whereas the Trend uses units of chronology.

Use Tracks for Anomaly Detection

The Track provides valuable debugging information by directly pointing to an area of interest. 

Notice the negative-going spike in the Track waveform in Figure 1. Figure 1 occurs at the point in time where the input waveform reaches its most narrow pulse width, and the Track instantly finds it, indicating when one measurement deviates from the others in the graph. The Track identifies the exact location in time where the narrowest or widest pulse width has occurred, and fully describes the measurement changes occurring throughout the entire waveform. Since oscilloscopes can acquire thousands or even millions of waveform edges within a single acquisition, the Track allows an engineer to quickly "find the needle in a haystack".

PCI Express 4.0 Error Detection Using a BERT and Oscilloscope

Figure 1: BERT and Oscilloscope connection
for PCI Express 4.0 error detection.

The PCI Express® 4.0 standard specification requires an oscilloscope with at least 25 GHz analog bandwidth and a BERT which can test bit rates of at least 16 Gbps. The BERT provides a known input pattern to the PCIe® device under test (DUT), and the DUT is instructed to regenerate the identical bit pattern while placed in loopback mode. Since a BERT can output a signal when a bit error is detected, this signal can be input to the oscilloscope to trigger a synchronized capture when an error occurs. By combining the capabilities of these two instruments, a powerful combination of real-time error detection and characterization can emerge.

Connecting the Instruments

In Figure 1, a known pattern is connected from the BERT PPG D1 output to the PCIe DUT input via a 2.92 mm K-type cable. The DUT attempts to regenerate the same data pattern, while the DUT output is routed to both the BERT error detector input and the oscilloscope channel 1 via 2.92 mm K-type tables and a power splitter. An error-free reference signal is connected between the BERT D2 output to the oscilloscope channel 2, along with the error trigger signal from the error detector output to the oscilloscope channel 3. An oscilloscope Edge trigger is set on the rising edge of the error detector output signal on C3.

When You Need a TDR and When You Need a WavePulser

Figure 1: S-parameters and TDR responses are two means to
the same end, characterizing interconnects.

Oscilloscopes are used to measure signals, usually as voltages vs. time, and signals come from active devices. But interconnects are passive structures that don’t produce their own signals. To characterize interconnects, you need a stimulus-response system.

There are two, principal types of stimulus-response systems used to characterize interconnects: Vector Network Analyzers (VNAs) used to measure S-parameters in the frequency domain, and Time Domain Reflectometers used to measure impulse responses in the time domain. Each uses a different type of incident signal and a different formalism, but as long as the interconnect is linear, passive and time invariant, both S-parameters and impulse responses yield the same information content in different formats and can be translated from one into another.

So, which do you need? We’ll look briefly at what each does and what are the criteria that might require you to have one versus the other.

Configuring Dynamic Oscilloscope Measurements Using Advanced Customization

Figure 1. The Advanced Customization option
lets you seamlessly and continuously update the
input of one parameter with the output of another.

With the installation of the Advanced Customization (XDEV) option on Teledyne LeCroy MAUI® oscilloscopes, you can create a measurement parameter whose input value is dynamically updated with each new trigger by the output value of another measurement parameter. All that is required is three simple lines of VBScript. 

To demonstrate, we’ll use the example of taking the x@max value measured on each acquisition and using it to dynamically populate the X position used by the measurement parameter lvl@x. However, these principles could be applied to any two parameters that share a logical/mathematical relationship, or to a parameter and a math function (for example, to use the output of a parameter as the multiplier for a Rescale function).

A Robust Method for Measuring Clock Jitter with Oscilloscopes

Figure 1. Clock jitter measured as a variation
of clock signal absolute period.

Clock jitter is the variation of a clock signal’s frequency or period. Either measurement carries the same information, but the period measurement is a simple time interval measurement easily performed using a real-time oscilloscope. If we have a robust way of measuring clock jitter, we have the basis for measuring the clock signal’s sensitivity to other features in the environment that can affect the period. Voltage noise on the power rail is just one external force that can affect clock jitter, which we'll show you how to measure in a future post.

In this post, we’ll demonstrate a robust method for measuring clock jitter using an example from Dr. Eric Bogatin’s webinar, “The Impact of Power Rail Noise on Clock Jitter.”  

The clock in our examples is a 5-stage ring oscillator which generates a square wave signal between 10 and 66 MHz. The test instrument is a WavePro HD 12-bit, 4-Ch, 8 GHz, 20 GS/s, 5 Gpts oscilloscope with 60 fs sample clock jitter.

In the process, we make a series of oscilloscope sample clock tests and timebase adjustments as  consistency checks. While measuring jitter is less about absolute accuracy than about the relative precision of measuring the time interval from cycle to cycle, a fundamental part of that is ensuring the absolute accuracy of the oscilloscope’s timebase.

9 Important Things to Know When Making Sensitive Measurements with Oscilloscopes

We've routinely posted on how you can characterize your total measurement system to gain important "situational awareness" when using an oscilloscope to make sensitive measurements. The knowledge gained from these tests helps you properly interpret your measurement results so that you can deduce what is actually going on with your circuit, versus what is an artifact of the measurement system. Listed here are nine important things you should know before making sensitive measurements with your oscilloscope, with links to blog posts that instruct you how to test them.

Transmission Lines for Oscilloscope Users, Part 4

Figure 1: Characteristic waveform when the source
impedance is lower than the cable impedance.

In Part 3, we saw the pattern of reflections that occur when both the source impedance and the oscilloscope input impedance are higher than that of the interconnect, and how those reflections affected the rise time measurement. Now let’s briefly consider what happens when the source impedance is lower than the impedance of the connecting cable. 

For this example, the source voltage is a 3.3 V square wave and the source impedance is 9 Ω. As before, our transmission line is a 50 Ω coaxial cable connecting the source to the oscilloscope. If the oscilloscope input termination is set to 1 MΩ, we see the interesting waveform shown in Figure 1.

Transmission Lines for Oscilloscope Users, Part 3

Figure 1: The Thevenin equivalent circuit model can be used
to characterize a voltage source with respect to the 
interconnect cable and oscilloscope input termination.

In Part 2, we demonstrated how to calculate the instantaneous impedance of a transmission line. However, any measurement made using an oscilloscope should consider not only the transmission line, but the source, the transmission line and the oscilloscope as a system. Therefore, characterizing your source, as well as knowing the effects of your oscilloscope input impedance, is important to developing the “situational awareness” needed to interpret measurements properly.

Two terms needed for us to characterize the source are the Thevenin source voltage and the Thevenin source resistance. Once we know these, we have all the pieces we need to fully understand what is happening with our measurements. This is true whether the signal source is a Cal terminal or a device-under-test.

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.