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.