Oscilloscope Basics: Multiplexed Front Panel Controls

Fig. 1. Modern,
slim front panel.

Most Teledyne LeCroy oscilloscopes are equipped with traditional front panel controls—knobs and buttons—that are a (literally) handy way to make basic acquisition settings such as gain, timebase and trigger level. While all these could be made using the oscilloscope software, using the front panel allows you to keep dialogs closed and more of the screen “real estate” available for viewing traces as you modify these settings.

In order to optimize that real estate, front panels have become increasingly slim, and many front panel controls on newer Teledyne  LeCroy oscilloscopes are multiplexed, meaning they have multiple functions or can be used to control multiple on-screen objects. Here is a list of tips to keep in mind when using the front panel.

Fundamentals of Power Integrity: Mutual Aggressors and Rail Transient Response Measurement

Fig 1. Rail droop in response to a load step is
a typical case of mutual aggressors in a PDN.

A third type of noise found in PDNs is what we call mutual aggressors, which is crosstalk coupling from one component of the PDN onto another.

An obvious example is a load step in the PDA, where something in the system being turned on pulls current from the VRM that supplies a rail. In Figure 1, you can see how the output voltage of the VRM supplying a 1 V rail droops in response to a load step before it recovers. This is still noise: it is a signal variation that we're not expecting and don't want.

We want to be able to characterize that noise, because too much droop could affect the operation of other components that are already consuming power from that device.

In order to do so, we’re going to measure the rail transient response to the load application. We need only look at two signals: the voltage and the current on the rail of interest. Figure 1 shows the voltage on C5 (the green trace) and the current on C8 (the orange trace).

Your Ground Bounce Questions Answered

Figure 1. Line set to "quiet low" shows ground
bounce occurring as I/O driver switches.

During an October 2020 webinar, Don’t Let Ground Bounce RuinYour Day, Dr. Eric Bogatin was asked several questions regarding his topic of presentation. Here are his answers.

Q: From what frequency should we consider ground bounce to be a problem?

A: Ground bounce is really due to a dI/dt. Generally, it becomes a problem with rise times shorter than 100 ns. The bandwidth of this is about 3.5 MHz. This means ground bounce can be an issue at relatively low frequency.