The Effects of Passive Probe Ground Leads

Figure 1: Teledyne LeCroy's PP108,
a representative passive probe

When you open the box containing your shiny new oscilloscope, one of the items you'll likely find inside is a set of basic 10:1 passive probes (Figure 1). Those probes have a ground lead that you'll want to use when you make measurements. Your probe has a bandwidth specification that's probably somewhere between a few hundred megahertz to 1 GHz; that spec was obtained at the factory with a specialized test jig having a specific ground inductance and source impedance. Now, the way in which you connect your ground lead can have a big impact on the real-world bandwidth and response of the probe.

Back to Basics: S-parameters

Figure 1: S-matrices for one-, two-,
and three-port RF networks

Suppose you have an optical lens of some sort onto which you shine a light with a known photonic output. While most of the incident light passes through the lens, some fraction of the light is reflected and some is absorbed (the behavior is also dependent on the wavelength of the incident light). You'd like to characterize that lens: Exactly how much light was reflected? How much passed through? What is it about the lens that prevented all of the light from passing through?

Is Your Testbench Mixed-Signal Ready?

Figure 1: A representative block diagram of
a mixed-signal embedded system

Mixed-signal design is ubiquitous these days, with hybrids of digital and analog circuitry turning up everywhere. A typical mixed-signal designer may be a hardware or software engineer with specific needs. They may be working with 4-bit, 8-bit, 16-bit, and 32-bit microcontrollers in a single embedded controller or across several embedded systems. They need to capture a host of different signal types and serial-data protocols and understand timing relationships between them. Then there's all the different sensor signals, power-supply signals, and PWM control signals to guarantee embedded system performance and reliability.

Applying Multi-Stage, Multi-Rate Digital Filtering

Figure 1: The input signal shows both the desired 63-kHz signal
along with a 60-Hz component. Zoom trace Z1 shows the
60-Hz component in detail.

A while back, we posted some basics on how to apply digital filters to sort out signals with undesirable elements riding on top of them, i.e. a square wave that's being corrupted by a sinusoidal signal creeping in from somewhere in your system design. Now, let's look at how to extend the range of cutoff frequencies for digital filters, allowing them to be used even more effectively.

Understanding Probe Calibration Methods (Part II)

Figure 1: Experimental setup for comparing a
source-referred signal to an unloaded signal

As many engineers and techs know, there's ample room for confusion when it comes to a proper understanding of oscilloscope probe loading. In an earlier post, we covered probe calibration methods and tried to sort out terms such as calibration, correction, compensation, and de-embedding.

Using Digital Filters

Figure 1: Using a band stop filter to remove a 5-MHz sine
wave from a 4-MHz square wave

In a perfect world, all of the signals we wish to view on our oscilloscopes would be as pure as the driven snow. However, in most scenarios, reality intervenes. As we know, everything in a system is an antenna, some being more efficient than others in terms of radiation and/or reception. Hey, crosstalk happens, and if you've got a sine wave and a square wave co-existing in the same general vicinity, it could make a true evaluation of the square wave difficult to do. But how do we get that nasty sine wave coming from one circuit to stop jumping on the  back of the square wave in another?

Tips and Tricks: Rescaling for Non-Voltage Measurements

Figure 1: Using math rescale to read a current probe's
output directly in Amperes

As we know, an oscilloscope is essentially an instrument that measures a constantly varying voltage as a function of time. Those measurements are subsequently analyzed to derive other properties of the input signal, such as frequency, amplitude, rise/fall times, and so on. But what about measurements of other electrical quantities as a function of time?