PCIe Electrical Testing: Where Are We?

Figure 1. PCIe specifications through Rev 5.0.

You may be wondering where we are in the roll out of PCI Express test specifications and active testing. Figure 1 shows the status as of March, 2020.

Six Ways Not to be Confused by S-parameters (Part II)

In Part I, we discussed three causes of confusion when working with S-parameters and what you can do to avoid them.  In this post, we’ll discuss three more ways not to be confused by S-parameters.

4. Know the difference between return loss and reflection coefficient

Figure 1. A 2-port device has two distinctly interesting
S-parameters, S11 and S21.  S11 is the reflection
coefficient of Port 1 (also called return loss), and
S21 is the transmission coefficient of Port 2
(also called insertion loss).

Let’s start by looking at a 2-port device illustrated in Figure 1.

There are two S-parameters of interest in a 2-port device.  The first is the reflection coefficient, S11, that measures the ratio of the reflection from Port 1 to the drive signal at that port.  The second is the transmission coefficient, S21, that is the ratio of the output of Port 2 to the drive signal into Port 1.  Confusion arises because historical measurements of return loss and insertion loss are often used interchangeably with reflection coefficient and transmission coefficient, respectively.

Six Ways Not to be Confused by S-parameters (Part I)

S-parameters describe the electrical properties of electronic interconnects, which can include connectors, printed circuit traces and vias, cables, and oscilloscope probes. Given that instruments such as Teledyne LeCroy’s WavePulser 40iX make measuring S-parameters relatively simple, there are still some aspects of S-parameters that can cause confusion, especially to new users. Here are six things you can do to avoid confusion when working with S-parameters.

1. Know where the fixture ends and the DUT begins

Figure 1. Characterizing a  microstrip line requires
two connectors, one at each end of the line. 
These connectors add their own characteristics to
the measurement. A TDR measurement can
determine the boundary between the connectors
and the microstrip PC line. 

Measuring S-parameters involves connecting a test instrument to the Device Under Test (DUT), placing the DUT in series with a number of other interconnect elements, as in Figure 1. The DUT is not isolated, and confusion can arise as to where you want the cables and fixtures to end and where the DUT begins. Do you want the DUT to include just the cable of the DUT? What about the connectors on its ends? What about the matching fixture or lead in in the circuit board?

Basics of Power Conversion: Power Semiconductors

Figure 1: The basic building blocks of power
conversion circuits are the power MOSFET and
the IGBT; shown are (left) an N-channel
enhancement-mode MOSFET and (right)
a P-channel (minority carrier) IGBT

There are a multitude of circumstances that make power conversion a necessity. Power conversion, of course, is the converting of electric power from one form to another, from one voltage to another, or one frequency to another; it also encompasses any/all combinations of these.

A Real-World FFT Example

Figure 1: Shown at top right is the output of a
5V switch-mode power supply acquired
with an RP4030 active voltage-rail probe

Performing a fast Fourier transfer (FFT) on an oscilloscope can be likened to driving a car. Just as there are two dominant strains of power-train transmissions, there are two dominant approaches to transferring signals acquired on an oscilloscope from the time domain to the frequency domain. There's the stickshift approach, in which the FFT parameters are set manually, and the automatic approach, in which you let the oscilloscope make decisions for you.

Fast Fourier Transforms: Automatic Edition

Figure 1: Shown is the user interface for Teledyne LeCroy's
Spectrum Analyzer software option

Many motorists love the experience of driving a sporty car with its convertible top down and a stickshift manual transmission. Others don't want the hassle of a clutch and prefer the user-friendly feel of a slick automatic that does some of the work for them. Oscilloscopes can provide the same sort of choice for many measurement tasks in the same instrument.

Fast Fourier Transforms: Stickshift Edition

Figure 1: Shown at left is a 50-kHz input sine wave
with the FFT of the same signal at right

Perhaps you're old enough to remember when more cars had stickshifts. They're a little bit more work to drive than cars with automatic transmissions, but the experience can be much more rewarding. Oscilloscopes these days are like cars with both types of transmissions, and you can use either one for many tasks. One of those tasks is fast Fourier transforms (FFTs), and in this post we'll take you through driving an oscilloscope to perform an FFT with a stickshift.