Checking Cable Performance with VNA


Reading time ( words)

In my December 2013 column Comparing Cable Shields,we showed that poor cable shields can result in significant noise pickup from the air, which can easily mask a few mV of noise voltage that we need to measure on a good power distribution rail. We showed a quick comparison of cable shield quality with a signal source and an oscilloscope. In this column, we will look at the same cables in the frequency domain, using a pocket-size vector network analyzer (VNA).

Vector network analyzers are similar to time domain reflectometry (TDR) instruments that many digital engineers may be more familiar with: They both transmit a known signal into the device under test (DUT) and measure the response. TDR instruments use a step waveform with a given rise time; VNAs use a sine wave source sweeping the frequency within a user-defined range. VNAs have long been popular in microwave engineering and more recently in high-speed digital engineering. They measure what are called scattering (S) parameters, which are the complex ratios of transmitted and reflected waves.

In recent years, small, low-cost, portable VNAs have become available. Measured data in this column was collected with a miniVNA Pro, a pocket-sized VNA. It operates over the 0. –200 MHz frequency range. It is battery-powered and features USB and Bluetooth connectivity (Figure 1). We hooked up a two-port DUT to the DUT and DET SMA connectors. The instrument injects sine waves (swept from 0.1–200 MHz or in any user-defined sub-band of it) into the cable connected to the SMA labeled DUT, measures sine waves propagating back from the DUT SMA (reflection) and the DET SMA (transmission), and compares the measured received sine waves to the injected sine waves to characterize reflection (e.g., S11) and transmission (e.g., S21). With this instrument, we can measure the full S matrix of a two-port DUT, though to get the full matrix, we have to manually set up four independent measurements. The instrument comes with open, load and short SMA calibration standards, shown on the lower left in Figure 1.

Read the full column here.


Editor's Note: This column originally appeared in the March 2014 issue of The PCB Design Magazine.

Share


Suggested Items

Martin Cotton Discusses Ventec’s New Book and Low-Loss Materials

04/11/2018 | Andy Shaughnessy, Design007 Magazine
During DesignCon 2018, Andy Shaughnessy sat down for an interview with Martin Cotton, director of OEM projects for Ventec. Martin was a PCB designer for years, so he has experience on both sides of the desk. They discussed Ventec’s reasons for coming to DesignCon, their expansion into low-loss materials, and Ventec’s new I-Connect007 book, The Printed Circuit Designer’s Guide to Thermal Management with Insulated Metal Substrates.

Mentor’s HyperLynx Automates SERDES Channel Design

04/09/2018 | Andy Shaughnessy, Design007 Magazine
Mentor recently released the newest version of its HyperLynx signal integrity software. This version may be the first SI tool in the industry to fully automate SERDES design channel validation. I spoke recently with Chuck Ferry, product marketing manager with Mentor, about the new HyperLynx and some of the new serial link design capabilities that customers have been demanding.

Part 2: EIPC’s 2018 Winter Conference in Lyon, Review of Day 1

02/19/2018 | Pete Starkey, I-Connect007
We continue with the rest of Pete Starkey’s report on Day 1 of the EIPC Winter Conference in Lyon, France. Included in this segment are presentations by Ventec, Ericsson, TTM and others, plus photos of their evening tour of Alstom.



Copyright © 2018 I-Connect007. All rights reserved.