Design007 Magazine

PCBD-Mar2014

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60 The PCB Design Magazine • March 2014 In my December 2013 column Compar- ing Cable Shields [1] , 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 an- alyzer (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 mea- sure what are called scat- tering (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 [2] , a pocket-sized VNA. It op- erates over the 0.1–200 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 con- nectors. 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 by Istvan Novak orACle QuIET POWER Checking Cable Performance with VNA column Figure 1: The usB-connected minivnA Pro pocket-size vnA.

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