Checking Cable Performance with VNA

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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.


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