Characterization and DC shield resistance tests can provide a quick check on the shielding quality and connector bonding of proposed test cables
Testing most products for radiated emissions usually requires all I/O and power cables to be attached to the equipment under test (EUT) and spread out in accordance with the specific product standard. In many cases, we test engineers simply grab the nearest cables and hope for the best during the compliance test. Unfortunately, poor-quality cables can lead to emissions failures due to poor shielding or poor shield termination (via “pigtails“) to the connectors.
In an earlier article, I related the issue of HDMI cable radiation due to shield pigtails (Reference 1). I also graphically demonstrate why cable shield pigtails lead to radiated emissions in the video in Reference 2.
In addition to the shield pigtail issue, coax cables are manufactured with varying qualities of shielding weave from tight to loose, depending on the cost. Cheaper cables can have very loose weaves, which allow signal leakage, both into and out of the cable. Higher-quality “MIL-Standard” cables often have two layers of woven shielding, which is ideal for emissions testing. However, use of cheaper cables can lead to compliance failures as well as measurement contamination from external sources during troubleshooting operations.
Relative shielding quality
One of my colleagues, Steven Sandler of Picotest, and I were exploring the shielding properties of coax cables and he demonstrated a unique method by connecting one end to a spectrum analyzer input and setting the span to cover the 2.4 GHz ISM (Wi-Fi/BT) band. Assuming you have Wi-Fi access points and computers nearby, scanning in max hold mode, poor quality shielded cable should be easily identified by observing an increasing level of received signals. Well-shielded cables will show relatively little signal (Figure 1).
Another important characteristic is a low DC resistance between each connector shield. Measuring the DC resistance also makes a gross check on shield conductivity and shield to connector bonding and is easy to do. Simply set a DC power supply to current limit at 1 amp and connect the output to each end of the cable under test (Figure 2). The output voltage will likely dip down to a low value, but this is OK, so long as the supply is designed for constant current operation.
Figure 2 Here is the block diagram of the DC resistance measurement setup.
A measure of the voltage drop (in mV) from one end of the cable to the other using a DMM is the DC resistance in mΩ according to Ohm’s Law. A low measurement cable DC resistance is especially important for accurate power distribution network (PDN) impedance measurements (Reference 3).
Relative shielding quality
Any spectrum analyzer should work well to characterize relative shielding properties of single coaxial cables. However, since we both had access to a Tektronix Series 6 oscilloscope with the new Spectrum View option, we could examine the shielding performance of four cables simultaneously by connecting each to the four input ports and turning on the spectrum analyzer function for each port.
I had several cables I wished to characterize; among them were some standard BNC patch cables and some higher-quality double-shielded RG-316 cables. I was also curious how well the Beehive near field probe cable performed, as I use it extensively during general troubleshooting.
Since the center pin of the free male connector end could potentially pick up stray RF signals, you may elect to terminate it with a shorting connector or 50 Ω termination, just in case. However, I found that terminating the free cable end did little to change the resulting display, so I didn’t bother.
Spectrum analyzer setup
For a standard spectrum analyzer, set the start and stop frequencies to 2.4 and 2.5 GHz, respectively, the resolution bandwidth to 100 kHz, and adjust the attenuation and reference level for a useable display. Cables with excellent shielding properties will technically show very little response, while poorly-shielded cables will clearly show Wi-Fi channels with Bluetooth signals adjacent.
Tektronix Series 4/5/6 setup (with Spectrum View)
Double-click on each channel badge, turn it on, and set the vertical for 1 mV/division and the input to 50 Ω. Then open up the Spectrum View panel, turn on the analyzer, and check off normal, max hold, and average. This will provide a roughly zero baseline, save the maximum signal, and show the actual measured values for each channel. Set the horizontal to 40 ns/division, but this is not that critical (Figure 3). For more information on this oscilloscope, see Reference 4.
Double-click the spectrum analyzer panel and set the span to 100 MHz and center frequency to 2.45 GHz. Set the resolution bandwidth to 100 kHz, leaving the rest at their default settings. Once the measurements start, I elected to grab the center line between the spectrum and time domain displays and pull it down to allow the four spectrum displays to dominate. Let the max hold measurement stabilize for a few minutes to show the maximum envelope showing the Bluetooth and Wi-Fi channels clearly.
I had a variety of coax cables I was hoping to test; some older HP general purpose BNC patch cables, an unmarked, inexpensive BNC patch cable, some RG-316 double-shielded SMA cables purchased from a vendor through Amazon, an SMA to SMB cable from Beehive Electronics I’ve been using a lot for general troubleshooting, and a custom high-quality triple-shielded PDN measurement cable from Picotest.
Prior to the testing, you might wish to connect a cheaper cable initially to get a sense of what to expect while adjusting the displayed results. Connecting cables 1 through 4 yielded the following results (Figure 4). Cable 5 (Picotest) was tested separately (Figure 5).
DC resistance test
One additional characterization test, suggested by Sandler, was to simply connect each end of the cable shield to a constant 1-amp source (Figure 6). Measuring the voltage drop with a DMM provides the mΩ directly. Good cables should read less than 10 to 15 mΩ, maximum. This checks both the shield itself and shield to connector bonding. Be sure to make the DC voltage drop measurement right at the coax connectors, as shown, so the drop across the two connecting test leads is ignored.
Table 1 shows the results of the five cables tested.
Table 1 Measurement of DC resistance for each cable tested
The DC resistance measurement for the Black BNC and Amazon RG-316 cables is suspiciously poor and indicates poor shield conductivity and/or poor shield-to-connector bonding. The HP cable measured slightly better than the Picotest cable, which was surprising considering the likely age. Both the Beehive and Picotest cables had excellent results.
Dissection of black BNC cable
I was curious why the black BNC patch cable picked up so much signal versus the others tested, so I dissected one end (figures 7 and 8).
It became immediately obvious why the black BNC cable had such poor shielding quality. While the cable does have a shield pigtail connection, it is very short. Much worse is that the shield weave is obscenely loose and allows the center conductor to show through the entire length of the cable. I estimate a coverage percentage at less than 40%, which is very poor. A coverage of 90 to 98% is considered very good. I would never use such a cable for compliance testing.
Poorly-shielded cables used for radiated emissions compliance testing can be quite risky and usually leads to emissions failures. Characterization tests such as the simple shielding test using commonly available Wi-Fi and Bluetooth signal sources, as well as DC shield resistance tests, can provide a quick check on the shielding quality and connector bonding of proposed test cables.
I was happy to have discovered the bad BNC patch cable and was very pleased the Beehive probe cable, HP, and Picotest cables performed so well. The Amazon-purchased cable shielding was not as good as the HP, Beehive, or Picotest and the DC resistance indicated poor shield bonding to the connectors.
This article was originally published on EDN.
—Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.