Benchtop troubleshooting of radiated immunity issues can be reduced to a few minutes or hours using this powerful technique.
You wouldn’t think that batteries could fail radiated immunity testing, but today’s more sophisticated Li-ion battery packs include battery management system (BMS) circuitry that monitors the charge/discharge state of each cell, as well as ensuring the battery is automatically disconnected from the system its powering should it detect a fault (figures 1 and 2).
Figure 1 This battery was failing radiated immunity by “disconnecting” itself from the system.
Recently, I was asked by a medical equipment manufacturer to characterize and troubleshoot a battery that was “disconnecting” automatically during the formal compliance testing of their product. The IEC 60601-1-2 standard (4th edition) for medical products was updated in 2014 and one outcome was that the test levels for radiated immunity were increased to as high as 10-20V/m, with a maximum of 28V/m in some wireless and two-way radio bands. This battery pack was failing at 5- 10V/m at 100 and 127 MHz.
I’ve previously described the method I developed for benchtop testing of radiated immunity in past books and articles (see references). The technique simply uses a USB-controlled RF synthesizer with connected H-field probe. The RF source I normally use is the Windfreak Technologies SynthNV ($599), which produces about +19 dBm (80 mW) and that usually has sufficient power to create a very intense RF field near the tip of the probe and suffices for most products. Sweeping the probe around your circuit board and associated wiring often reveals sensitive circuit nodes.
I’ve successfully used this technique in dozens of client projects that have similar RF immunity issues. More than once, a project manager said that they had wished I’d been asked in weeks earlier after helping their team isolate and resolve the issue within a few hours. This has proven to be a very powerful troubleshooting technique!
In the case of this particular battery pack, sweeping the H-field probe around the board revealed many spots of sensitivity. Every sensitive circuit node caused the main MOSFET switch to disconnect the battery and the output voltage would drop towards zero (Figure 3).
Figure 3 When RF was applied to the battery, the voltage would drop to zero.
After evaluating the results, it became apparent the most obvious “antenna-like” structure that was picking up the RF and coupling it into the circuit board was the main battery cable itself. I determined that simply coupling the H-field probe directly to the battery cable didn’t have sufficient power to cause the failure, so I had to think of alternatives.
One immediate thought was to use a standard current probe to couple the RF energy directly into the battery cable. That is, we’ll simulate a radiated RF immunity test by using a conducted RF immunity test setup. I confirmed the battery cable was indeed the issue by temporarily clamping a #43 material ferrite choke around the cable between the current probe and battery, which fixed the issue. Unfortunately, there was no room in the product for ferrite chokes.
While there was no easy way to compare this test with a conventional RF immunity test using transmitting antennas at the product, the important thing when troubleshooting any problem is asking, “Am I able to simulate the failure?” Once the failure is duplicated, then various mitigations can be tried.
The normal RF synthesizer I was used to didn’t have the power required to reliably cause the failure when using the current probe coupling, so I turned to a new product from Tekbox Digital Solutions. They make a series of high-powered RF amplifiers with built-in modulation, ranging from 22 dBm (160 mW) to 37 dBm (5W) output. These are “modulated” amplifiers and when the modulation is turned on, it can supply 1 kHz 80% AM modulation and pulse modulation at either 1 kHz or 217 Hz (for testing TDMA mobile devices).
I chose to use the Tekbox TBMDA3 (5W) amplifier (10 to 1000 MHz) and to drive it with my Signal Hound VSG25A vector signal generator. The VSG25A can also produce a variety of modulations, so it is very handy by itself, although the highest output is +10 dBm (10 mW) and I generally need more than that for normal troubleshooting.
The maximum recommended RF input power to the TBMDA3 amplifier was +3 dBm for 37 dBm (5W) output, so the combination was a good match. I just ensured the output of the VSG25A was set below +3 dBm. Most of the testing just required a drive level of -15 to zero dBm.
Later, when I was conferring with Tekbox on this test setup, they recommended inserting a 10W 3 dB attenuator between the amplifier output and either H-field probe or current probe in order to keep the RF output better matched to 50 Ohms. While the amplifier is rated to operate safely into a short, best practice dictates a safer, more reliable termination.
I connected a scope probe to channel 1 of my Agilent MSO-X 3102A oscilloscope and connected a medium-sized (1-cm diameter) H-field probe stuck partway into the current probe to monitor the RF on/off state to channel 2. By triggering on channel 2 and selecting a slow sweep, I could use the RF to start the sweep while observing the battery voltage on channel 1. Every time I applied RF, I could watch the battery voltage decrease during the failure. See the test setup in Figure 4.
Figure 4 In the test setup, the DMM and oscilloscope monitored the battery voltage and the scope also monitored when RF was applied. The laptop controlled the RF synthesizer, which was amplified and connected to the RF current probe that injected the RF into the battery cable.
A photo of the test setup may be seen in Figure 5. Note, I’ve used several turns of the battery cable wound around a ferrite toroid. This helps direct the RF towards the battery, rather than having it split in two directions. Figure 6 shows a closeup of how the current probe, extra inductance, scope probe, and monitoring probe are arranged.
This test setup made it especially easy to perform the troubleshooting. Apparently, the RF energy was coupling into the dual MOSFET power switch, which when biased off, disconnected the battery. By adding a 0.01 μF filter capacitor to that area of the circuit, I was able to decouple the RF from affecting the switch (Figure 7).
Figure 7 Applying a steady RF injection into the battery cable, I could either connect or disconnect the filter capacitor and see the results in real time.
By monitoring the oscilloscope, I was able to perform troubleshooting and mitigation experiments in real time. In Figure 7, we see that upon application of RF, the failure response varies between 0.8 and 3.2 seconds in this example.
I have found that benchtop troubleshooting of radiated immunity issues is fast and easy when applying intense and localized RF fields (either unmodulated or modulated) to the circuit board or system cables through the use of H-field probes or current probes. Once the failure mode can be simulated and the area of sensitivity is identified, it becomes much simpler to try various mitigations to resolve the issue. Sometimes, as in this case, the RF level requires a boost through the use of a broadband power amplifier, such as the Tekbox TBMDA3. What can easily take weeks of trial and error, and repeatedly cycling back and forth between your facility and the compliance test lab, can now be reduced to a few minutes or hours using this powerful technique.
This article was originally published on EDN.
—Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.
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