Introduction
Last month, we discussed how to use an inexpensive handheld license-free walkie-talkie to help simulate radiated susceptibility. We also introduced the use of benchtop RF generators or smaller RF synthesizers, along with near-field probes to generate a strong localized field to simulate specific disruptive frequencies to cause system upset.
There are times, however, when it’s necessary to use more than the +20 dBm (100 mW) maximum RF level produced by typical RF generators or synthesizers in order to cause disruption. An additional RF power amplifier may be required. Some of these techniques will require testing within a shielded room to avoid interference to existing broadcast or communication services. We’ll review three case studies where small benchtop power amplifiers were required.
Case Study #1 – Client wanted to assess product to 50 V/m
In this case, the client wanted a quick assessment on whether their commercial product could meet the required 50 V/m test level for a military application. Normally, you would take this requirement to a 3rd-party test facility, but they really wanted to know in advance whether their product could even come close to passing this high a field level.
We set up a test on their bench, consisting of a small antenna, RF synthesizer, E-field monitor, and RF power amplifier (Figure 1). We used an antenna to try duplicating what would be done during the formal testing by immersing the EUT with an RF field. When using an antenna to transmit an amplified RF field, the test should be performed within a shielded room.
The antenna had to be placed directly against the EUT in order to achieve 50 V/m. As a bonus, we also identified an interior connecting cable that was susceptible. The test setup diagram used is shown in Figure 2. The E-field probe was located adjacent to the EUT to confirm the transmitted field level.
The laptop not only controlled the frequency but monitored both the E-field level and RF level into the power amplifier via a 20-dB coupler.
The test was performed quickly on the bench top and we were able to confirm most frequencies would achieve 50 V/m, so the client was more comfortable spending time and resources to perform the actual compliance testing for a product not normally designed for a military environment.
Case Study #2 – Battery failure during radiated immunity
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 3 and 4).
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 to 20 V/m, with a maximum of 28 V/m in some wireless and two-way radio bands. This battery pack was failing at 5 to 10 V/m at 100 and 127 MHz.
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 5).
After evaluating the results, it became apparent that 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. Because the H-field loop was unable to couple enough RF power into the cable, I decided to use a standard RF 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 connected a scope probe to channel 1 of my 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.
The test setup used may be seen in Figure 6. 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 7 shows a close-up 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 MOSFET power switch, which, when biased off, disconnected the battery. By adding a 0.01 mF filter capacitor to that area of the circuit, I was able to decouple the RF from affecting the switch. See Figure 8.
By monitoring the oscilloscope, I was able to perform troubleshooting and mitigation experiments in real time. We can see that upon application of RF and the failure response delay varies between 0.8 and 3.2 seconds as I connect or disconnect the 0.01 µF capacitor.
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, “Am I able to simulate the failure?” Once the failure is duplicated, then various mitigations can be tried.
Case Study #3 – Blood analyzer pump “takes off”
In this last case study, I was called in to help troubleshoot a radiated immunity issue for a blood analyzer. The analyzer used a series of slow pump motors to extract blood from a patient, perform the analysis, and then pump it back in. These pumps are designed for very low speeds, approximately 2 to 4 RPM. However, at certain transmitted frequencies, two of the pump motors would take off at 10,000 RPM – probably bad for the patient!
The designers had been working for several weeks, running back and forth to a 3rd-party test lab and trying shielding, bonding, and ferrite chokes, to no avail.
I set up a test station in Figure 9 using an RF synthesizer feeding a 3W RF power amplifier. After trying the commercial near-field probes and small antenna, I could not seem to duplicate the failure. Then, I constructed a larger H-field loop using a piece of coax cable. After sweeping it around, I noticed that when it approached the digital encoder mounted to the back of the affected motor, it took off at 10,000 RPM (Figure 10)!
The solution was to install a single ferrite choke around the encoder cable (the client decided to use two just to ensure reliability). See Figure 11.
The end result was that we located the root cause and installed a low-cost mitigation, all before lunchtime! The project manager for the team admitted privately that he should have called me in weeks ago. Sometimes the job of an EMC consultant is rather satisfying!
Summary
I have found that bench-top 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 these case studies, the RF level requires a boost through the use of a broadband power amplifier. 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.
