Introduction
For the last several months, we’ve discussed issues like radiated and conducted emissions and how to make and characterize these measurements. Let’s deal with some common immunity issues for the next series of articles. One that’s becoming more prevalent is radiated immunity (or radiated susceptibility, in MIL-STD-461 terms).
It seems this problem has been increasing over the last decade, and the reasons are threefold: (1) electronic devices are getting smaller and using plastic enclosure, (2) the proliferation of electronic and body-worn devices has increased exponentially, and (3) as we’re powering circuits with lower voltage levels (3.3V, or less) resulting in a greatly reduced noise margin. That is, it takes less energy to disrupt sensitive digital circuits.
Interestingly, antenna-like structures, like attached cables, seams in shielded enclosures, or internal cables, not only serve to transmit radiated emissions but can also act as receiving antennas to couple external RF energy sources into the product. This coupled energy is what can disrupt sensitive electronic circuits.
Common failure modes include scrambling of LCD displays, changing instrument states or modes, or upsetting measurements. It can even reset processors. The problem from a systems viewpoint is that any number of entry points can be allowing the RF energy into the product. Once you find the coupling mechanism, you still can’t be sure what specific components are being affected. This can be frustrating to troubleshoot.
Troubleshooting at the Test Lab
Troubleshooting system radiated immunity issues at the compliance test lab is time-consuming and expensive. Figure 1 shows the general test setup for consumer and industrial products using the IEC 61000-4-3 standard. A gradually-stepped frequency (30 to 6000 MHz or higher) is transmitted towards the product under test at levels of 3 to 20 V/m (as high as 200 V/m for MIL products).
This is performed inside a semi-anechoic chamber to reduce reflections, and personnel are not allowed inside during the testing. So, troubleshooting requires applying some sort of fix, then exiting and re-running the test. This is repeated, usually unsuccessfully, until your time slot ends. Testing one frequency sweep can take hours. Even knowing the failing frequencies can take a long time to mitigate.
Troubleshooting on the Workbench
A better method is to take everything back in-house and test it locally on the workbench, where plenty of resources exist to attack the issues. The trick is to inject high intensity but localized RF fields into various parts of the system. I’ve used this technique successfully for many years and have reduced weeks of work by my clients down to a few hours. By using an RF generator or other source and coupling this energy into the product, the issue can readily be identified.
Methods to Inject RF Energy
There are several methods I’ve used to try to simulate the radiated immunity test. I also found that even using a source far off in frequency caused system upset (for example, FRS walkie-talkie at 465 MHz) when brought close enough to a circuit, can generally duplicate the failure.
License Free Family Radio Service (FRS) Walkie-Talkie – These small transceivers can be purchased at Walmart and other sources. They are regulated to 0.5 watts and transmit about 465 MHz in the FRS band. Figure 2 shows how I’m using one to test a small circuit board.
One client had worked for weeks trying to tame the LCD display and measurement in an industrial tool. They noticed they could simulate the issue with an FRS radio. So, we decided to use that as the only troubleshooting tool.
By sweeping it around the system, it was apparent a strain gauge seemed to be the most sensitive area. This strain gauge was connected to a small amplifier via a shielded cable. Disassembling the area around the cable revealed the shield was never bonded to the metal structure (Figure 3)! It turned out that neither end of the cable shield was bonded to the chassis structure.
Once we discovered that, we peeled back the insulation and made a proper bond to the metal structure using copper tape (Figure 4). After bonding the other end of the cable, we retested the system and confirmed the fix by successfully transmitting right next to the affected cable without causing disruption in the display.
While I brought a cartload of test equipment and probes along for this job, all we really needed was a $30 walkie-talkie! The client had been troubleshooting this for weeks, and we had it located and mitigated before lunch. Sometimes, it doesn’t take thousands of dollars of equipment to solve EMC issues!
RF Generator and H-Field Probe – A more controlled method for injecting RF energy into a product is to use an RF generator and H-field probe. Generators that can produce +10 to +20 dBm are perfect for most cases. We’ll discuss how to achieve higher localized power in Part 2 of this series.
One time, I was called out to a medical products client in the Bay Area. They were developing a handheld blood glucose meter that was failing radiated immunity at about 950 MHz. The failure mode was scrambled data in the LCD display. This is rather close to a paging band back in that day, so was not a good spot to have susceptibility.
I suggested that if we could find an RF generator, we could inject 950 MHz into the device to simulate the failure mode. In the Bay Area, used test equipment was readily available, and the client was able to locate one quickly.
We set the generator for the failing frequency of 950 MHz and connected the smallest H-field probe because the product circuit board was so small. Adjusting the RF output to +11 dBm, we scanned the probe around on the circuit board to no avail.
Then, we started scanning the very short ribbon cables attached to the board. There were several, and one of the shortest (1 cm) was susceptible! A simple bypass capacitor to divert the RF away from an op-amp was sufficient to mitigate the issue. Again, the client had been troubleshooting for weeks, and we narrowed it down and mitigated it within a day.
RF Synthesizers – Since I didn’t want to lug my 40-pound RF generator around to client locations, I started looking for something smaller and equally powerful. It didn’t take long to locate companies that were starting to sell small USB-controlled RF synthesizers that could output +10 to +19 dBm. Connecting one of these to my H-field probes became my “go-to” test and troubleshooting solution.
Figure 6 shows some of these that are on the market for around $500. The really nice feature of most of these is the ability to produce AM or pulse modulated RF at 1 kHz. This is just what’s required by both IEC 61000-4-3 and MIL-STD-461 RS102 testing.
Figure 7 shows how I tested a small embedded processor for radiated immunity using 80% AM from 30 to 1,000 MHz with modulation of 1 kHz. The field from the H-field probe drops off quickly at about 3 cm away, so there’s no issue with interference.
These probes can be swept around components and traces on circuit boards or coupled to interior or exterior cables. I generally set the RF output as high as +20 dBm for most troubleshooting. Reducing the level or using a smaller loop probe for higher resolution can help zero in on specific components or circuitry that is being affected.
Summary
This article has described an efficient method for identifying circuit sensitivities to RF energy. By identifying the likely coupling paths via cables or specific circuits and coupling strong localized RF fields directly into circuitry or cables, you can perform radiated immunity troubleshooting right on your workbench!
Part 2 of this series will describe methods for testing to much higher field levels on the bench top. Of course, some of these techniques will require testing in shielded rooms.
