Design Con 2015

Inexpensive ways to test radiated immunity

-April 29, 2013

There are several EMC issues I run into all the time. Radiated emissions is nearly always at the top of the list. However, radiated immunity (RI) seems to crop up much more frequently, and it is the second-most prevalent issue for me.

Nothing beats a controlled and formal RI test, but this requires an expensive chamber and time-consuming testing. Most manufacturers will not have the means to perform this test in-house. But is there a way to evaluate a product more quickly using simple instrumentation? Yes, there is. I'm a big proponent of pre-compliance testing, and I'd like to describe how you could perform some simple testing right at your own facility.

Bear in mind, the FCC and other regulatory agencies don't allow full immunity testing to be performed outside a shielded chamber, due to potential for interference with existing services, so the actual compliance test levels and frequency range may not be accommodated during these limited RI tests. However, my experience has been that if a product has immunity issues, they usually occur in multiple frequency bands. Therefore, limited testing often reveals issues, which may then be resolved right on the workbench. To really ensure accurate radiated immunity levels, the product under test will eventually have to be taken to a test facility.

There are three primary tools I use to assess RI; license-free two-way radios, RF signal generators driving small H-field probes, and a "chattering relay." All these simple RF generators will work well to at least give you a general feel for whether your circuitry is immune to RF.

Two-Way Radios
One handy gadget I’ve used for a general look at immunity is a hand held Family Radio Service (FRS) transceiver. These may be purchased in most electronics stores for $20 to $30 for a pair. They are small and will fit right into your troubleshooting kit. While their transmitted frequency is limited to 462 to 467 MHz at just 0.5 watts of power, they are still useful for inducing product failures when held very close to the product circuitry or I/O cables. Note that many “FRS” radios also transmit on the General Mobile Radio Service (GMRS) frequencies (also 462 to 467 MHz, but 5 watts of power). These frequencies require an FCC license and should not be used for immunity testing, due to possible interference to established GMRS communications.

Other good choices for different frequencies would include a handheld Citizen's Band (CB) radio, which operates near 27 MHz. I also use a TriSquare FM radio that operates in the license-free ISM band about 915 MHz. Between these three radios, I can cover the low, mid and high portions of the typical consumer RI band.



Figure 1 - Here are several of the handheld two-way radios used to inject RF at different frequencies.



Figure 2 - FRS radio inducing circuit upset in a handheld medical device. Because the product under test was so small, we had to refine the technique by utilizing an RF generator and small H-field loop probe (Figure 4).


RF Generator
I also use an RF generator with small H-field probe to inject a more controlled and localized RF field I to circuit boards and I/O cables. The best generators will output +10 to +20 dBm for a typical field strength at the probe of about 1 to 3 V/m. See Figure 3 in the case studies below.

"Chattering Relay"
This simple DIY hookup will produce an amazingly broadband and strong RF field. I've measured energy out to nearly 1 GHz with field strengths of 10 to 30 V/m close in to the relay and power cable. See more detail on the construction.


Figure 3 - Components required for a DIY chattering relay.

>>"Look over Ken's shoulder" in case studies using these techniques on page 2 >>

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