I have a board that is failing radiated EMC tests. There are narrowband spikes with 50 MHz intervals (300 MHz, 350 MHz, 400 MHz, etc.)

I believe it has something to do with a clock (I suspect so since 50 MHz seems to be the "base frequency" and that is a very even number).

EMC chamber time is very expensive so I thought I would look for the problem with near-field probes. I have access to a spectrum analyzer, but I haven't used these probes much before.

So what is the logical choice when one is looking for this 50 MHz clock (might be some internal clock on some chip since I have no such crystal on the board), H-field or E-field probes?

What are the two types of probes usually used for? Some example? Is one for current and the other for voltage?


1 Answer 1


As clock signals are generally low current and high impedance sources in nature, an E-field probe would be the first choice for measurements in the near-field. Oscillators and clock signals are actually the classic example for E-field emissions.

H-field probes are usually used for low impedance and high current sources, e. g. things related to power distribution.

Fundamentally, E-field probes respond to voltage changes and H-field probes respond to current changes.

Having said that, I would still recommend to have both types at hand. You can buy near-field probe sets that usually have an E-field probe and a set of H-field probes (different sizes). Those sets are not very expensive these days.

  • \$\begingroup\$ OK, great answer. Thank you! \$\endgroup\$
    – aaq
    Mar 2 at 7:52

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