EMC Bench Notes

EMC Bench Notes: Troubleshooting with RF Current Probes

June 6, 2024

Introduction

Last month, we showed how to use near-field probes to characterize and interpret dominant harmonic energy sources on PC boards. This time, we’ll discuss a more advanced troubleshooting tool for assessing radiated emission issues, the RF current probe. These are most useful for measuring RF common mode harmonic currents on cables.

I suspect most product designers are familiar with the smaller current probes designed for oscilloscopes or digital multimeters (DMMs). These typically have smaller apertures that fit a wire or small cable and generally extend from DC to 100 MHz at best. There are also current probes for electrical measurements with larger apertures that range up to only a few MHz and are really designed for mains frequencies.

Figure 1: A typical RF current probe from Tekbox with useful frequency range of 30 kHz to 400 MHz (3dB bandwidth).

Figure 1: A typical RF current probe from Tekbox with useful frequency range of 30 kHz to 400 MHz (3dB bandwidth).

RF current probes usually have a hinged aperture that can accept everything from a single wire to large-diameter cables (Figure 1). When their 50Ω port is connected to a spectrum analyzer, you’ll observe an RF spectrum similar to that when using a near-field probe. Many manufacturers make these probes, but for this article, we’ll use the affordable Tekbox Model TBCP2-30k400 ($679). See Reference 1.

Various harmonic energy sources on your circuit boards or system can couple to attached cables and are the main causes of radiated emissions from products. We’ll use the RF current probe to characterize and reduce these coupled RF currents by clamping it around each I/O and power cable (Figure 2). The typical RF current probe is sensitive enough to measure µA of RF current, and only 6 to 8 µA of harmonic current can fail the FCC class B limit.

Figure 2: Using an RF current probe to measure the common mode currents on a USB cable.

RF Current Probe Measurements

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The RF current probe is merely a current transformer that measures RF currents in the primary (wire or cable to be measured) and couples that to the secondary, which is loaded by the 50Ω input impedance of the spectrum analyzer (Figure 3). This produces a voltage across 50Ω that is usually in terms of dBµV. I usually insert a bit of “bubble wrap” within the probe aperture to keep the wire or cable centered and away from the metal probe case in order to minimize measurement errors.

Figure 3: Schematic diagram of a typical RF current probe.

Because of resonances on cables, I like to slide the RF current probe back and forth on the cable or wire in order to maximize the dominant harmonic or harmonics. Once the harmonic is “peaked up,” I tape the probe down to the table to minimize variables while I try different mitigations to reduce cable coupling to the board.

Mitigations could include rerouting internal cables, improving bonding of cable shields to chassis or digital return plane, adding or improving common mode filtering at the I/O or power connectors, shielding energy sources using local shields, etc.

Estimating Pass/Fail

One important use for the RF current probe is to provide an estimate of passing or failing specific emission test limits. By knowing the current in an I/O or power cable, we can calculate the E-field at the test distance per the standard used. While this won’t necessarily be precise, it still gives us a “ballpark” estimate to compare to the test limit at that frequency.

Figure 4: The transfer impedance calibration chart for the Tekbox TBCP2-30k400 RF current probe.

Commercial RF current probes come with a calibration chart of transfer impedance versus frequency (Figure 4). Using Ohms Law, we can use this chart to calculate the measured common mode current in the wire with respect to the voltage measured at the probe output port, assuming a 50Ω system. This is based on work by Dr. Clayton Paul (Reference 2) and further refined by Henry Ott (Reference 3). I also have example calculations in References 4 and 5.

Let’s assume we measure one of the dominant harmonics in a cable as 28 dBµV at 120 MHz at the spectrum analyzer. We can also read of a transfer impedance of about 3 dBΩ from the calibration chart in Figure 4.

Using Ohms Law, we can calculate the common mode current (Icm) in the cable:

Icm (A) = E (V) / R (Ω), or, in converting to terms using log identities,

Icm (dBµA) = Vprobe (dBµV) – 3 dBΩ = 28 – 3 = 25 dBµA

Now using the E-field equation from Paul and Ott:

where,

Ec is the calculated E-field in V/m due to common-mode current flowing on the cable,
Ic is the current through the wire or cable (A),
f is the harmonic frequency being measured (Hz),
L is the length of the cable in meters and
d is the measured distance during the compliance testing (usually 3 or 10m).

Converting the measured values to basic units and plugging into the E-field equation, we get 8.94E-4 (V/m). Converting this back to log units, we get 59.03 dBµV/m. Comparing this with the FCC class B limit at 120 MHz (43.5 dBµV/m) indicates we may be over the limit by 15.5 dB.

Figure 5: A simple Excel spreadsheet can perform all the math required to estimate the E-field in dBµV/m from the measured harmonic current in a wire or cable.

I developed a simple Excel spreadsheet to streamline all these calculations, which may be downloaded from my Dropbox (Reference 6). Figure 5 shows an example calculation. By entering the specific probe transfer impedance, the frequency of concern, the cable length and test distance (typically 3 or 10m), the E-field in dBµV/m is calculated and may be compared to the appropriate test limit.

Summary

The RF current probe is not only a useful tool for general troubleshooting but may also be used to determine potential passing or failing due to a radiating cable. While they may be a bit pricy, I find the RF current probe is one of my most used tools for troubleshooting emissions. I also have a short video showing how to use these RF current probes (Reference 7).