Wires and cables tend to behave as transmitting antennas. When troubleshooting a system for EMI, you must identify which cables have these undesirable "common mode" currents. Current probes let you measure the RF currents flowing in cables or wires, which can help you identify and quantify emissions from wires and cables. Then, you need to find the cables that couple common-mode currents to these sources of harmonic energy.

Tekbox Technologies recently introduced an inexpensive and calibrated TCBP1 current probe characterized from 10 kHz to 250 MHz with a transfer impedance of an advertised 18 Ω nominal. The price? Just $295; not quite one-tenth the cost of some of the competitive probes. So how does this compare with the more expensive Fischer Custom Communications F-33-1, which I use a lot and like very much?

First, let's compare some advertised specifications:

Characteristic Tekbox TBCP1 Fischer F-33-1
Frequency range 10 kHz to 250 MHz 10 kHz to 250 MHz
Aperture diameter 25 mm 32 mm
Outside diameter 76 mm 71 mm
Height 31 mm 19 mm
Weight 320 g Not specified
Connector type N N
Transfer impedance (dBΩ) 18 nominal 16 nominal
Maximum primary current 80 A (DC to 400 Hz) 100 A (DC to 400 Hz)
Maximum primary current 3 A (RF) 2 A (RF)
Peak Pulse Current Not specified 50 A
Maximum Core Temperature 125°C Not specified

Unboxing the Tekbox probe reveals a neat wooden box for storage, which reminds me somewhat of the nice walnut boxes HP used to use with their various probes and RF accessories (Figure 1).

Tekbox RF current probeFigure 1. The Tekbox current probe with wooden storage box (Photo by Kenneth Wyatt)

The probe itself appears robust and good quality. Unfortunately, it's missing a hinge, making it impossible to merely clamp the probe around a wire or cable. That's a definite disadvantage when troubleshooting large systems with internal cables. It is also thicker and has an aperture of 25 mm, which is smaller than others. Figure 2 compares the Tekbox probe to one from Fischer Custom Communications, which lets you clamp around cables.

Tekbox TBCP1 current probeFigure 2. The Tekbox TBCP1 current probe compared with the Fischer Custom Communications F-33-1 (Photo by Kenneth Wyatt)

The probe can, however, be used by sliding over an attached cable, so long as one end isn't connected to anything and connectors are small enough. Ethernet, USB, and HDMI connectors seem to work. Larger strip-type and other connectors may not.

Comparing the transfer impedance curves with my Fischer F-33-1 (10 kHz to 250 MHz) reveals a little slight downward slope from 300 kHz to 100 MHz and then a sudden drop-off from 100 to 250 MHz. This would require looking up the transfer impedance for each of the higher harmonic frequencies, rather than assuming a constant impedance, as with the Fischer (Figures 3a and 3b).

textFigure 3a. The Tekbox TBCP1 advertised transfer impedance versus frequency (Source: Tekbox TCB1-150 manual)
textFigure 3b. The Fischer Custom Communication F-33-1 advertised impedance versus frequency (Source: Fischer Custom Communications)

To calculate the current through the probe in dBµA, just subtract the transfer impedance (dBΩ) from the spectrum analyzer reading in dBµV.



It's possible to take the current and calculate the E-field generated by the cable at a specific distance (assuming the length of the cable is "short" compared to a wavelength). See Using current probes to estimate E-fields.

I tried measuring the both the Tekbox and Fischer probes using the same setup, which consisted of a demo board producing several harmonics. I used a bit of "bubble wrap" to keep the cables centered within the aperture (Figure 4).

Figure 4. A simple test setup for comparing the relative performance of the two probes (Photo by Kenneth Wyatt)

There was a little discrepancy between the two probes in the area of 200 MHz and 320 MHz (where neither are really characterized), but they mostly compared well, considering the transfer impedance was slightly different between the two (Figure 5).

textFigure 5. A screen capture shows the two probe measurements, as measured from 10 to 500 MHz. Most measured within 3-4 dB.

To summarize the advantages and disadvantages of the Tekbox TBCP1:

Pluses:

  • Appears to be very well constructed.
  • Includes a protective wooden box for storage.
  • Measures similarly enough to the Fischer F-33-1 for accurate EMI troubleshooting.
  • Low price.

Minuses:

  • Lack of hinge forces the probe to be "strung" onto a cable.
  • Smaller aperture prevents measuring very thick cables or cables with large plugs.
  • Thicker probe body may make measuring shorter cables troublesome.

Overall, this probe may be just the answer for companies with very limited budgets who require a reasonable tool for measuring harmonic currents on cables during EMI troubleshooting. The issue really surfaces when attempting to troubleshoot interior system cables where you'd need to power down the system and disconnect one end of a cable and string the probe on prior to making a measurement. This could slow down the troubleshooting process a great deal. The 25 mm aperture may also be too small for some connectors. On the bright side, most radiated emissions problems occur with external cables, over which, the Tekbox probe may easily be threaded.

I recommend the Tekbox current probe for troubleshooting applications on a tight budget. The U.S. distributor for Tekbox products is Saelig.

Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.

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