What is an absorbing clamp in an EMC Test Setup?

As per EMC standards, the conducted emission measurement test is done up to the frequency range of 30 MHz. These standards instruct the usage of a LISN (line-impedance stabilization network) to measure the conducted emission from a EUT (equipment under test). Above the frequency range 30 MHz, radiated emissions are predominant.

Figure 1: Absorbing clamp

An absorbing clamp is an EMC measuring instrument that is used to measure the radiated disturbance (emission) from a small EUT at a frequency range above 30 MHz (typically, 30 MHz to 1000 MHz/ 1GHz) by measuring interference power on power supply cables. Small EUTs mainly emit disturbance via closed cables (e.g., power cables). The absorbing clamp method is an alternative method to the standard method that uses an antenna system to measure the radiated emission from the EUT. The standard method (antenna system) to measure the radiated power is preferred when the equipment has several connected cables or when its dimensions are greater than a quarter wavelength (i.e., it is physically large, and/or the frequency of interest is high).

The absorbing clamp can also be used to measure the screening attenuation of RF coaxial cables. 

The standards such as CISPR 13 [EN 55013], CISPR 32 [EN 55032], and CISPR 14 [EN 55014], and some more standards prefer absorbing clamp for disturbance power measurements. In addition, the absorbing clamp can be used as a coupling unit for the immunity test also.

Construction and working of the absorbing clamp:

Figure 2: Absorbing clamp construction details

An absorbing clamp consists of a ferrite current transformer that operates at a frequency range above 30 MHz and two sets of ferrite rings. 

Both of these ferrites are split into two halves. One half mounted in the absorbing clamp base, and the other half mounted in the hinged top cover. The mainline/power cable to the EUT is can easily be laid into the bottom halves of the ferrites by opening the clamp. When the clamp is in closed stage, the two halves meet, forming a ring around the line(s) under test.

The current transformer is clamped over the lead/power cable and provides a voltage proportional to the RF current through the power cable. The current transformer faces the EUT, and the far end of the power cable is connected to a power supply or to an ancillary device.  One set of ferrite rings surround the power cable acts as an absorber to isolate EUT from power line RFI or RFI from the ancillary device (i.e., acts as resistance for the high-frequency RFI power and dissipates the RFI power as heat) and also stabilizes the impedance of the power line.

Some EMC tests utilize extra ferrite rings (optional) to provide better isolation between EUT and power supply/ancillary device (Figure 3). The second set of ferrite rings surrounds the lead/cable that connects the current transformer output to the measuring device (EMI receiver) help to minimizes standing waves

How to measure the interference power?

Figure 3: Test set up

To measure the disturbance power of a EUT, first, the EUT is placed on a non-conductive surface (test table) as per EMC standard requirements. The power cable is inserted horizontally so that the absorbing clamp surrounding the cable can be easily slid along the cable. The absorbing clamp output is connected to the measuring receiver or spectrum analyzer. Now, the EUT is switched on, and the measuring EMI receiver is set to a frequency of the disturbance spectrum. The absorbing clamp is moved along the cable length under test until to get the maximum reading (dBµV) in the display of the measuring receiver or spectrum analyzer.

Now the disturbance power (Pico watts) is calculated by using the following formula. Here, the insertion loss value is provided in the manufacturer's data sheet.

Disturbance power (dBpW) = Indicated voltage reading (dBμV) + Correction factor in dB 

Correction factor = insertion loss (dB) -17

Specification details of absorbing clamp:

Frequency range: Represents the useable frequency range of the absorbing clamp. Usually, the frequency range of the absorbing clamp can vary from 30 MHz to 1000MHz/1GHz.

Clamp factor/correction factor: It helps to include the insertion loss of the absorbing clamp (dB) in the disturbance power calculation.

Insertion loss: Represents the power loss due to the insertion of the clamp or power loss in the clamp. It is represented in dB.

Decoupling factor: Represents the amount of decoupling of the far end of the EUT LUT(lead under test) from the near end of the EUT LUT).  It is represented in dB.

Receiver input impedance: Represents the input impedance of the measuring system (EMI receiver). It is represented in ohm. Usually, 50 ohms preferred.

RF connector output/input: Represents the RF connector type. Usually, it is a 50 ohm-BNC (female).

Operating current: Represents the maximum operating current of the clamp in Amperes (A).

Maximum cable diameter: Represents the maximum diameter of the cable that can be tested with the absorbing clamp. It is represented in cm.

Aperture Diameter:  Represents the aperture diameter of the clamp. It is represented in cm.

Maximum input power (for immunity test): Represents the maximum limit of input power that accepts by the absorbing clamp. It is represented in Watts.

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