What are Near-Field Probes?
EMC emission tests are conducted to check whether the radiated electromagnetic energy is within the emission limits prescribed by international/national EMC standards. If the product emission exceeds emission limits, then the product fails the EMC compliance test.
Fig. 1 Near-field probe kit
Near-field probes help engineers to quickly identify the source of EMI emission at the development stage and help the product developer to solve EMI issues at the factory, thereby reducing the cost of the overall product compliance.
In order to pass the EMC compliance test, the product must be investigated for causes of higher emissions, and the causes are needed to be fixed. To find the exact EMI emission source, it is necessary to conduct emission measurements closer to the circuit. This is not possible with an antenna. Near-field electric (E-field), and magnetic (H-field) probes let product developers "sniff" around the circuits, cables, and enclosures to find the EMI emission source.
Near-field probes are generally immune to background noise or hand position and are available in a kit. The kit contains Near-field probes (E-field & H-field probes) of different sizes so that a user can use a smaller size probe in order to narrow the area of measurement. Usually, the probe output is connected to a spectrum analyzer for measurement. Sometimes a preamplifier may be connected between the probe and the spectrum analyzer to improve the measurement sensitivity.
The near-field probes are ideal for radiated EMC measurement, RF immunity testing, contactless (load free) relative measurement of RF signal chains, and contactless (load free) relative measurement of oscillators, modulators, etc.
Fig. 2 Emission measurement of a PCB
How does an E-field Probe Work?
Fig. 3: E-field probe
An E-field probe acts as a monopole antenna and responds to voltage changes or electric field changes. It produces an output voltage across its output terminal, which is proportional to the E-field under measurement. To conduct a measurement, it is important to place the E-field probe perpendicular to the plane of measurement. E-field probes are used to identify the sources with relatively high voltages as well as sources with no termination, such as high impedance nodes and circuits, cables, and un-terminated PCB traces.
How does an H-field probe work?
Fig 4: Magnetic field probe
Fig 5: Magnetic field probe placement
H-field probes have a unique loop design and respond to the magnetic fields (i.e., current changes). Recall the magnetic field is produced by the current changes. To conduct magnetic measurements, the loop of the H-field probe should be perpendicular to the magnetic field under measurement. So, when searching for an emission source, the probe is often held with the plane of the loop parallel to the circuit board surface. During emission measurement, the magnetic field passing through the probe loop generates a voltage (Faraday's law). i.e., the induced voltage is proportional to the rate of change of magnetic flux that passes through the circuit loop. The diameter of the loop decides the Sensitivity of the probe, frequency response, and spatial resolution.
H-field probes are used to identify the sources with relatively high currents, such as low-impedance nodes and circuits, transmission lines, power supplies, terminated wires, and cables.
Key Specifications of Near-field Probes:
H-Field (magnetic) probe length: Represents the length of magnetic field probe in mm.
H-Field (magnetic) probe loop diameter: Represents the diameter of the loop in the magnetic field probe in mm.
E-Field (electric) probe length: Represents the length of electric-field probe in mm.
E-Field (electric) probe tip length: Represents the tip length of the electric-field probe in mm.
E-field probe maximum resonance frequency: If the signal is above the resonance frequency, the e-field probe may go into resonance. Usually, the e-field probe has a maximum resonance frequency is in the range of GHz.
H-field probe maximum resonance frequency: The H-field probe has the maximum resonance frequency is in the range of MHz
E/H or H/E Rejection: Represents the rejection of unwanted E/H field while measuring the H/E field. It is measured in dB.
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