Effectively Using the EM 6992 Near Field Probe Kit to Troubleshoot EMI Issues

Transcription

1 Effectively Using the EM 6992 Near Field Probe Kit to Troubleshoot EMI Issues Introduction The EM 6992 Probe Kit includes three magnetic (H) field and two electric (E) field passive, near field probes and an extension handle designed to locate sources of EM emissions. Near field probes are an essential tool for quick and efficient location of EMI sources that might result in compliance issues. The probes can be used with any spectrum analyzer or oscilloscope. Pairing the probes with the J2180A wideband preamplifier improves the sensitivity and noise floor of the measurements. Using near field probes and an oscilloscope you can: Identify the source and location of a radiation source. Identify characteristics of the EMI signals that can be used in far field compliance testing to determine the signal sources. Reduce expensive EMI lab test time by pre screening. Often the near field reading will be dramatically different than would be expected based on an extrapolation of the far field reading. Near field readings will seem higher than expected due to the presence of the reactive field; alternately, it may be lower than expected because of nulls created by the interference pattern set up near the unit. A reflection pattern is often established near the unit by the direct wave combining with the reflection off parts of the unit and other items in the vicinity.

2 Figure 1, Test setup for EMI Troubleshooting using the Picotest J2180A, J2130A, and the Lecroy 640zi. Magnetic (H) Field Probes The EM 6992 includes three H field probes of varying size and sensitivity: models EM 6993, EM 6994 and EM These probes are highly selective of the H field while being relatively immune to the E field. The H Field probe is used to detect current induced signals. The larger the probe, the greater the sensitivity, but this comes at the expense of selectivity. The smallest probe can be used to identify signals down to the PC board trace level. The J2180A preamplifier can be used to increase the sensitivity, especially for the smaller probes. Electric (E) Field Probes The EM 6992 includes two E field probes: the EM6997 stub probe and the EM 6996 ball probe.

3 The E Field probe is used to detect voltage induced signals. A good example might be the radiated noise from the switching voltage of a Mosfet. Due to the small sensing element, the stub probe has high selectivity and low sensitivity. Again the J2180A preamplifier is ideal for amplifying these weaker signals while the ball probe is much more sensitive at the expense of selectivity. The larger sensing element does not offer the highly refined definition of the source location which the stub probe allows, but it is capable of measuring much weaker signals. Probe Characteristics EMI Probe EM 6993 H Field Loop Probe EM 6994 H Field Probe E Field rejection EM 6995 H Field Probe E Field rejection EM 6996 E Field Probe H Field rejection EM 6997 E Field Probe H Field rejection SPECIFICATIONS 6 cm Loop E Field rejection: 41 db Upper resonant Frequency 790 MHz 3 cm Loop 29 db Upper resonant Frequency 1.5 GHz 1 cm Loop 11 db Upper resonant Frequency 2.3 GHz 3.6 cm Ball 30 db Upper resonant Frequency > 1 GHz 6 mm Stub Tip 30 db Upper resonant Frequency > 3 GHz Probe Selection Choosing the correct probe is determined by the following: Whether the signal is E or H: If the signal is primarily is E field, use the ball probe or stub probe. If the signal is primarily H field, use one of the loop probes. If unknown, try one of each and select the one that best picks up the signal. The strength of the signal: Select a probe that adequately receives the desired signal of interest. Respectively, the ball probe and the 6 cm loop are the most sensitive of the E field and H field probes. The stub probe and the 1 cm loop are the least sensitive. The frequency of the signal: If the signal is above 790 MHz, the 6 cm probe may resonate, and so a smaller probe should be used. See the upper resonant frequency listed for each probe above.

4 The physical size of the space where the probe must fit: If the desired probe cannot fit into the space available a smaller probe may need to be selected, and the use of a preamplifier may be required to improve the sensitivity. How closely you want to define the location of the source: Choose the probe that gets as close to the signal source as required. Select a large probe and begin outside a unit, then move closer to the source and switch to smaller probes to identify the location of the source. For example, the smallest probes should allow you to determine exactly which circuit on a printed circuit board is radiating. This kind of refinement provides the ability to stop the radiation at the source rather than shielding an entire unit. Approximate Field Level Determination Probe factor is defined as the ratio of the field presented to the probe to the voltage developed by the probe at the BNC output connector. By adding the probe factor (db(s/m)) to the voltage measured from the probe (db(uv)), the field amplitude (db(uv/m))may be obtained. All probes in the EM 6992 Probe Kit were originally calibrated and defined in a transverse electromagnetic mode (TEM) cell which presented a field with a characteristic impedance of 377 ohms. The H field probes only respond to the H field; however, the equivalent E field response is graphed. This transformation can be made if the field is assumed to be a plane wave with an impedance of 377 ohms. The reason for graphing the factors in this manner is to allow estimation of the field strength in the far field. If H field amplitude is desired, subtract db from the probe factor indicated on the graph, and the resultant data will be in the units of db(ua/m).

5 Using Sniffer Probes Typically, there are several possible sources for a given signal. To identify the particular one in question, use the sniffer probes. 1. From a set of loop probes of varying sizes, start with the largest, which is also the most sensitive. Begin several feet from the unit and look at the signal of interest. Search for the maximum and approach the unit along the line of maximum emission. If you cannot see a signal, use a preamplifier, such as the J2180A. A Spectrum analyzer has much better sensitivity than an oscilloscope FFT, but an oscilloscope with a preamplifier can often be used in place of a spectrum analyzer 2. As you near the unit, switch to the next smaller probe; this probe will be less sensitive but will differentiate the signal source more narrowly. Often the initial probing locates where the signal is escaping from the unit, indicating the point of escape from the housing. 3. Once inside the unit and inside any shielding, look for the source of the signal; use the smallest diameter H field probe or if looking for E field signals switch from the ball probe to the stub probe. Once the point of escape from the unit is located and the signal source is identified you may decide to improve the shielding or to suppress the source.

6 4. If it is possible disable portions of a circuit to aid in the determination of the location of the source, or to rule out circuits. For example, disable a line driver to see if the radiation is coming from the base unit or from a cable. When disabling parts of a circuit, use a sensitive probe and take readings several meters from the unit. Remove any connected scope probes or cables that extend out of the unit when making radiated readings; an attached scope probe can easily radiate and mask the real problem. Locating Radiation Causes and Sources Current related (h field) problems are normally associated with differential mode situations. Likewise, voltage (E field) problems are normally associated with common mode circuit situations. Solutions that are effective for differential mode are seldom effective against a common mode problem, so it is important to determine whether the offending source is H field or E field before attempting to determine a solution. References 1. Steve Sandler, Troubleshooting EMI: Use Versatile Instrument And Preamp To Search For Embedded Noise, May 2012, How2Power 2. Steve Sandler, Measuring EMI with the MDO, December Steve Sandler, Measuring Qi power with the MDO, Jan. 2012