Page 3
ETS-Lindgren Inc. reserves the right to make changes to any product described herein in order to improve function, design, or for any other reason. Nothing contained herein shall constitute ETS-Lindgren Inc. assuming any liability whatsoever arising out of the application or use of any product or circuit described herein.
Table of Contents Notes, Cautions, and Warnings ..........vi Safety Information ..............vi 1.0 Introduction ................7 Magnetic (H) Field Probes ................8 Electric (E) Field Probes ................8 Ball Probe ................... 9 Stub Probe ..................9 Standard Configuration ................9 Optional Items (Sold Separately) ..............
Page 5
Signal Demodulation ................. 29 Examples ..................30 Using Sniffer Probes ................. 32 Diagnosing Radiation Causes ..............33 Common and Differential Mode Current Flow ........35 Differential Mode Techniques ............40 Common Mode Techniques .............. 42 Pre-Screening Alternate Solutions ............43 Evaluating Alternate Solutions ............
Notes, Cautions, and Warnings Note: Denotes helpful information intended to provide tips for better use of the product. Caution: Denotes a hazard. Failure to follow instructions could result in minor personal injury and/or property damage. Included text gives proper procedures. Warning: Denotes a hazard.
1.0 Introduction The ETS-Lindgren Model 7405 Near-Field Probe Set includes three magnetic (H) field and two electric (E) field passive, near-field probes designed for use in the resolution of emissions problems. The Model 7405 provides a self-contained means of accurately detecting H-field and E-field emissions, and includes a 20 cm extension handle to provide access to remote areas in...
Magnetic (H) Field Probes The Model 7405 includes three H-field probes of varying size and sensitivity: models 901, 902, and 903. These probes are highly selective of the H-field while being relatively immune to the E-field. Each H-field probe contains a single turn, shorted loop inside a balanced E-field shield.
ROBE The shaft of the model 904 ball probe is constructed of a length of 50 ohm coax. The coax is terminated with a 50 ohm resistor in order to present a conjugate termination to the 50 ohm line. The center conductor is extended beyond the 50 ohm termination and attached to a 3.6-cm diameter...
Optional Items (Sold Separately) • Preamplifier, including wall-mounted power supply (115 VAC or 230 VAC available) ETS-Lindgren Product Information Bulletin See the ETS-Lindgren Product Information Bulletin included with your shipment for the following: Warranty information • • Safety, regulatory, and other product marking information Steps to receive your shipment •...
2.0 Maintenance Before performing any maintenance, follow the safety information in the ETS-Lindgren Product Information Bulletin included with your shipment. Maintenance of the Model 7405 is limited WARRANTY to external components such as cables or connectors. If you have any questions concerning maintenance, contact ETS-Lindgren Customer Service.
Page 12
This page intentionally left blank. Maintenance www.ets-lindgren.com...
4.0 Operation Before connecting any components, follow the safety information in the ETS-Lindgren Product Information Bulletin included with your shipment. Typical Configuration Choose the appropriate probe from the Model 7405 Near-Field Probe Set. See Probe Selection on page 16. Connect a coaxial cable from the probe to the signal analyzing device; typically, an oscilloscope or spectrum analyzer.
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.
• 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.
Page 18
This page intentionally left blank. Operation www.ets-lindgren.com...
5.0 Typical Performance Factors The following graphs represent typical measurement. Individual probe results may vary. Probe performance factor is defined as the ratio of the field presented to the probe to the voltage developed by the probe at the BNC connector, PF = EN. By adding the performance factor to the voltage measured from the probe, the field amplitude may be obtained.
6.0 Common Diagnostic Techniques Before connecting any components, follow the safety information in the ETS-Lindgren Product Information Bulletin included with your shipment. Obtaining accurate, repeatable results from EMI testing requires a carefully established and characterized test setup, usually an open field test site or a shielded room.
Page 28
The various specifications are given in the frequency domain, so there are many dBuV at a particular bandwidth over a given frequency range. However, most EUT operations are characterized in the time domain: 150 ns memory access time, 300 V/ms slew rate, and so on. This section presents a technique that will aid in linking emissions with the signals that create them.
IGNAL EMODULATION To demodulate a signal: Set the spectrum analyzer for a 0 Hz frequency span and tune to the signal of interest. This essentially changes the spectrum analyzer into a tuned receiver and makes the display a frequency filtered oscilloscope.
Obtain a clear picture of the signal produced on the oscilloscope. You now have a good representation of what you are looking for when you start sniffing with the probe. Produce scope photos of the demodulated trouble frequencies and then use the sniffer probes to look for similar signals in your equipment.
Page 31
If the problem is caused by a rise or fall time, you may be looking for a waveform component which is between a wavelength and 1/8 of a wavelength of the radiating frequency. Example: In the 208 MHz example a wavelength is 1/13 of the 16 MHz clock;...
SING NIFFER ROBES Typically, there are several possible sources for a given signal. To identify the particular one in question, use the sniffer probes. 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.
Diagnosing Radiation Causes A small sniffer probe can help diagnose the cause of an electromagnetic interference problem. This section addresses using sniffer probes for a rough estimate of field impedance, which is used to diagnose the radiation physics of a given situation.
Page 34
Radiation is caused by an instantaneous change in current flow, causing a magnetic field, or by an instantaneous change of a potential difference, causing an electric field. Experience has shown a high degree of correlation between magnetic fields with differential mode current flow. Although a change in voltage will cause a change in current and vice versa, one of these vectors will predominate.
OMMON AND IFFERENTIAL URRENT From the local perspective of the unit, this is a common mode situation; EMC/EMI problems may be classified principally as current-related or voltage-related. Current-related problems are normally associated with differential mode situations. Likewise, voltage problems are normally associated with common mode circuit situations.
Page 36
To review the physics of the situation: In a far-field that is more than about one wavelength from the source, the ratio of the E-field and H-field components to the propagating wave resolve themselves to the free space impedance of 377 ohms.
Page 37
Distance: A to B = Propagating Field: –3.52 dB –6.02 dB –9.54 dB Reactive Field: –10.57 dB –18.06 dB –28.63 dB After the source is identified, two or three angles of approach are measured. A typical situation would record two points at 0.5 meters and 1.5 meters from the source along two radials from the source.
Page 38
Differential mode data is generally well behaved. The amplitude • measured with the H-field probe will be significantly higher than that measurement with the E-field probe. Also, the H-field will drop off at a much faster rate than the E-field. •...
Page 39
A qualitative knowledge of the field impedance indicates how to approach the EMC/EMI design for the problem. By determining the dynamics of the radiating structure, it can be surmised what kinds of designs will be effective is solving the radiation problem. A primarily H-field problem signifies that current flow predominates.
IFFERENTIAL ECHNIQUES Some traditional differential mode techniques do not work in common mode situations When differential mode solutions are applied to a common mode problem; many of the techniques will prove ineffective. For example: • Reducing circuit loop area: The radiating signal is on the signal and return path, so this will be ineffective.
Page 41
• Reducing the signal voltage swing: This will be ineffective when the radiating potential is developed deep in the circuitry, not at the output signal driver. At times the radiating potential will be built up on the power or ground system through the additive effects of a number of gates.
OMMON ECHNIQUES Some traditional common mode techniques do not work in differential mode situations Once a common mode problem is determined, use techniques which have a good potential for success. Start by analyzing the ground and power distribution system. Understand what RF impedances these systems present, and then reduce the excessive impedance.
• Increasing decoupling of power to ground. Reduce lead or trace inductance by reducing their length or making • them wider. Inserting ground and power grids or planes. • • Shielding, using a ground separate from signal ground. Relocating I/O cables to a lower impedance area on the ground •...
Page 44
Evaluating various solutions requires great skill and awareness, and it is in this area that the far-field/near-field effects can be the most misleading. The E-field and H-field vectors are initially determined by the source impedance. As you move away from the source, these vectors increasingly balance until the radiating field is isolated as a plane wave with a characteristic impedance of 377 ohms.
Page 45
The probe becomes part of the circuit during near-field measurements. There is capacitance and inductance between the circuit being measured and the probe with the associated cabling. The probe will re-radiate the received field, altering the field being measured. However, technical imprecision does not necessarily eliminate a method. Sometimes an attenuation of the field strength in the near-field will translate into an attenuation of the far-field reading.
VALUATING LTERNATE OLUTIONS There are two approaches that yield good results when evaluating alternate design solutions: The first step in each procedure is to choose a set of points; for example, two to six points. Since the object is to determine what the far-field results will be, most of the points should be one to four meters away.
Page 47
The more distant measurement points may lose the signal into the system noise; a given solution may only redirect the beam. Especially with narrow beam problems, solutions frequently only shift the beam so that it radiates in a different direction. After measurement points are chosen, baseline the unit by measuring each point with an E-field and an H-field probe.
Page 48
The technique was improperly applied. An outside factor is involved, such as a second source radiating at the same frequency. Example: A solution that worked in the lab and on the range before 10:00 AM failed later in the day. Analysis revealed that the rise in temperature was affecting the values of decoupling capacitors, making them less effective at higher temperatures.
Need help?
Do you have a question about the ETS-Lindgren 7405 and is the answer not in the manual?
Questions and answers