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Introduction
Seminars
Services
EMI SnifferTM Probe
Other Products
Resources
Order

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APPNOTE
EMI SNIFFERTM PROBE - APPLICATION NOTE

The EMI SnifferTM Probe is used with an oscilloscope to locate and identify magnetic field sources of electromagnetic interference (EMI) in electronic equipment. The probe consists of a miniature 10 turn pickup coil located in the end of a small shield tube, with a BNC connector provided for connection to a coaxial cable.

The EMI SnifferTM Probe output voltage is essentially proportional to the rate of change of the ambient magnetic field, and thus to the rate of change of nearby currents.

The principal advantages of the EMI SnifferTM Probe over simple pickup loops are:

  1. Spatial resolution of about a millimeter;
  2. Relatively high sensitivity for a small coil;
  3. A 50 ohm source termination to minimize cable reflections with unterminated scope inputs;
  4. Faraday shielding to minimize sensitivity to electric fields.

The EMI SnifferTM Probe was developed to diagnose sources of EMI in switchmode power converters, but it can also be used in high speed logic systems and other electronic equipment.

SOURCES OF EMI

Rapidly changing voltages and currents in electrical and electronic equipment can easily result in radiated and conducted noise. Most EMI in switchmode power converters is thus generated during switching transients, when power transistors are turned on or off.

Conventional scope probes can readily be used to see dynamic voltages, which are the principal sources of common mode conducted EMI. (High dV/dt can also feed through poorly designed filters as normal mode voltage spikes, and may radiate fields from a circuit without a conductive enclosure.)

Dynamic currents produce rapidly changing magnetic fields which radiate far more easily than electric fields, as they are more difficult to shield. These changing magnetic fields can also induce low impedance voltage transients in other circuits, resulting in unexpected normal and common mode conducted EMI.

These high dI/dt currents and resultant fields can not be directly sensed by voltage probes, but are readily detected and located with the EMI SnifferTM Probe. While current probes can sense currents in discrete conductors and wires, they are of little use with printed circuit traces, or in detecting dynamic magnetic fields.

PROBE RESPONSE CHARACTERISTICS

The EMI SnifferTM Probe is sensitive to magnetic fields only along the probe axis. This directionality is useful in locating the paths and sources of high dI/dt currents. The resolution is usually sufficient to locate which trace on a printed circuit board, or which lead on a component package, is conducting the EMI generating current.

For "isolated" single conductors or PC traces, the Probe response is greatest just to either side of the conductor where Figure 1 the magnetic flux is along the probe axis. (Probe response may be a little greater with the axis tilted towards the center of the conductor.) As shown in Figure 1, there is a sharp response null in the middle of the conductor, with a 180 degree phase shift to either side and a decreasing response with distance. The response will increase on the inside of a bend where the flux lines are crowded together, and is reduced on the outside of a bend where the flux lines spread apart.

When the return current is in an adjacent parallel Figure 2 conductor, the Probe response is greatest between the two conductors as shown in Figure 2. There will be a sharp null and phase shift over each conductor, with a lower peak response outside the conductor pair, again decreasing with distance.

The response to a trace with a return current on the opposite side of the board is similar to that of a single isolated trace, except that the probe response may be greater with the Probe axis tilted away from the trace. A "ground plane" below a trace will have a similar effect, as there will be a counter-flowing "image" current in the ground plane.

Figure 3

The Probe frequency response to a uniform magnetic field is shown in Figure 3. Due to large variations in field strength around a conductor, the Probe should be considered as a qualitative indicator only, with no attempt made to "calibrate" it. The response rolloff near 300 MHz is due to the pickup coil inductance of 75nH driving the total terminated impedance of 100 ohms, and the mild resonant peaks (with a 1 M ohm scope termination) at multiples of 80 MHz are due to transmission line reflections.

PRINCIPLES OF PROBE USE

The EMI SnifferTM Probe is used with at least a two channel scope. One channel is used to view the noise whose source is to be located (which may also provide the scope trigger), and the other channel is used for the EMI SnifferTM Probe. The probe response nulls make it inadvisable to use this scope channel for triggering.

A third scope trigger channel can be very useful, particularly if it is difficult to trigger on the noise. Transistor drive waveforms (or their predecessors in the upstream logic) are ideal for triggering; they are usually stable, and allow immediate precursors of the noise to be viewed.

Start with the Probe at some distance from the circuit with the Probe channel at maximum sensitivity. Move the probe around the circuit, looking for "something happening" in the circuit's magnetic fields at the same time as the noise problem. A precise "time domain" correlation between EMI noise transients and internal circuit fields is fundamental to the diagnostic approach.

As a candidate noise source is located, the Probe is moved closer while the scope sensitivity is decreased to keep the Probe waveform on-screen. It should be possible to quickly bring the probe down to the PC board trace (or wiring) where the Probe signal seems to be a maximum. This may not be near the point of EMI generation, but it should be near a PC trace or other conductor carrying the current from the EMI source. This can be verified by moving the Probe back and forth in several directions; when the appropriate PC trace is crossed at roughly right angles, the probe output will go through a sharp null over the trace, with an evident phase reversal in probe voltage on each side of the trace (as noted above).

This EMI "hot" trace can be followed (like a bloodhound on the scent trail) to find all or much of the EMI generating current loop. If the trace is hidden on the back side (or inside) of the board, mark it's path with a felt pen and locate the trace on disassembly, on another board, or on the artwork. From the current path and the timing of the noise transient, the source of the problem usually becomes almost self-evident.

Some of the more common EMI problems are discussed in this short form ap-note to illustrate typical probe uses.

TYPICAL dI/dt EMI PROBLEMS

Rectifier Reverse Recovery

Reverse recovery of rectifiers is the most common source of dI/dt related EMI in power converters; the charge stored in P-N Figure 4 junction diodes during conduction causes a momentary reverse current flow when the voltage reverses. This reverse current may stop very quickly (<1 ns) in diodes with a "snap" recovery (more likely in devices with a PIV rating of less than 200V), or the reverse current may decay more gradually with a "soft" recovery. Typical EMI SnifferTM Probe waveforms for each type of recovery are shown in Fig. 4.

The sudden change in current creates a rapidly changing magnetic field, which will both radiate external fields and induce low impedance voltage spikes in other circuits. This reverse recovery may "shock" parasitic L-C circuits into ringing, which will result in oscillatory waveforms with varying degrees of damping when the diode recovers. A series R-C damper circuit in parallel with the diode is the usual solution.

Output rectifiers generally carry the highest currents, and are thus the most prone to this problem, but this is often recognized and they may be well snubbed. It is not uncommon for unsnubbed catch or clamp diodes to be more of an EMI problem. (The fact that a diode in an R-C-D snubber may need its own R-C snubber is not always self evident, for example).

The problem can usually be identified by placing the EMI SnifferTM Probe near a rectifier lead. The signal will be strongest on the inside of a lead bend in an axial package, or between the anode and cathode leads in a TO-220, TO-247 or similar type of package, as shown in Fig. 4.

Using "softer" recovery diodes is a possible solution, and Schottky diodes are ideal in low voltage applications. However, it must be recognized that a P-N diode with soft recovery is also inherently lossy (while a "snap" recovery is not), as the diode simultaneously develops a reverse voltage while still conducting current. The fastest possible diode (lowest recovered charge) with a moderately soft recovery is usually the best choice. Sometimes a faster, slightly "snappy" diode with a tightly coupled R-C snubber works as well or better than a soft but excessively slow recovery diode.

If significant ringing occurs, a "quick-and-dirty" R-C snubber design approach works fairly well: increasingly large damper capacitors are placed across the diode until the ringing frequency is halved. We know that the total ringing capacity is now quadrupled, or that the original ringing capacity is 1/3 of the added capacity. The damper resistance required is about equal to the capacitive reactance of the original ringing capacity at the original ringing frequency. The "frequency halving" capacity is then connected in series with the damping resistance and placed across the diode, as tightly coupled as possible.

Leakage Inductance Fields

Transformer leakage inductance fields emanate from between Figure 5 primary and secondary windings. With a single primary and secondary, a significant dipole field is created, which may be seen by placing the EMI SnifferTM Probe near the winding ends as shown in Fig. 5a. If this field is generating EMI problems, there are two principal fixes available:

  1. Split the Primary or Secondary in two, to "sandwich" the other winding, and/or:
  2. Place a shorted copper strap "electromagnetic shield" around the complete core and winding assembly. Eddy currents in the shorted strap largely cancel the external magnetic far field.

The first approach creates a "quadrapole" instead of a dipole leakage field, which significantly reduces the distant field intensity. It also reduces the eddy current losses in any shorted strap electromagnetic shield used, which may or may not be an important consideration.

External Air Gap Fields

External air gaps in an inductor, such those in open "bobbin core" inductors or with "E" cores spaced apart (Fig. 5b), can be a major source of external magnetic fields when significant ripple or AC currents are present. These fields can also be easily located with the EMI SnifferTM probe; response will be a maximum near an air gap, or near the end of an open inductor winding.

"Open" inductor fields are not readily shielded, and if they present an EMI problem the inductor must usually be redesigned to reduce external fields. The external filed around spaced E cores can be virtually eliminated by placing all of the air gap in the center leg. Fields due to a (possibly intentional) residual or minor outside air gap can be minimized with the shorted strap electromagnetic shield of Fig. 11, if eddy current losses prove not to be too high.

A less obvious problem may occur when inductors with "open" cores are used as second stage filter chokes. The minimal ripple current may not create a significant field, but such an inductor can "pick up" external magnetic fields and convert them to noise voltages, or be an EMI susceptibility problem.

Poorly Bypassed High Speed Logic

Ideally, all high speed logic should have a tightly coupled bypass capacitor for each IC, and/or have power and ground distribution planes in a multi-layer PCB.

At the other extreme, I have seen one bypass capacitor used at the power entrance to a logic board, with power and ground led to the ICs from opposite sides of the board. This created large spikes on the logic supply voltage, and produced significant electromagnetic fields around the board.

With an EMI SnifferTM Probe I was able to show which pins of which ICs had the larger current transients in synchronism with the supply voltage transients. (The logic design engineers were accusing the power supply vendor of creating the noise. I found that the supplies were fairly quiet; it was the poorly designed logic power distribution system that was was the problem.)

SPURIOUS CAPACITIVE RESPONSE

The electrostatic Faraday shielding of the EMI SnifferTM Probe is excellent, despite the open end of the Probe. (This end of the pickup coil is grounded to enhance shielding.) The spurious capacitive pickup is only about 4 fF (0.004 pF), based on the measured capacitive feedthrough. The effect is so slight that it can be ignored in virtually all applications; it is actually very difficult to measure, requiring a special test jig to minimize pickup of associated capacitive "displacement" currents in the vicinity, while maximizing the "true" capacitive coupling.

Due to the 75 nH inductive "loading" of the pickup coil the capacitive response is not proportional to the derivative of the voltage (dV/dt) but to the second derivative of the voltage up to about 200 MHz.

NOTES ON SIGNAL INJECTION

Some EMI sensing probes have also been used to test for EMI susceptibility by injecting a current into the probe and placing it near potentially sensitive circuits. This miniature probe is not particularly suitable for this application, due to its small coil and limitation to low drive levels; more than 1/8W input can cause damage.

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