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:
- Spatial resolution of about a millimeter;
- Relatively high sensitivity for a small coil;
- A 50 ohm source termination to minimize cable reflections with unterminated scope inputs;
- 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
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
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.
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
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
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:
- Split the Primary or Secondary in two, to "sandwich" the
other winding, and/or:
- 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.
