Locating and measuring sources of EFD and ESD burst interference - Embedded.com

Locating and measuring sources of EFD and ESD burst interference

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

Most electronic products must pass the electrical fast transient (EFT)and electrostatic discharge (ESD) standard tests. EFT and ESD are twotypes of burst interference. The monopulse peak voltage of an EFTsignal can reach 4kV with a rising edge of 5ns. The peak voltage of anESD signal, on the other hand, can be as high as 8kV under a contactdischarge test with a rise time of 1ns. Both types of burstinterference have an outburst, high voltage and broad spectrum.

It is thus important to supervise the working mode of equipmentsimultaneously when coupling the interference pulse from outside toinside the equipment. If the equipment does not pass the standard test,the test itself cannot offer any feasible solutions.

To locate the position in which the equipment under test issusceptive to burst interference, a signal measurement should be madein advance. However, using an oscilloscope can cause changes in theinternal interference. Connecting a metal conductor probe to theoscillograph may generate an extra path of interference current,affecting the test result and making it difficult to locate ESD- orEFT-related problems.

EFT/ESD interference
The relevant burst interference should be imposed on the power cord,signal line or casing when conducting an EFT/ESDimmunity-to-interference test.

Figure1: Connecting a metal conductor probe to the oscillograph may generatean extra path of interference current.

The interference current may flow into the EUT's internal circuitvia cables or casing and weaken its performance. The interference-prooffeature of electronic products depends on the IC's susceptibility andthe PCB design. The position in which the circuit is susceptive toEFT/ESD signal can be accurately located. Sensitive spot formations maybe caused by the shape of the GND/VCC or the type of IC used. EFT/ESDoccurs when the interference current flows to the low-impedance powersystem.

The interference current can directly enter the GND system andcouple out from another position via line connections – it can enterthe GND system and couple out in electric field mode (field beam) viaelectric induction with a metal block (e.g. casing). As shown in Figure 2, below , the interferencepulse electric current (I) penetrates the PCB via cables or capacitors.

The interference current generates electric field interference(strength of electric field E) or magnetic field interference (magneticintensity B ). Magnetic pulsefield (B ) or electric impulsefield (E )primarily affects PCB. Sensitive spots are generally caused by eitherthe magnetic field or the electric field only. I flows into theequipment. A part of the electric current (I A ) leaves the EUT due tothe existence of a bypass capacitor and the reduction of the internalinterference current (Ii ).

The magnetic field B generatedby the interference current Ii mayaffect some circuit modules within several centimeters around it. Ingeneral circuit modules, only a few signal lines are susceptive to B field. Note that the magnetic field is generated not only by theinterference electric current from the power cord and rows of cables,but also by the current path of the bypass capacitor (C ) and theelectric current of the internal GND and V CC , which can expand theinterference range.

Figure2: The interference pulse electric current penetrates the PCB viacables or capacitors.

The interference current flowing through the power system generatesvery intense frequency spectrum EMF, which can interfere with IC orsignal lines within several centimeters around it. The presence ofsusceptive signal lines or devices such as reset signal, chip selectsignal or crystals around the interference current path may cause thesystem to become unstable.

Generally, there is only a limited amount of sensitive spots. Youcan detect them and improve the interference-free performance ofproducts by applying some solutions.

To determine the reason behind an EUT's failure in an EFT/ESD test,we should first locate the current path of the burst interference inthe system and find out which susceptive signal lines and devices(sensitive spot) exist around the path. We should then improve theearthed system to change the current path, or shift the position of thesusceptive signal lines and devices to solve the EFT/ESD problems atthe least possible cost.

E1 disturbance immunity
Since the EFT/ESD signal has a high voltage and wide frequencyspectrum, it is very difficult to measure the path of interferencecurrent using a traditional oscilloscope and spectrum analyzer. TheE1disturbance immunity development system is used exclusively inmeasuring EFT/ESD and solving relevant problems.

The E1 system is composed of four essential parts: the SGZ21, whichsupplies the signal source of burst interference; the MS02 transientmagnetic field probe; the electric- and magnetic-field source group;and an IC sensor.

SGZ21. Thecontinuous interference pulse generated by SGZ21 is similar to theinterference pulse of EFT/ESD, with a rising edge time of 2ns and afalling edge time of 10ns. The energy contained in the pulse is smallerthan the pulse of EFT/ESD, thus it can couple the interference to theEUT's internal PCB without destroying any components.

Figure3: Figure shows the composition of the E1 disturbance immunitydevelopment system.

The pulsing signal output by SGZ21 has continuous varying impulseamplitude with a peak value from 0 to 1,500V in statistical averagedistribution. This method allows for a particularly fast assessment ofthe disturbance immunity of modules in conjunction with sensors. Acounter with an optical input is included in the generator for thispurpose.

The SGZ21 has an electrically isolated (no PE reference) andsymmetric output. The disturbance pulses are capacitively coupled outwith alternating polarity. This allows a variety of coupling-invariants such as:

1. Disturbance current canbe fed through sections of the GND system by directly connecting bothgenerator outputs with the EUT's GND system.

2. Disturbance current canbe injected into GND and returned via VCC.

3. Disturbance current canbe injected into the primary side of atransformer, transmitter or optocoupler and returned via the secondaryside.

Figure4: The SGZ21 imposes interference on the PCB and measures theinterference current path.

Transientmagnetic field probe MS02 . The interference current flow throughthe EUT generates the magnetic field. Distribution of interferencecurrent can be determined from the intensity and direction of themagnetic field.

The MS02 transient-state magnetic- field probe, which is passive,can detect the relative strength of the outburst EMF from the readingof the counter if it is connected to the input of the SGZ21 counter viaoptical fibers. If a pulse of the magnetic field is detected, MS02 willgive out a light pulse which can be read from the SGZ21counter; thereading value is in proper proportion with the average magneticintensity.

Since only magnetic lines of force going through the probe loop canbe detected, we can rotate the probe to obtain a maximum count value atwhich we can detect the direction of magnetic lines of force. After theconfirmation of the direction of magnetic lines of force, it will beeasy to detect the direction of the interference current.

Electric- andmagnetic-field source group. Changing the electric output ofsignal source to EMF. The field probe group includes magnetic- andelectric-field probes of different shapes and sizes with minimumresolutions of less than 1mm. They can be connected to the output ofSGZ21. To locate the exact position of a circuit's sensitive spot, weshould inject an interference- to-earth system, power system,integrated circuit, pin, discrete component, key electric wiring cablesand connectors.

After finding the path of interference current using SGZ21 signalsource and a transient magnetic field probe, we can also determinewhether there are susceptive signal lines or devices using the probe;if it is a device, we must check which pin of the device it is. Somefield sources in E1 can generate a magnetic field, while others cangenerate an electric field, so we can confirm which kind ofinterference field the EUT is susceptive to.

IC sensor forsusceptibility detection. The IC sensor is specially designedfor evaluating the validity of modifications to a circuit. It caninduce the effect of burst interference on digital logic and can eventransmit the interference condition to the counter via optical fibers.The E1 disturbance immunity development system is configured withdifferent kinds of EMC sensors, which can detect the interferencesituation of the key signal line, power, earth, cables and connectorson a PCB.E1 interference-proof
The E1 disturbance immunity development system emulates theinterference process inside the equipment. Different methods areapplied to directly inject the interference current, electric field andmagnetic field to electronic modules. After the electromagnetic weakspot of the PCB is determined, we can understand the coupling mechanismand optimize the design.

Figure5: It is important to measure the interference-proof features of thetransient state magnetic field on key signal lines.

Since the system cannot give a compatibility test according to acertain standard, it is suggested that a standard interference-prooftest be conducted on the EUT to determine and analyze the source of theproblem. We can thus evaluate the modifications and improvements on thedesign at the product development site using E1. We can also redisplaythe functional faults under a standard interference-proof test, thenconfirm and evaluate the path-coupled I/O.

The following are general steps in measuring and locating EFT/ESDusing the E1 disturbance immunity development system:

Step #1: Check every circuit module of the EUT. This includes as a single blockof PCB, interconnection cables between PCBs and circuit functionmodules inside the PCB. Take a block of PCB or a part of a circuit anddirectly inject interference to the GND of this module.

Step #2: Injectinterference by connecting the two poles. Connect the twooutputs of SGZ21 signal source to the circuit module's GND separatelyto determine whether it is susceptive to the magnetic field. Ifexpected functional faults occur in the EUT under this mode, we canconclude that there exist some sensitive spots, which are susceptive tothe magnetic field around the interference current path between theGND's two nodes.

Step #3: Injectinterference under monopole connecting mode. Connect one outputport of the SGZ21 signal source to the GND of the circuit module andconnect the other to the EUT casing – electric field source can be usedto emulate the casing. We can then find out whether it is susceptive tothe electric field.

Under monopole connection, the functional faults may be caused bythe EUT and field source probe (in electric field) or the electriccurrent flow to the electric field generates a magnetic field, which iscoupled to the signal ring circuit and causes trouble.

To differentiate them, we must establish a very short connection oflow impedance between the EUT's GND and a metal object nearby toexclude the effect of the electric field; if the given functional faultdoes not occur, we can conclude that it is caused by either theelectric field or the magnetic field.

Step #4.Confirm the sensitive spot seen via broad fault location and thesusceptive feature of the circuit. You can measure the relativeintensity of the outburst magnetic field inside the EUT and thedirection of interference current via a transient state EMF probe.

Using this device, you can discover where the outburst magneticfield exists in the EUT, how the EUT's internal interference currentcomes about, whether the interference current field source afterdetecting trouble in the modules and measuring the current path. First,select the field source according to the previous measurement resultand decide whether to adopt the magnetic field or the electric field.

Step #5. Injectinterference to the EUT along the path of interference currentdirection according to the “current path” detected. Nine kindsof field source with different resolutions are equipped in the E1interference- proof development system.

When selecting a field source, do it from a large area to a smallarea; when selecting intensity, move the probe slowly and approach theEU, coming from afar, to confirm the position of the sensitive spot.

Step #6: Modifythe circuit to improve the EUT's interference- free performance .Advanced pulsation-rate measurement applied in the E1interference-proof development system can help in the prompt evaluationof the validity of circuit modifications. In pulsation ratemeasurement, the SGZ21 generator and transducer are required. SGZ21generates out-of-order pulsing signal with hypodispersion at peaklevel.

Thus, a generator is needed to ensure synchronization with thecounter. For instance, a transducer used inside the EUT to supervisesusceptive signal lines will give a light pulse once any interferenceis detected on the signal line. The SGZ21 counter will indicate thelight pulse. The count value of the light pulse detected in a completealternation signal sequence (1s) represents the position ofinterference threshold – the EUT's susceptibility.

If 11 pulses are detected in a complete alternation pulse sequence,the interference threshold is u1. It means that the area will sufferinterference if a burst interference with a voltage of u1 is injected.The interference threshold is u3 if three pulses are detected. Thefewer pulses detected, the better the module's design is. Onerepresentative application is to measure the waveform of the filter.

Inject the interference current generated by SGZ21 to the EUT,measure the interference signal to the EUT while reading the countvalue on the SGZ21counter. After modifying the filter to measure againand comparing the two measurements, you can see whether or not yourdesign modification is effective.

Step #7: Whenconducting an immunity test, quickly detect the functional faultsinside the EUT.   EUT troubles are often invisible fromoutside and are discovered after quite a while. For instance, even whenthe processor inside the EUT has been frozen, normal information isstill displayed on the screen. For effective trouble loca tion, it isnecessary to use the S31 sensor to provide relevant information on EUTfunctions.

We can use it to supervise post-position triggering signal of awatchdog open circuit and chip select signal to manage the EUT'sworking mode. The pulse counter can be used to determine whether theequipment is working normally.

You can also connect the optical fiber output to the optical fiberreceiver – the latter will change the optical signal to an electricalsignal – then connect it to the oscilloscope to make an observation andanalysis.Functional faults
The data stream in a bus system or interface can always reflect theoperating status of the system. Supervision via an oscilloscope orlogic analyzer, however, can be time-consuming and expensive. A fasterand easier way to determine a system's operating status is to use theSGZ21 counter.

The value on the counter changes, since the content of data ischangeable and the counter is out of step with the data packet.However, it can still reflect the EUT's working mode, allowingengineers to assess the working mode of the equipment.

For instance, after resetting the EUT, the recorded values representthe working mode when it is restarted. In this way, engineers can seewhether the EUT has been reset or resending is needed when transmittingdata. They can also detect other problems caused by interference duringimmunity testing.

Functional faults may arise if the interference pulse occurs in thekey stages of EUT programs (e.g. when data is being transmitted viainterface).The frequency of functional faults depends on the EUT'sstructure. Thus, to avoid or minimize functional faults in the EUT, wemust ensure that there is enough time and that the voltage level iscorrect when doing measurements.

We can take the counting values of the transducer as the referencefor susceptibility if we are installing a transducer inside the EUT,particularly in a position where the interference threshold has noconnection with time. This method is especially designed for evaluatingthe effects of filter and shield.

The IC has a different susceptibility to interference from that ofthe transducer. Thus, it is possible that as the interference voltageincreases, the EUT encounters an interference while the transducer doesnot, or vice versa. In this condition, we must establish a relation ofinterference threshold between the EUT and the transducer; the relativerelation is stable only when we modify the shield or filter.

It is difficult to measure outburst, high voltage and broad spectruminterference such as EFT/ESD using the traditional method. IfEFT/ESD-related problems are encountered in electronic products,engineers can solve them by relying on past experience. The E1disturbance immunity development system offers engineers a new andeffective measurement method that can help locate the sensitive spot ina circuit. Through design modifications, the EUT can pass relevant EMCstandard tests at the lowest possible cost.

Shen Xueqi is Chief Representativeat Compliance Direction Systems Inc.

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