Introducing
The Testability Director Version 3.2
 

Making Electric Field Measurements with E-Field Sensors

 By:  

You will be linked directly to the listings you select below after you have  logged in. If you are not logged in, go to http://www.besttest.com/Register/disclaimer.cfm 

This issue's focus is Sensors. 
You can view and add to our existing list of Test Products/Services, Test Literature, Test Definitions, Test Vendors, containing "Sensor"

Come to the tutorial

Cost Effective Tests Using ATE, DFT and BIST

by Louis Y. Ungar at APEX on February 18, 2007 

By:  

Electromagnetic field probes (field sensors) are an invaluable tool for engineers who perform electromagnetic compliance (EMC) measurements and tests.  Available since the 1980’s , they are now in common use at labs worldwide.

This article discusses how to select and use electric field (E-field) probes used in the feedback loops of immunity test setups, and wherever field strength values need to be measured.  

The Difference between Probes and Antennas

Probes can be broadly thought of as antennas, but they are different in a number of aspects. Antennas transmit or receive electromagnetic signals or power with maximum coupling to the electromagnetic field.  As a result, antennas will typically perturb the electromagnetic field.  Probes on the other hand, are designed to measure the electromagnetic field with minimal field perturbation.  To do this, they need to be as physically and electrically small as practical to minimize coupling with the field of interest. The E-field sensor in a probe is usually an electrically small dipole coupled with a diode.

Isotropic Probes

For most EMC test applications non-directional or isotropic probes are the best choice. They offer more accurate measurement and are less critical about their orientation in the field of interest.  This results from the geometry used in their construction.  Three independent broadband sensing elements are placed at right angles to each another in an X, Y, Z configuration. Each element’s output is measured, the vector sum is determined, and the result is the total field value.

A three-axis probe provides flexibility. Ordinarily, the probe would be programmed  to calculate and send root mean square (RMS) values and then use this measurement to control the field inside a test chamber in a feed back loop to and amplifier.  But for a characterization, the probe can be programmed to send readings from each of the three axes. With that data, the field polarization as well as magnitude can be calculated.

Isotropic Deviation

Isotropic deviation (isotropy) describes how well a probe measures field intensities without concern for the polarization of the field. A lower deviation is always better. The best probes typically have an isotropic deviation of 0.5 dB or less.

Manufacturers typically specify isotropic deviation at a particular frequency, but this can be misleading. The reason is the value given can vary with changes in frequency, usually increasing as the frequency increases.  

Minimize the effect of isotropic deviation by knowing the specifics of your test setup, and take measurements from each of the probe’s sensing elements.  For example, if you know the polarization of the field, you can position the probe so that one of the elements is aligned with the field.  Measuring the field strength is then a matter of making a measurement from that element. The measurements from the other elements tend to be very small and can usually be ignored.

Dynamic Range and Linearity

Dynamic range is the maximum to minimum power level or field intensity that can be sensed by the probe over its specified frequency range.  Measurements are expressed in dB and represent a logarithmic power or voltage ratio.  For example, a 10:1 field strength ratio is 20 dB.   Check the manufacturer’s specifications against your requirements and make sure you have some headroom as a buffer.  

Linearity is a measure of how accurately the probe indicates actual values over its dynamic range. Manufacturer’s specifications of ± 0.5 dB are common. But consider whether the specification is referencing the reading or the full scale. For example at 0.5 dB, if you’re making a 1 V/m reading with a probe that has a 10 V/m range, and the linearity specification is referenced to the reading, then the maximum linearity error is 60 mV.  On the other hand, if the specification is referenced to full scale, the linearity error is ten times that, or 600 mV – a big difference!

Frequency Response

A probe’s response will not be perfectly flat across its entire frequency range. Manufacturer’ compensate by providing calibration data for their probes. If the calibration contains many data points across the probe’s frequency range, it will be easier to compensate for frequency variations with greater precision. However, if the calibration data provided is not sufficient for your application, it might be necessary  to pay extra for a more detailed frequency response plot for your particular application.  The calibration factors can often be entered into the control program and factored into the system calculations.

Response Time

Typical EMC susceptibility tests sweep the test signal across a specified frequency band. Because E-field probes are used in the control loop to regulate the field intensity, their response time is a factor in how quickly you can sweep the test signal. Probes used should be able to sample more than 50 times per second. Faster response time will save time and let you finish the test more quickly.  At the highest sample rates (now in excess of 200 readings per second) the limiting factor may be the ability of the readout device or computer to accept and process the data.

Probe response time is not the only factor determining how fast you can sweep the test frequency. Other factors include the settling time of the signal generators and amplifiers you use to generate the field. Take these factors into account when writing a test program to ensure that the field is stable before performing tests. If your application includes mode stirred testing in a reverberation chamber, sampling speeds greater than 50 times per second will be required.

Measurement Range

For some applications, it’s important to use a probe with a single measurement range. In an environment with a constantly fluctuating field (such as a reverberation chamber) changing ranges can be time consuming or impossible. This effectively reduces the probe’s sample rate.  If the length of the test is dependent on the sample rate of the probe, test times will increase.

Size Does Matter

Small probes will perturb the field less. A probe will begin to perturb fields when the dimensions of its housing approach ¼ wavelength of the frequency of interest. Some manufacturers have been able to miniaturize their probes by having the signal conditioning performed outside of the probe.  Properly done, the size of the probe can be reduced without sacrificing accuracy.

Conclusion

While not an exhaustive list of every possible consideration, this article has provided a good basis to select, evaluate and use E-field probes.

• If your friend forwarded this newsletter to you, please register as a member and receive The BestTest Newsletter -- absolutely free!
• If you wish to update your news preferences or cancel the subscription, please unsubscribe.
• If you have any questions, please email experts@BestTest.com

Get the widest selection of test related books and software at the BestTest Online Store.

Find any text on test. Search the topic below:

Search Now:

Reach the thousands of test professionals we mail to.
Place Ad here.