Testing a comb generator for conducted emissions measurements

Article By : Kenneth Wyatt

This harmonic comb generator is designed expressly for testing conducted emissions measurements.

I’ve always been an advocate of harmonic comb generators for everything from validating emissions measurements to using them as controlled harmonic sources for various measurements and experiments (Reference 1).

There are many companies that make commercial comb generators for radiated emissions chamber validation and I’ve reviewed several (References 2 and 3). However, I know of only one company, Com-Power, that expressly makes one to validate conducted emissions measurements (Figure 1).

photo of the Com-Power CGC-510E conducted comb generatorFigure 1 The Com-Power model CGC-510E conducted comb generator validates conducted emissions measurements.

Why is this important? One example comes to mind. Early in my career with Hewlett-Packard, one of our divisions was planning to transfer an RF generator product line over to us at the Colorado Springs division. For all product transfers, we would routinely double-check the EMC performance to ensure continued compliance before marketing for sale.

To our surprise, the unit was failing the conducted emissions test by over 20 dB! Now, conducted emissions was typically a fairly benign test, as all our products used good line filtering modules, so failing this test was unexpected.

When we asked the other division for test data, it indicated it was passing with good margins, so something was obviously amiss. Ultimately, it was discovered that the other division had used improper calibration factors in their measurement software.

About this same time, we HP EMC engineers were aware of reported large variations in radiated emissions test results from site to site and had started round-robin testing using the same calibrated battery-operated RF source. This helped expose some test setup and measurement software issues and helped convince management to start standardizing the way radiated emissions were measured across the company.

Today, with most EMC test labs complying with the IEC 17025 standard for quality management, we’re required to validate tests prior to formal compliance testing. For radiated emissions, we have a variety of comb generators. Com-Power’s harmonic comb generator nicely fills a need for conducted emissions.

The CGC-510E conducted comb generator

Com-Power’s CGC-510E harmonic comb generator is designed expressly for testing conducted emissions measurements and simply plugs into the LISN port. It is a small (5×2.2×2.2 inches) box and comes with instructions and a battery charger. It produces stable harmonics at either 100 or 500 kHz spacings, which vary from 60 to 80 dBμV. Another model (CGC-255E) provides steps at 50 or 250 kHz. Amplitudes are specified as less than ±1 dB. A calibration chart from 150 kHz to 115 MHz is provided. Figure 2 shows a display of 500 kHz harmonics from 10 kHz to 30 MHz.

graph of typical display harmonicsFigure 2 This graph shows a typical display of harmonics at 500 kHz spacings, from 10 kHz to 30 MHz.

Battery life seemed excellent during the extended testing I performed. A green LED indicates a good battery level and a red LED indicates charging is required. An internal voltage regulator assures a stable operating voltage.

Evaluation testing

I set up a temporary measurement area using a layer of aluminum foil taped down to a flat surface. The comb generator was measured using two different LISNs with a Siglent SSA 3032X spectrum analyzer. The LISNs used were the Tekbox TBLC08 and an older EMCO 3810/2, and both were bonded to the foil and plugged into a Solar Electronics isolation transformer. I recorded both line and neutral voltages, but for simplicity, only plotted the line voltage, as the two readings were virtually the same. Figure 3 shows the test setup.

photo of the comb generator test setupFigure 3 This was the basic test setup for evaluating the comb generator (isolation transformer out of the picture).

Figure 4 shows the results of the comparison measurements of the harmonic combs. Both LISNs measured very close to each other out to 30 MHz.

graph comparing LISNFigure 4 This graph compares the measured harmonics (line output) from the Tekbox TBCL08 (red) and the EMCO 3810/2 (green) LISNs to the calibration chart provided with the comb generator (black).

Using the Com-Power comb generator to validate your conducted measurement setup is quick and easy. Just plug it in to the LISN, make a quick frequency sweep, and ensure your system is measuring correctly prior to performing compliance testing. Using this prior to formal testing also fulfills the validation requirement of the IEC 17025 quality management standard.

This article was originally published on EDN.

Kenneth Wyatt is president and principal consultant of Wyatt Technical Services.

References

  1. Wyatt, Harmonic comb generators are useful tools, Interference Technology.
  2. Wyatt, Review: The AET USDS spherical harmonic comb generator, EDN.
  3. Wyatt, Review: Picotest J2150A harmonic comb generator, EDN.

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