DTV Signals Mask Radiated Emissions

Digital TV broadcast signals add yet another ambient signal to the mix.

Martin Rowe, Senior Technical Editor -- Test & Measurement World, 2/1/2000

If you live in a major metropolitan area, then your local TV stations have already begun broadcasting in digital format. As of November, 100 DTV stations were on the air, and by 2006, all TV broadcasts in the US will be digital. While viewers may enjoy the digital television picture, EMI test engineers find DTV to be just one more aggravation that impedes their work.

Both the VHF and UHF bands are within the range of microprocessor clocks, so EMI test engineers already account for local analog TV signals when they are performing radiated emissions testing in an open-area test site (OATS). The DTV broadcasts, which currently operate in the 470 MHz to 806 MHz UHF band, add yet another set of ambient signals to the electromagnetic spectrum.

Figure 1. A spectrum plot of an analog TV signals shows distinct narrowband subcarriers. (Courtesy of Tektronix.)

Figure 2. The flat spectrum of a DTV signal consumes most of the 6-MHz channel. (Courtesy of Tektronix.)

 What makes DTV broadcast signals different from conventional TV? Plenty. Although both occupy a 6-MHz bandwidth, the analog TV signal (Fig. 1) has three distinct narrowband peaks—video subcarrier, color subcarrier, and audio subcarrier (from left to right). In contrast, the DTV spectrum (Fig. 2) is flat across the entire channel’s bandwidth, so it masks more radiated EMI than do the narrow peaks of analog signals.

So what can you do about radiated EMI measurements in the presence of ambient DTV broadcast signals? The obvious way around OATS testing in the presence of DTV signals is to leave the OATS—perform your testing in a semianechoic chamber. Unfortunately, few test labs have chambers large enough to perform EMI measurements at 10 m.

For OATS measurements, you have a few options. First, you should find out if you have any DTV broadcasters in your area. In the US, you can check the Web sites of the FCC (www.fcc.gov/mmb/vsd/ files/dtvonair.html) or the National Association of Broadcasters ( www.nab.org/Newsroom/PressRel/
DTVStations.asp). I suggest you also check the local DTV broadcaster’s Web site to see which programs are broadcast in DTV format and plan your tests around those programs. (Right now, many stations are broadcasting in digital format during evening prime time hours only, but that will gradually change as stations add more programs to their DTV schedules.) You can also check the FCC Web site (www.fcc.gov/oet/dtv/tvchfreq.html) for channel frequency allocations.

For precompliance tests, you can detect EMI by narrowing the resolution bandwidth of your spectrum analyzer or EMI receiver to, say, 1 kHz. The narrow bandwidth will effectively reduce the wideband DTV signal within that bandwidth.

You can’t narrow the bandwidth for actual compliance tests, because FCC Part 15¹ and CISPR 22:1997² require you to set the resolution bandwidth of your receiver to 120 kHz, says Roland Gubisch, chief engineer for EMC and telecom at Intertek Testing Services (Boxborough, MA). Yet, by reducing the resolution bandwidth for precompliance testing, you can determine whether your product is likely to pass compliance tests.

You can also try reducing the distance between your EUT and receiving antenna to boost EUT signals relative to the DTV background. Standards require testing at 3 m or 10 m for compliance tests, but you can—in limited cases—reduce the distance to 1 m or 2 m for precompliance testing of 3-m measurements. A scale factor of 1/d relates emission limits and distance, but you can’t apply it at low frequencies (those less than 30 MHz) or for large EUTs (see FCC Part 15 section 15.31(f) for details). The same scale factor applies to CISPR 22 measurements referenced to 10 m, but you can use the scale factor for Class B equipment only (see clause 10.2.1 of CISPR 22:1997).

If you do not have access to a semianechoic chamber and if the scaling of distance and bandwidth still does not yield the information you need, you do have another alternative: noise cancellation. This alternative requires two antennas, one near the EUT and another at a greater distance. The near-EUT antenna receives emissions at amplitudes high enough to detect them over the ambient signals. Then, you can measure the power of the emissions at the second antenna’s distance and use the result to perform a calculation to cancel the ambient signals. T&MW

FOOTNOTES

1. Title 47 of the Code of Federal Regulations, Chapter I, Subchapter A (General), Part 15, Radio Frequency Devices, Federal Communications Commission, Washington, DC, www.fcc.gov/oet/info/rules/. Test methods for Part 15 can be found in ANSI C63.4-1992, IEEE Methods of Measurement of Radio-Noise Emissions from Low-Voltage Electrical and Electronic Equipment in the Range of 9 kHz to 40 GHz, ANSI, New York, NY, www.ansi.org.

2. CISPR 22:1997 (EN55022:1998), Limits and methods of measurement of radio disturbance: characteristics of information technology equipment, CENELEC, Brussels, Belgium, www.cenelec.be.  Also see CISPR 16-1:1993 (latest edition: 1999), Specification for radio disturbance and immunity measuring apparatus and methods - Part 1. Radio disturbance and immunity measuring apparatus.