Design Con 2015

LTE devices may interfere with cable TV

-August 07, 2014

Prior to June 13, 2009, terrestrial broadcast TV stations in the U.S. used analog signals and cable TV networksoften rebroadcast local TV stations on the same frequencies. Thus, it was possible for over-the-air signals to leak into home cables and splitters, causing interference on the analog signals coming from the cable network. That happened to me because I had a complicated setup with two VHS recorders, a splitter, and an A-B coax cable switch. The interference ended when the local VHF analog stations went digital and moved to higher frequencies.

After the DTV transition, the FCC auctioned off some of the former UHF TV frequencies, the so-called "700 MHz band," to cellular providers. Cable TV networks continued to use the vacated frequencies. And why not? After all, cables are shielded, right?

Not so fast. There have been reports of LTE phones interfering with consumer set-top boxes, causing video streams to pixellate and lose picture. Problems have also occurred with cable modems, resulting in dropped packets. A paper presented sat the 2014 IEEE EMC Symposium, Characterizing a Device’s Susceptibility to Broadband Signals: A Case Study by Jason B. Coder and John M. Ladbury from NIST and David F. Hunter of CableLabs, describes the problem and a method of immunity testing.

EMC immunity standards such as IEC 61000-4-3 specify a test procedure that covers the frequencies in question. But, current standards call for a swept CW (carrier wave) signal, which is clearly narrowband. LTE channels, however, are 10-MHz wide (Figure 1) and LTE-A signals will go to 20 MHz channels. So, just because a set-top-box or cable modem passed regulations, it could still be susceptible to interference from broadband signals.

Figure 1. Engineers used this simulated LTE to test for susceptibility on home cable-TV equipment.

The engineers devised a test using simulated LTE signals in a reverberation chamber. They tested several set-top boxes, cable modems, RF cables, and splitters. Figure 2 shows test results for eight set-top boxes tested at three center frequencies: 627 MHz, 711 MHz, and 819 MHz. The bars indicate the field strength at which each box failed. The definition of failure is, however, vague and subject to a consumer's tolerance. The paper discusses how the engineers decided on that's a failure and it shows more test results. Because the paper is U.S. Government work, it's not subject to copyright and we can make it available for download.

Figure 2. Test results showed failures occurring over a field-strength range of
40 dBmV/m to 80 dbMv/m.

Have you run into cable-TV interference? How did you solve it?

—Martin Rowe, Senior Technical Editor Circle me on Google+ Follow me on TwitterVisit my LinkedIn page

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