With the end of the FCC’s spectrum auction in January, it’s time to measure its damaging effects on broadcasters.

What damage, you ask? Interference to reception of your signal after the “Great Repacking.”

The FCC claims negligible interference is predicted after repacking. If so, that would be great, but wouldn’t your station manager like to know? The FCC says only co-channel and (first) adjacent channel signals cause interference. Others are worried about interference generated in DTV receivers. There are three such sources for this interference:

● Desensitization not due to receiver nonlinearity;
● Third order intermodulation (IM3) due to receiver nonlinearity; and
● Triple beats, which require three signals, also due to receiver nonlinearity.
● Triple beats are generated when there are three or more strong undesired (U) signals at the receiver’s input. They come from:
Fa + Fb – Fc, Fb + Fc – Fa and Fc + Fa – Fb

The easiest way to work with triple beats is to let Fa equal the lowest channel number, Fb equal the middle channel number and Fc equal the highest channel number.

For example, suppose the FCC allocates four UHF channels to your community. You have three U signals on channels 24, 27, and 29. Triple beats will be centered on channels: 24 +27 – 29 = 22. Also on channel 27 + 29 – 24 = 32 and Channel 29 + 24 – 27 = 26. If your station is to operate on any of these three channels, there may be interference; if your station is not going to operate on any of these three channels, you are in luck!

Fig. 1THE EFFECTS OF TRIPLE BEATS
Fig. 1 shows the number of triple beats generated by all triplets of U signals in the UHF band. There can be no triple beats where the number of U signals is less than three (that is where the name “triple beats” comes from). Fig. 1 shows this and it also shows that where the number of U signals in the UHF band is greater than four, the number of triple beats increases at an amazing rate. There will be some communities with 11 or more UHF channels; the number of U signals will be 10, 11, or more. The number of triple beats is 360, but there aren’t that many UHF channels so there will be multiple triple beats falling in some channels.

Fig. 2 shows that the spectrum of triple beats can be extremely wide. In fact, channels 37–49 won’t really be TV channels anymore, they will belong to broadband operators. Nevertheless, triple beats will fall in the broadband spectrum where formerly channels 38–51 have been auctioned. Channel 37, which was never assigned to a TV station, was—and will continue to be—used for medical telemetry, primarily in hospitals and radio astronomy.

Fig. 2 Let’s look at the column for channel 7 in Fig. 2. The lowest trace is for three U signals and is centered on channel 35. Channel 37 will have six triple beats, which will increase the noise floor of telemetry receivers.

That was the good news. The bad news comes where the number of U signals is higher. With 10 U signals, the number of triple beats falling in channel 37 is 20.

The noise power of a single triple beat is 4 dB greater than a IM3 product, assuming the same power is in the U signals. This 4 dB difference is due to the fact that three signals are involved while only two generate IM3.

Fig. 3 Fig. 3 compares noise power density per channel of triple beats with respect to the noise power of a single IM3 for a number of U signals. For seven U signals that means eight channels allocated to the repacking process. This exceeds 15 db for eight channels! It gets worse with more channels allocated. The best case, of course, is three U signals, but even then the difference is 5 dB. All of this suggests a strategy by which the channels allocated to a given community can be used to minimize such interference.

If the chief engineers in a given large DMA— where there will be more than five stations in the UHF band—were to agree on a repacking scheme to minimize the number of triple beats, perhaps the FCC could be persuaded to grant their petition. If that is to be done, now is the time to start such a program.

Note: My friend and colleague, Stanley Knight, generated the data and created the three graphics for this column. Readers might like to know that Stan I have never met face-to-face. Nevertheless he collaborates with me generating all my graphics. Thanks, Stan.

Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via email atcwr@bootit.com.

In the past, I have written about out-of-band (OOB) interference between ATSC signals in different bands: the low VHF band into a high VHF channel and a high VHF band ATSC into a UHF channel. Another example of OOB is where FM radio signals (88–108 MHz) interfere with reception of an ATSC signal on a high VHF channel. More recently, I have been concerned with interference to ATSC signals on a UHF channel being jammed by LTE signals in the 600 MHz band after repacking, which is no longer far into the future. But why worry about OOB from LTE signals? Why wouldn’t the FCC protect TV broadcasting from such interference? We will come back to this later in this column.

NEW LTE FILTER

Channel Master recently introduced a filter designed to eliminate interference to ATSC reception in the present UHF Band (470-698 MHz) from LTE signals in the 700 MHz band. You can find this filter (CM 3201) on the Channel Master website, www.channelmaster.com. Don Everist, president of Cohen, Dippell and Everist,P.C., a prominent Washington, D.C.-based consulting firm, ordered two of them and sent them to me.

Channel Master has dedicated four pages to the the new LTE filter on its web site, most of which are testimonials from satisfied customers around the U.S.

This is an unexpected development. OOB may have already arrived already in the U.S.; at least there is enough of a need now for such a filter that a prominent U.S. electronics firm invested in it.

In the case of LTE signal interference to ATSC reception, it results when a base station transmitter nearby goes to maximum power each time it seeks to establish communications with a subscriber. For 1ms, when the base station transmitter is establishing communication with another subscriber, interference may result. In our experience, up to 20 seconds can pass between consecutive interruptions, so testing for OOB requires more than the usual 20 seconds. This OOB doesn’t usually take out an entire frame of video; just a small portion of a frame may be affected/But who wants such interruptions? We did not listen to the program audio; with perfect hindsight, we would do so if ever we run more tests. I will not speculate whether disruptions to the picture or its audio will be most objectionable.

This is a 75 ohm Low Pass filter which according to the literature should be installed at the antenna and before any Low Noise Amplifier (LNA) if there is an LNA in the customer’s receiving system.

My colleague, Linley Gumm and I measured the frequency response of a CM 3201 filter.

Table 1: The results of measuring the frequency response of a CM 3201 filter Table 1 presents our results.

As my colleague and I had never heard of this OOB interference from an LTE signal in the 700 MHz Band, I would welcome readers having any experience with it to contact me (cwr@bootit.com). Specifically, please cite the channel number that was suffering interference and the distance between the transmitter and the receiver sites if it is known.

Fig. 1: A plot of a CM 3201 filter’s swept frequency response. Fig. 1 is a plot of this filter’s swept frequency response. The vertical orange line at 725.5 MHz is a recent artifact of the display of my spectrum analyzer. The insertion loss increases to about 60 MHz at 750 MHz. LTE signals may appear anywhere between 700 and 750 MHz.

My own observation is that this filter’s insertion loss above 600 MHz may cause problems where the desired signal is weak if the manufacturer’s recommendation is followed by placing the filter ahead of an LNA. In such cases, place the filter after the LNA.

Now, back to the FCC rules.

WHAT IS ‘HARMFUL’ INTERFERENCE?

In 2015, the FCC created a new and novel definition of harmful interference to the reception of DTV signals in the 600 MHz band.

The definition is found in Part 27 of the FCC Rules, specifically “Sub-Part N in the 600 MHzBand,” paragraph 27.1310: protection of broadcast television service in the 600MHz band from wireless operations:

“Licensees authorized to operate wireless services in the 600 MHz Band must cause no harmful interference to public reception of the signal broadcast stations transmitting co-channel or on the adjacent channel.

“Such wireless operations must comply with the D/U ratios in Tables 7–13 in OET Bulletin No. 74 if the 600 MHz licensee causes harmful interference to the public reception of a broadcast that is operation co-channel or on an adjacent channel, that licensee must eliminate the harmful interference.”

Please note that according to this definition, harmful interference can only exist between an undesired signal on the same channel as the desired ATSC signal or on either channel adjacent to the desired ATSC channel. If this is correct, then the licensee of the wireless system is not required to remedy OOB interference.

Furthermore, what about interference arising from third order intermodulation products generated by two undesired signals? Or from interference caused by multiple undesired signals desensitizing the affected receiver?

I should give credit to my colleagues, Linley Gumm and Stanley Knight for without whose help and encouragement, this column and the three IEEE Broadcast Society Newsletters we produced could never be written. The fall 2016 newsletter will soon be published. IEEE Broadcast Society members will receive our latest, and perhaps final work product. IEEE members who are not members of the IEEE Broadcast Society should look into joining this group (http://bts.ieee.org/).

Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via email atcwr@bootit.com.