There are 42 symmetrical triplets in the FCC Table of Allotments, and 161 asymmetrical triplets. Of the two kinds, Murphy’s Law says that the more numerous kind are also the worst kind from an interference point of view and Professor Murphy is once again correct. Fig. 1 is the most compact triplet, but it and its sideband splatter occupy nine channels. Fig. 2 is another symmetrical triplet. Its sideband splatter extends from Channel 22 to 44. Can DTV signals on any of these channels be received?
THE BROADCASTER’S NEMESIS
(click thumbnail)Fig. 1: A symmetrical triplet of DTV signals in adjacent channels. Note: IM3 products extending over nine channels.The 141 asymmetrical IM3 in the FCC’s UHF channel plan are those where the offset from the middle channel to the others is less than 10 channels and the offsets are unequal. For example, in the triplet shown in Fig. 3, the lowest channel number is offset by two channels from the middle channel and the highest channel is offset by five channels below the middle channel. Such a triplet can be generalized as a 2, 5 triplet. If you know the distribution of third-order distortion products for any 2, 5 triplet you also know them for all the others. This is why I devised this classification scheme. A triplet of Channels 31, 33 and 38 would also be a 2, 5 triplet. A triplet of Channels 27, 32, and 37 would be a 5, 5. One hundred eleven of the triplets in the FCC channel plan have offsets of less than five, meaning that even with good RF selectivity receivers operating in the UHF band will not usually protect the mixer from overload. If most of the triplets did involve large offsets then improved RF selectivity would have been more effective to protect the mixer but alas, that is not reality we confront. I do not think the FCC deliberately chose close-spaced triplets over wide spaced triplets, but that is the way the process happened to work. After all, until you read this, there was nothing I know of in the literature on multiple undesired broadband signals causing interference.
The symmetrical triplets (shown in blue in Table 1) include the channel pairs whose third-order distortion products fall in what may be the desired channel (N or N+3K). These are the channel pairs of the form N+K, N+2K. For example the triplet of channels in Fig. 2: 30, 33 and 36 is of this form, K=3. In Table I this is the 3, 3 triplet shown in blue. It could as easily be the triplet 41, 44 and 47. They behave in the same way. The signals on Channels 30, 33 and 36 generate IM3 in Channels 27 and 39. Symmetrical triplets generate such interference as was reported by both the CRC and FCC Laboratories in 2007, which has been described in this column and can be reviewed by downloading previous columns from www.tvtechnology.com/rhodes.
(click thumbnail)Fig. 2: A symmetrical triplet of DTV signals on Channels 30, 33 and 36. Note: IM3 products extending over 21 channels.Readers can use these algorithms to determine the channels in which “Bee-Hives” of IM are centered which could cause maximum interference to their station. For example Chicago has four triplets: No. 1: 21, 27, and 29—a 6, 2 (asymmetrical); No. 2: 19, 21, and 27—a 2, 6 (asymmetrical); No. 3: 27, 29, and 31—a 2, 2 (symmetrical); and No. 4: 43, 45, and 47—also a 2, 2 (symmetrical).
In Table 1, let’s see where the IM3 generated by triplet No. 1 falls. This simple procedure has identified two Chicago channels subject to interference from this triplet and that Channels 19 and 31 may get interference from both IM3 and X-M. It has also found that one X-M generated by this triplet falls on Channel 27 of this triplet. However, there are a few “near misses” that is Bee-Hives are three channels wide so a local station might be side-swiped by a Bee-Hive. The noise in each side channel of a Bee-Hive is only 6 dB below the noise power in the middle channel which mitigates the effect of interference from noise in either side channel. I hope that readers will use the above model to calculate the interference generated by the triplets allocated in their own community.
Possible interference from other DTV signals on pairs of channels can also be investigated in this way, just ignore the Fc terms in the above algorithm for IM3.
NOW THE ‘KILLER’ TRIPLETS
(click thumbnail)Fig. 3: An asymmetrical triplet of DTV signals with seven “Bee-Hives” of IM3.Fig. 3 shows one of the many asymmetrical triplets. Notice that the noise floor in Fig. 3 from Channel 22 up to Channel 46 is elevated by third-order distortion products. Reception of DTV signals in this range may be affected by this asymmetrical triplet. I wonder how the receivers in the proposed unlicensed devices are going to work with such high noise levels over this broad range of channels. Perhaps the consortium advocating the sharing of broadcast spectrum would tell you that their receivers will not generate third-order distortion products. That means they have found better frequency agile receiver front-ends (tuners) than DTV receivers have. If so, receiver manufacturers will simply have to use these better front-ends too. But do they exist? Does the consortium understand such RF problems?
By now you may be wondering why I have not mentioned interference from unlicensed devices (UD) operating on TV channels as you must already know the FCC fully intends to allow on or before Feb. 18, 2009. The reason is that this column deals with DTV-DTV interference. Such interference is constant, either you don’t receive a given station (or several stations), or you do enjoy reliable reception. That may change when: NTSC transmissions by full power stations end on Feb. 17, 2009, and DTV stations have shifted to their permanent channel assignments and maximized their facilities, i.e. increased their power. As you know, this column has been concerned with DTV-DTV interference due to third-order IM generated in receivers. Well-designed receivers will not suffer such interference. But such receivers are scarce according to the FCC Laboratory tests.
(click thumbnail)Table 1: Let’s see where IM3 generated by triplet No. 1 falls. Channels in red are allotted to Chicago.Triplets present two interference problems: 1) with three undesired signals of equal power, the total power overloading the receiver (usually the mixer) is 4.8 dB higher that the power of each received signal. That was the good news, the bad is that this 4.8 dB increase in total signal power results in a 14.4 dB increase in the third-order IM products. 2) Even worse, is the spectrum spreading of the IM products. In Fig.1, the three contiguous channels and the sideband splatter occupy only nine channels. In Fig. 2, (30, 33 and 36) the spreading extends over 19 channels, while in Fig. 3 (30, 32 and 37) the spreading extends more than 25 channels. This is not the worst case. The algorithms given in this column above will allow you to determine the spectrum spreading of triplets that you may be interested in. Spectrum spreading means more and more channels are subject to interference especially by triplets with large offsets. I counted 111 triplets with offsets of five channels each.
When, and not if, UD become widely deployed, additional interference is predicted by Dr. Oded Bendov and me, who are working independently and sometimes in collaboration. Today, in writing this column, I was impressed with the number of channels that can cause interference to DTV receivers like those recently tested by the FCC. Look again at the listing whose heading is Bee-Hives. You now know that one triplet of channels allocated to Chicago can generate co-channel interference in receivers tuned to either Channel 19 or 31, and to a lesser extent channels surrounding the “worst case” channels. There are other combinations of channels which can create IM3 which appears on a local DTV channel, causing co-channel interference. Multiple signals can generate interference on the channels listed above. Reception of DTV programs being broadcast on these channels is subject to interference from such IM products. An UD may operate on a channel which, in combination with one or more DTV signals or another UD signal generates “IM3 which just happens” to fall in your DTV channel. Too bad!
In my last column, “Single Distorted DTV Signals and Pairs” in the Dec. 19, 2007, issue of TV Technology, Fig. 2 was the same image as Fig. 1. For the correct Fig. 2, please refer to the article here.
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