More Interference Due To Signal of Triplets - TvTechnology

# More Interference Due To Signal of Triplets

Had the FCC been able to anticipate this problem of receiver-generated IM3 (noise) in ���vacant channels,” perhaps it could have established minimum performance standards for consumer DTV receivers.
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Triplets of undesired signals on sets of channels such as 30, 33 and 36, which comprise a symmetrical triplet, create a significant increase in the noise level on Channels 23 to 44—a total of 21 channels—as is shown in Fig. 1. In Fig. 2 an asymmetrical triplet of Channels 30, 32 and 38 is shown to raise the noise level in Channels 21-47 inclusive. Some 24 so-called “vacant channels” can be jammed by the third-order intermodulation products this asymmetrical triplet can generate in a receiver front-end. Is this good spectrum management?

Had the FCC been able to anticipate this problem of receiver-generated IM3 (noise) in “vacant channels,” perhaps it could have established minimum performance standards for consumer DTV receivers, and perhaps it couldn’t do so, but alas, that knowledge was not available then. So we have a problem of 20/20 hindsight. I don’t know how many tens of millions of DTV receivers have been sold in the United States, but there are millions more abuilding now, and millions more will be built before long.

WHAT’S THE NUMBER?

(click thumbnail)Fig. 1: A symmetrical triplet with third-order distortion products, which appear as “beehives” centered on Channels 24, 27, 39 and 42.So how prevalent is this problem? Table 1 shows the number of triplets in the FCC proposed permanent channel allotment plan. I devised a scheme to describe all triplets by two integers. The first integer is the difference between the middle channel number and the lower channel number of a triplet, while the second integer is the difference between the upper channel and the middle channel. For example, Fig. 1 is a 3, 3 triplet (Channels 30, 33 and 36) while Fig. 2 is a 5, 3 triplet (Channels 30, 35 and 38). This scheme is useful because any 3, 3 triplet has the same bandwidth of its IM3 as any other 3, 3. A triplet of Channels 28, 31 and 34 acts just like that of the 3, 3 shown in Fig. 1. I arbitrarily limited Table 1 to channel differences ranging from one to, and including, nine.

As you can see, the FCC allotments bunch these triplets into the upper left quadrant. There are fewer allotments in the lower left and upper right quadrants, and fewer still in the lower right quadrant. There are 203 instances of a DTV channel allotments of triplets in Table 1. There are 161 asymmetrical triplets (shown in red) and 42 symmetrical triplets, such as 3, 3 (shown in blue). There are 111 triplets with offsets of five or less. No community with fewer than three channels can have a triplet of channels, so we can conclude that these 203 triplets exist in medium and large cities. A lot of viewers are involved.

The implication of this bunching of channels with small offsets (< 6) means that the intermodulation products generated cannot be controlled with improved RF selectivity (prior to the mixer stage). So there are only two ways to reduce this generation of intermodulation products in the next generation of DTV receiving appliances (receivers, video recorders, etc). If the RF automatic gain control circuit is “smart,” meaning it can sense interfering signals on nearby channels, it can reduce the gain of the RF amplifier, which means these undesired signals will be weaker when they reach the mixer. This will reduce IM3 very significantly.

(click thumbnail)Fig. 2: An asymmetrical triplet with third-order distortion products, “beehives” centered on Channels 21, 24, 27, 33, 41, 43 and 45.
Reducing the undesired signals by say 10 dB will, of course, reduce the signal-to-noise power ratio by 10 dB, but will reduce the generation of IM3 by 30 dB. Better mixers would generate less IM3 for a given level of undesired signal power. The combination of these two strategies is the winning strategy. It is one that my lecture at the International Conference on Consumer Electronics tried to bring to the consumer electronics engineers and their managers in Las Vegas last month.

LAYING OUT MY CASE

OK, I’ve tried to make my case that such interference may affect a significant number of viewers of free off-air TV. It may also affect viewers who do not have their own antenna, being subscribers to CATV service providers. In many cases, they still fetch signals for subscribers at their headend with (of all things), an antenna.

Let’s look again at Fig. 1, the symmetrical triplet of Channels 30, 33 and 36. If all three are DTV signals at roughly the same power at the CATV headend site, and your signal is on either Channel 27 or 39, CATV operators may have a big problem in receiving your signal. If you are on Channel 24 or 42, the CATV people should have a somewhat easier time of it. If your signal at the headend is really weak, compared to this triplet, and you are on any of these Channels: 23, 25, 26, 28, 29, 31, 32, 34, 35, 38, 40, 41 or 43, you have a big problem. That is what Fig. 1 is telling you. Fig. 2 says the same thing, as all those channels and more can be pretty noisy at the CATV headend due to third order IM generated in the receiver there. If the operator understands the problem he can probably fix it.

(click thumbnail)Table 1: Frequency of Triplets in the FCC Channel Plan. Numbers in red are symmetrical triplets and numbers in blue are asymmetrical triplets. FCC Table of Permanent Allotments (August 2007)OK, you say, we don’t have any triplet which threatens our DTV channel. If that’s the case, you are indeed fortunate. But wait; perhaps there is a pair of DTV channel allotments such as 30 and 33 in your community. What if unlicensed devices operate on Channel 36 creating the triplet in Fig. 1 or one similar to it? Welcome to the club. While it is true that any one unlicensed device can jam DTV reception over a small radius, these gadgets are consumer devices, expected to be sold to millions of homes, offices and businesses.

Triplets can also be formed entirely by unlicensed devices.

The minimum effective radiated power for DTV signals in the UHF band is 50 kW (17 dB above 1 kW) while the maximum is 1,000 kW (30 dBK). Stations operating at the low end of this 13 dB range are much more vulnerable to jamming than those near the upper end. Stations that are the least vulnerable are those operating at similar high power, with similar antenna heights and located in the same antenna farm.

Put another way, in those cities where all stations are close to each other, (co-sited) and both their ERP and antenna height are similar, signals will not interfere with each other, or if they do, a simple 75-ohm attenuator at the receiver’s “F” connector will fix the problem inexpensively. But an attenuator cannot deal with the guy(s) next door and across the street who are operating unlicensed devices.

As Table 1 shows, there are a lot of DTV stations clustered close to each other in terms of their channel numbers. It also shows that the asymmetrical triplets out-number symmetrical triplets 161 to 42. Fig. 2 shows that these asymmetrical triplets blanket a lot of TV channels with IM3 or X-M. In Fig.2 we find six channels in which there is a peak due to IM3 and three more channels in which there is a peak due to X-M. The peaks due to X-M are higher by 3 dB than those peaks due to IM3 because each X-M product is generated by two different sets of signals. Asymmetrical triplets do not conceal distortion products under a signal, which is why we see so many peaks in Fig. 2.

Symmetrical triplets as shown in Fig. 1 do conceal to some degree. Fig. 1 shows four “beehives” of IM3, while we know there are six IM3 products. Two are being masked by the signals on Channels 30 and 36. Two X-M products fall into Channel 33 where they are also concealed in Fig. 1 because it is symmetrical. If you are wondering about quadruplets of undesired signals, I have avoided mentioning them until I can generate “quads” in my own laboratory. Then I can illustrate the problems associated with them. However, the situation can only get worse, as this column will demonstrate in a few months.

WORST IS YET TO COME

Some months ago, I warned about unlicensed devices on first-adjacent channels. Let’s look at that one again. As you must know, DTV transmitters generate IM products and they radiate them on both channels adjacent to their allotted channel. It is well known that the total power in each adjacent channel is 44.5 dB below the power radiated in the allocated channel. Let’s see what this means. The IF selectivity of DTV receivers provides about 2 dB of rejection of this sideband splatter, so it is really 46.5 dB below the U power.

Let the undesired signal on channel N-1 be received at –15 dBm. The IM spillover into channels N-2 and N is –61.5 dBm. This means that the minimum usable (received) power for channels N-2 and N equals –61.5 dBm plus 15.2 dB (the threshold SNR for the North American DTV standard). Dmin then equals –46.3 dBm due to sideband splatter into your channel N-2 or N. By way of contrast, Dmin in a noise-limited environment is –84 dBm, according to the FCC.