In my October column ("Cellphone, DTV Interference Issues Examined"), I promised to conduct tests of the 26 NTIA-approved converter boxes in my laboratory simulating the interference to cellular phones, which operate in the lower portion of the 700 MHz Band, former TV Channels 52, 53, and up.
This might give us some clues to the potential for interference to cellphones in this part of the 700 MHz Band assuming they use tuners similar to those in the 26 converter boxes, some of which have a tuner-on-a-chip where it is rather difficult to provide RF selectivity.
My colleague, Linley Gumm and I ran experiments with a pair of ATSC signals on Channels 32 and 35. They can generate third-order distortion products centered in Channel 38. However they also generate some third-order distortion products in Channels 37 and 39.
We put our desired ATSC signal in Channel 38 at –68 dBm, which gives a 31 dB signal-to-noise (SNR) power ratio. This is well above the ATSC SNR threshold of 15.2 dB absent interference and absent multipath.
The noise from the two undesired ATSC signal generators in the desired channel (38) was removed by a really nice band stop filter designed by Linley. This is absolutely necessary for such tests.
We knew that the threshold for interference to our desired signal at –68 dBm is somewhere around –28 dBm. Put another way, our signal generators have an SNR of 50 dB. So when the undesired signal is at –28 dBm, there is co-channel noise in Channel 38 of about –78 dBm per each undesired signal or –75 dBm for the pair. Add 15.2 dB to –75 dBm and you get the minimum usable desired ATSC signal power, 60 dBm.
But we wanted to test where everyone else tests, –68 dBm. So Linley's filter created a hole about 40 dB deep and we dropped the noise into that hole. Now the noise level in our experimental set up is perhaps –115 dBm, far too small to be measured or to have any detectable effect on our results.
Fig. 1: This plot shows that for a desired DTV signal power of –68dBm about 90 percent, 23 out of 26 units, will operate with a pair of undesired signals below 40 dBm. The green and blue traces show that when the interference falls in either first adjacent channel, the threshold changes by only 3 dB.(Editor's note: The following paragraph was updated from the print version that appeared in the Dec. 21 issue of TV Technology)
We found that the maximum undesired signal at which these units could perform was between –22.2 dBm for the best box, to –28.7 dBm for the worst.
PASSING THE TEST
There was no NTIA performance specification for any pair of undesired signals, so every one of these models passed all tests required for NTIA certification. The units we tested are approved models; we did not test any unit previously tested at the FCC Lab. We have never looked inside any of these boxes, so we don't know what tuner is in any of them as it doesn't matter. There are more than 30 million of these "type accepted" NTIA converter boxes in the United States.
(Editor's note: The following paragraph was updated from the print version that appeared in the Dec. 21 issue of TV Technology)
As the third order distortion products are three channels wide—in this case Channels 37, 38, and 39—we shifted our desired ATSC signal to Channel 37 and retuned all 26 DTV converter boxes to Channel 37. We then found the maximum undesired signal power at which these units worked was –17.7 dBm to –27.7 dBm, roughly the same undesired levels when the desired signal is on Channel 37 as when it was on Channel 38. The same result was also obtained with the desired signal on Channel 39. So, indeed third order distortion products cause interference over three channels.
Set-top converter box with 5-inch portable TV used in the field to measure DTV signal levels. But what is most interesting is the number of these converter boxes that can be tuned to a desired signal and lock to that signal to produce stable pictures and unbroken sound when the third-order distortion products in the desired channel are as shown in Fig. 1.
Note that we started from a desired signal power at which no converter box worked and then we attenuated the combined undesired signal power in 0.5 dB steps to measure the acquisition threshold for signal to interference (S/I). Receiver-generated noise was negligible, so this is truly S/I, not S/N+I. Fig. 1 is a plot of the number of units that operated at each specified undesired signal per channel.
Comparing the range of undesired signal thresholds for Channels 37, 38 and 39, one begins to appreciate the fact that third-order distortions jam three consecutive 6 MHz channels and/or blocks in the lower 700 MHz Band.
Similar tests were conducted by Steve Martin at the FCC Laboratory and reported in the December 2010 issue of "IEEE Transactions on Broadcasting."
Stay tuned; maybe we will soon have news from the retesting of interference to GPS receivers that was scheduled to be completed on Nov. 30.
Charles Rhodes is a consultant in the field of television broadcast technologies and planning. He can be reached via e-mail at email@example.com.
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