The Plotting Thickens - TvTechnology

The Plotting Thickens

In January, Gary Sgrignoli of Zenith and I co-authored a paper presented at the International Conference on Consumer Electronics (ICCE), the ideal forum to communicate with engineers designing future DTV receivers. We are concerned over the extent of DTV-DTV interference when all DTV transmitters soon go to maximum authorized power.
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In January, Gary Sgrignoli of Zenith and I co-authored a paper presented at the International Conference on Consumer Electronics (ICCE), the ideal forum to communicate with engineers designing future DTV receivers. We are concerned over the extent of DTV-DTV interference when all DTV transmitters soon go to maximum authorized power.

The following summarizes our paper, "Interference Mitigation Strategies for Improved Terrestrial DTV Receiver Designs."

Strong TV signals up to -8 dBm at the receiver input will be present five miles from a full-power transmitting facility. The maximum symbols of the 8-VSB signal are transmitted 4.8 dB above average power, so the DTV signal maximum symbol power is 34.8 dB above 1 kW. For analog TV, the peak power is 37 dBK, only slightly higher from the interference perspective. Spectrum plots where undesired signals are -19.59 dBm display third-order intermodulation in blue. I calculated the third-order intercept power (IP3) of the amplifier under test at 0 dBm, so the maximum signal power was about 20 dB below IP3. Note that I used 8-VSB DTV signals, not unmodulated carriers as in the classical two-tone testing of intermodulation.

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Fig. 1 shows the spectrum of an 8-VSB signal from a slightly overloaded amplifier. Note how it resembles the spectrum of a DTV transmitter before the RF mask filter. The signal power output is shown at -19.67 dBm.

Fig. 2 shows the third-order intermodulation products (IM3) in the lower adjacent channel (-58.67 dBm) for this signal output power. The IP3 can be determined from this data to be 0 dBm. IP3 is normally determined using a twotone test signal, but this technique is of much greater interest to broadcasters than is two-tone testing because we can predict DTV-DTV interference with actual DTV signals.

Fig. 3 shows two 8-VSB signals each at -19.59 dBm on adjacent channels with IM3 in the channel above the signals at -53.32 dBm. The noise IM3 in this channel is 6.35 dB higher due to the second 8-VSB signal of equal power. That is, a 3 dB increase in power input results in a 6 dB increase in distortion (noise). Theory suggests this increase would be 9 dB for a two-tone test signal, but it's not so with DTV signals because of spectrum spreading.

Fig. 4 also shows two distorted 8- VSB signals, but this time, they are separated by one channel. In this case, they are on the channel pair n-4 and n- 2. The noise (IM3) in the desired channel- shown in light blue (n)-is due to IM3 generated in the receiver and measures -53.53 dBm. Note the noise spreads above the desired channel. Five channels will have significant noise with two undesired signals spaced by one channel. Having done that, the minimum usable DTV signal power is determined by adding 15.2 dB (the SNR of our 8-VSB signal) to the total noise in the channel. This demonstrates why the receiver should not generate IM3, as radiated sideband splatter is significant without any receiver-generated IM3.


(click thumbnail)Fig. 5 also shows two 8-VSB signals of equal power but these are the n-6, n- 3 channel pair with n-6 not shown. The IM3 in the desired channel (n) is -52.97 dBm. With these -20 dBm undesired signals present, the minimum usable desired signal power is > -38 dBm.


(click thumbnail), Fig. 6 shows the n+3 and n+6 (though it is not shown) channel pair whose IM3 in the blue-tinted channel (n) is -53.71 dBm. Note that there is significant noise on both n-1 and n+1. Spectrum spreading is even greater with increased spacing between two undesired signals.

Fig. 7 gets more interesting. Three undesired DTV signals on Channels 35, 36 and 37 are shown. Each alone would have been at -19.67 dBm, but the three together have increased the total signal power and hence overloaded the amplifier under test.. These signals suffer compression because they are at -20.46 dBm instead of -19.59 dBm. The effect of signal compression never has been fully studied as it applies to 8-VSB, but I believe that 1 dB of signal compression is too much. That is, this amplifier works well with -19.59 dBm power output, but it couldn't handle more.

Fig. 8 shows that the three 8-VSB signals produce IM3 in an adjacent channel at -45.01 dB, a big increase over the IM3 with two 8-VSB signals. The desired signals not only suffer from IM3 noise, but also from signal compression. This topic will be the subject of future studies.

Fig. 9 shows the IM3 in the other adjacent channel, -43.91 dBm.

Finally, fig. 10 shows the spectrum spreading when two or more undesired DTV signals of comparable power are present and spaced by one channel. The noise power in the blue-tinted (desired) channel is -53.15 dBm. It's about the same two and three channels above the higher undesired channel, and of course for two channels below the lower undesired channel. This is a "blockbuster," with five channels subject to jamming.

Having demonstrated that a receiver with an IP3 of 0 dBm will be in serious trouble, we demonstrated that by attenuating the antenna signal by even 5 dB, the IM3 would decrease by 15 dB and the minimum usable desired signal power would decrease by 7.7 dB. More RF attenuation brings even lower minimum desired signal power levels and this will be needed.

This is too cumbersome for most viewers, so RF attenuation should be automatically switched by the receiver without viewers having to do something as they channel surf. We then showed how this can be done with wideband RF automatic gain-control (AGC) circuitry. We then compared this to what could be done with higher IP3 mixers (these require excessive LO power and can be costly) and high IP3 RF amplifiers, which are needed unless wideband RF AGC circuit is used.

Now you know a bit of what the ICCE attendees learned (I hope) and you didn't have to fly to Las Vegas to find out.

I want to acknowledge the extensive assistance rendered by Mark Aitken and Harvey Arnold of Sinclair Broadcast Group for the use of their spectrum analyzer and their help in producing the spectrum plots. Stay tuned.