[opendtv] DTV Interference Could Be Mitigated by Receivers

From: Monty Solomon <monty@xxxxxxxxxx>
To: undisclosed-recipient: ;
Date: Fri, 24 Sep 2004 02:36:31 -0400
08.18.04
Digital TV: Charles W. Rhodes

DTV Interference Could Be Mitigated by Receivers

My previous column introduced readers to a new parameter, third-order 
intermodulation (IM3), which is the sideband splatter from a DTV 
transmitter into both adjacent channels.

This splatter is limited by the DTV RF mask required of all DTV 
transmitters. What the RF mask filter cannot remove is the splatter 
close to the DTV signal channel. This splatter is what causes 
interference into adjacent channels, not poor selectivity of the 
victim receiver. The maximum IM3, radiated in each adjacent channel, 
is 44.5 dB below the power radiated in the DTV channel. I define this 
as the co-channel noise afflicting reception of an adjacent channel, 
calling it (I):

I = U -46.5 dB.

Did I just contradict myself? No. The maximum power radiated by a DTV 
transmitter is U -44.5 dB, but a DTV receiver tuned to either 
adjacent channel has some selectivity that discriminates against 
noise just inside the desired channel; hence, the bottom line is I = 
U -46.5 dB. I'll try to pick up a dB of interference rejection 
anywhere I can find it.

Suppose the D/U ratio at your home is -30 dB. The co-channel noise 
due to IM3 radiated by one adjacent DTV channel transmitter is D 
-16.5 dB. This means that a 1.3 dB fade or a little echo will put 
your signal-to-noise + interference (S/N+I) at threshold, 15.2 dB. If 
two DTV adjacent-channel signals are at the same level, I = U -43.5 
dB, so your S/(N+I) falls to 13.5 dB, and reception fails. Why?

But that was just the first part of the story. Even at a moderate U 
signal level, there may be additional IM3 generated in your DTV 
tuner. The ATTC tests demonstrated this at a U level of -25 dBm. 
Receiver-generated IM3 adds directly to the IM3 that accompanies the 
adjacent-channel DTV signal into your tuner; receiver-generated IM3 
attacks your desired signal, unattenuated. You might have had 
reliable reception until a DTV transmitter signed on at its maximum 
authorized power. Fortunately, if you put a 3 dB attenuator in the 
downlead right at the receiver, you will attenuate the 
receiver-generated IM3 by 9 dB! With only a 3 dB loss in desired 
signal power, that may take care of the problem. Suitable 75-ohm, 3 
dB attenuators are readily available and inexpensive. That simple 
remedy may fix your reception problem, but what will happen when your 
viewers encounter such problems? Perhaps broadcasters should make a 
brochure available to help viewers with DTV reception problems. They 
might look to the NAB for such help.

...

http://www.tvtechnology.com/features/digital_tv/f_DTV_interference.shtml

 
 
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[opendtv] DTV Interference Could Be Mitigated by Receivers

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