[opendtv] Re: Demand for free DTV rising in Australia

Dale Kelly wrote:

> Unfortunately this is not as straight forward as it might
> seem. Adjacent channels are not the only IM issue but has
> lessened due to the upgraded DTV Mask filter requirements.
> However, if the D/U ratio is moderately high it is still a
> problem and there is more than a 10db spread in DTV
> channel assignments - many have 1000KW while others are
> 100KW or less. Also, the potential for non licensed device
> interference is very high.
>
> See the following remarks from a highly qualified
> independent RF engineer, they are instructive and include
> the recent FCC receiver test report URL.

> This 200+ page Report can be downloaded from the FCC
> website:
>
> http://www.fcc.gov/oet/info/documents
>
> Scroll down to the Reports section, click Reports. The
> Report is at the top of the list that will appear.

Well, that will take some time to digest. I keep coming to the
conclusion that the combination of dual conversion tuner and tracking
filter up front should make a great tuner. Perhaps that was what the
"cold fusion" LG prototype did. This should solve the IM3 problems, at a
possible cost of some ultimate sensitivity (due to receiver noise, if no
extra cost is expended). It seems all the 5th gen tested here were
single conversion designs.

The other question is how do the European deployments differ. Or do
they? Given that, at least in some countries, it's not true that all
transmitters share the same tower or same location. Is there something
different, or is it a case of "making do."

I went to the summarized tables in Appendix A. Just looking at the worst
performer among the "5th gen," here's what I see:

In general, as the undesired channel moves away from the desired one, it
is better attenuated. But at N-6, N-2, N+2, and N+7, the max undesired
signal rejection drops down, for that single undesired channel. Then the
selectivity recovers as you keep moving away.

With one exception, the worst of the worst example of this is a
rejection of 23.3 dB (for N-2 at Dmin + 1 dB). Which means, if the
receiver is picking up a desired signal with 1 dB of margin over the
sensitivity threshold, a station at N-2 which is 23.2 dB stronger will
cause reception failure. (There is no A/74 recommendation for this
case.)

The exception is for strong desired signals (-28 dBm). In this case, the
worst performer is good for "only" about -20 dB rejection of N+1 and
N-1, but then again, A/74 only recommends -20 dB.

With combined undesired signals, the worst performer rejects worst case
27.3 dB stronger undesired signal, when desired is weak, and 16.6 dB,
when desired is strong. This last to be compared with the A/74
recommendation of -20 dB, though.

The worst performer in general does do a better job of rejecting
undesired signals when the desired signal is weak. But A/74 wants better
still. My guess is that this is because there is a lot more opportunity
for a strong undesired channel when the desired channel is weak. Whereas
it gets close to impossible for a much stronger undesired channel to
exist when the desired one is already strong (-28 dBm + 20 dB = -8 dBm,
after all).

If I had to pick two receivers for the site revisit test, I'd choose the
worst performer as one of them. Just to see what this means in the real
world. And I'd also like to see a dual-conversion design *with* tracking
filter, just to see if that is "cold fusion."

Bert
 
 
----------------------------------------------------------------------
You can UNSUBSCRIBE from the OpenDTV list in two ways:

- Using the UNSUBSCRIBE command in your user configuration settings at 
FreeLists.org 

- By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word 
unsubscribe in the subject line.

Other related posts: