[opendtv] Re: Demand for free DTV rising in Australia
- From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Fri, 29 Jun 2007 19:34:30 -0400
Dual-conversion receivers have their own problems with phase noise.
Single-conversion receivers are now being designed using Clarence Hansell's
1936 technique for image rejection, I am told. It was not too attractive in
vacuum-tube electronics, but is easy to implement with FETs.
Al Limberg
----- Original Message -----
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Friday, June 29, 2007 7:10 PM
Subject: [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
>
>
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