[opendtv] Re: White paper from CEA
- From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 3 Nov 2005 08:37:30 -0500
The more significant problem is strong adjacent channel interference, where
AGC on desired channel does not reduce the strong adjacent channel. Then,
the first mixer generates 3rd-order intermodulation that is strong enough to
disrupt digital demodulation.
Broadband AGC of the front end helps this problem, as pointed out by Charles
Rhodes & others.
Al Limberg
----- Original Message -----
From: "Richard Hollandsworth" <holl_ands@xxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Wednesday, November 02, 2005 7:11 PM
Subject: [opendtv] Re: White paper from CEA
> The CRC test results for single signals (multiple signals not tested):
> Samsung/ALPS Tuner (8/05): Overload of +1.5 dBm, Sensitivity of -84.7 dBm
> Zenith/LG PROTOTYPE (9/03): Overload of -2.3 dBm, Sensitivity of -78.4
dBm.
> LINX not tested.
>
> So the Samung had over 3 dB higher overload point for single signal input.
> Unfortunately, no data was taken for multiple signals. [Big
oversight!!!!]
>
> However, Mark said that for many tests they inserted RF attenuation in
order to potentially reduce intermods, with no significant advantage:
> //www.freelists.org/archives/opendtv/08-2005/msg00116.html
> So that would seem to generally rule out simple overload arguments.
> --------------------------------
> So what could a manufacturer do to make a higher overload tuner without
degrading sensitivity?
> If it were easy, wouldn't we have seen something on the aftermarket by
now, like there has always been for AM, FM and HAM bands???
>
> Someone should test the Funai/Sylvania 6900DTE with DX DTA-5000 Smart
Antenna/Variable AGC Preamp in a high signal level environment. The ATI
patent disclosure described a procedure whereby "bad" antenna combining
directions were discarded. It also looks possible to "hack" the DTA-5000 in
order to use your own choice of external antennas.
> [My test was in the 'burbs, although now I've learned that there is 50 KW
COFDM MediaFlo "jammer" only a few miles away. Unfortunately Funai has so f
ar been unable to fix the DD5.1 outputs.]
>
http://www.avsforum.com/avs-vb/showthread.php?p=5979741&highlight=6900dte+4bay#post5979741
>
> At least one OTA STB (I don't recall which one) had an internal Preamp
enable/disable switch to select for either maximum sensitivity or strong
signal environment.
>
> Maybe add an external variable RF attenuator to supplement the internal
varactor? [But that didn't work...]
>
> Maybe add an external tuned, passive RF filter to supplement the internal
tuned RF front end? [Did you try THAT yet???]
> [Sounds like a feature for that High-End OTA STB I mentioned earlier....]
>
> --------------------------------
> One theory that I have not seen expressed in earlier discussion is
possible RF leakage into the tuner module from the high ambient electronic
field in NYC.
> This could be more of a problem for PROTOTYPES, which are likely to have
the tin can popped on and off dozens of times.
>
> <holl_ands>
>
> //////////////////////////////////////////
> "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx> wrote:
> Mark Schubin wrote:
>
> > I, too, eagerly await the commercial introduction of
> > the one prototype that worked so well in my apartment
> > 15 months ago, but not everyone lives in a
> > problematic-multipath environment.
>
> I remember you telling me years ago that people from
> several organizations had come to your apartment to take
> measurements, but that you could not divulge what they
> had found. Would have been instructive to see.
>
> Since your rabbit ear OTA reception is 100 percent from
> reflected signal, I have to believe that one big problem
> with receivers through 3rd generation was their inability
> to cope with pre-echo. If the receiver can't handle
> pre-echo, it needs a stronger direct path than anything
> reflected. Ergo, no reception.
>
> When you reported on the new Samsung receiver initially,
> I had not seen the CRC measurements, so I couldn't put
> that report in perspective. Now that we see that the
> Samsung measures better than the LG (caveat -- the 2003
> measurements of the LG, which might not apply), I have to
> conclude that in your specific case, further improvements
> in echo tolerance might not help. If antenna placement
> away from the window doesn't work for the Samsung, when
> it did work for LG prototype, then my guess is that IM
> distortion products in the tuner are eating into the C/N
> margin. So while the C/N margin is exceeded with a window
> placement of the antenna, it's not exceeded when the
> signal (including multipath) becomes a little weaker away
> from the window.
>
> If my guess is right, what I don't comprehend is why
> receiver makers don't use the very best front ends in
> their initial attempts. And then attempt cutting corners
> only after they have done the obvious as well as possible.
>
> Bert
>
>
>
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