[opendtv] Re: Latest S/N test
- From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 27 Jan 2005 15:13:13 -0800
Bert Said:
> If this third order interference created within the
> receiver is a function of the cube of input power,
> seems to me that regular old single conversion tuners
> without some other provision to increase selectivity
> are not going to do well. Even wide band AGC might
> not help. A weak channel surrounded by loud ones
> can easily drop below the 15 dB margin in this sort
> of case, yes?
That seems correct to me Bert. You notice that Charley
Rhodes indicates that a tracking filter is required for
the required single conversion selectivity.
Zarlink recently announced design of a single ended tuner
for Thompson that they say exceeds the ATSC's A74
recommendation. That is nothing exceptional given that it
is a marginal spec. I was advised last year that Zarlink, RMT,
and others, supply triple conversion receivers, with significantly
better specs than A74, to the European COFDM markets,
where they are serious about OTA performance.
See these web page.
http://products.zarlink.com/product_portlets/thomson_pnp.htm
http://www.rwt.com.uk
Tthe Zarlink spec sheet doesn't give one a clue
about how their single conversion image rejection scheme
functions but I did note that the Noise Figure is a paltry 13db.
As I recall, the DVB-T spec. is 8db or less and a typical
DVB receiver is in the 5db range.
----- Original Message -----
From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
Sent: Thursday, January 27, 2005 1:44 PM
Subject: [opendtv] Re: Latest S/N test
> Ron Economos wrote:
>
>> Rhodes' current column is now on the website
>>
>> =
> http://www.tvtechnology.com/features/digital_tv/Features_Rhodes-01.19.05.=
> shtml
>
> And, right on topic, in it he states:
>
> "This brings us to another great unknown: What will
> the RF selectivity be of future receivers built under
> FCC's DTV tuner mandate? In some cases, RF selectivity
> may be nothing more than switchable bandpass filters,
> Channels 2-6, or 7-13 or 470-806 MHz.
>
> "A better solution is a tracking, tunable RF filter,
> but the problems of such filters include the physical
> size of the inductors, the costs of factory alignment,
> the Q factor of the resonant circuits and the small
> fractional bandwidth desired at UHF (1 percent).
>
> "Zenith understood these problems well enough to use a
> double-conversion tuner, but it too has problems with
> noise and cost.
>
> "I understand that most DTV receivers have single
> conversion tuners. Will DTV tuners have tracking or
> bandpass filters next year?"
>
> If by "Zenith" Rhodes means the 5th gen LG, then
> evidently the noise/selectivity tradeoff favors
> dual conversion tuners. At least, as of now. I now
> wonder how sensitivity measures in the 5th gen LG.
>
> If this third order interference created within the
> receiver is a function of the cube of input power,
> seems to me that regular old single converion tuners
> without some other provision to increase selectivity
> are not going to do well. Even wide band AGC might
> not help. A weak channel surrounded by loud ones
> can easily drop below the 15 dB margin in this sort
> of case, yes?
>
> Bert
>
>
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