[opendtv] Re: Latest S/N test

Allen wrote:
> The problem with dual-conversion receivers for DTV is
> phase-modulation noise

What is the trade off between this increased phase noise and it's ability to
reduce destructive image signals - considering our tightly packed DTV
spectrum's unimpeded usage of high powered taboo channel assignments? A
single conversion receiver must also deal with this serious issue - how,
other than thru good AGC design?

> Front-end overload problems are alleviated by using wideband AGC on the
> RF amplifier and first mixer.

I agree, good AGC performance is critical and I seem to recall some *much
older* receivers having uncoupled dual AGC stages (RF/IF).
It occurs to me, however, that an unfiltered wideband RF AGC operating in 
our
hostile RF environment could allow strong undesired signals to set the
amplifiers gain, thereby reducing the C/N of the desired signal (a condition
which resulted in the COFDM receivers poor performance during the MSTV
tests). Is not preselectivity needed in a receiver designed for it to 
perform
acceptably in the current and future RF environments?

In such maters I certainly defer to the knowledge of CB, Oded, Charley and
yourself. During the last couple of years CB and I have engaged in numerous
highly instructive discussion regarding DTV receiver design issues but I am
clearly a novice. There are very few left who truly understand this *art*,
other than those who you mentioned.



----- Original Message ----- 
From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Thursday, January 27, 2005 7:11 AM
Subject: [opendtv] Re: Latest S/N test


> The problem with dual-conversion receivers for DTV is
> phase-modulation-noise
> from the oscillators used in the early mixers.  The phase modulation is
> greater the higher the frequency of the local oscillations.
>
> Front-end overload problems are alleviated by using wideband AGC on the RF
> amplifier and first mixer, rather than delayed AGC developed from the
> narrowband AGC used on the IF amplifiers.  The use of wideband AGC on the
> RF
> amplifier and first mixer was used in Hallicrafter receivers during WWII,
> Charles Rhodes told me a few years ago.  I hear via the grapevine that he
> has recently authored a paper on the matter.  Oded Bendov and C. B. Patel
> recently wrote a paper "Television Receiver Optimization in the Presence
> of
> Adjacent Channel Interference" for the IEEE  Transactions on Broadcasting,
> but I am unsure whether it has been published yet or will soon.  My U.S.
> patent application 2003-0007103 publishe 9 January 2003 and titled
> "DIGITAL
> TELEVISION RECEIVER WITH REMOTE TUNER FOR DRIVING TRANSMISSION LINE WITH
> INTERMEDIATE-FREQUENCY SIGNAL" describes wideband AGC to avoid overload of
> the RF amplifier and first mixer in the remote tuner located at the
> antenna
> site.
>
> Al
>
> ----- Original Message ----- 
> From: "Manfredi, Albert E" <albert.e.manfredi@xxxxxxxxxx>
> To: "OpenDTV (E-mail)" <opendtv@xxxxxxxxxxxxx>
> Sent: Wednesday, January 26, 2005 4:14 PM
> Subject: [opendtv] Re: Latest S/N test
>
>
>> Dale Kelly WROTE:
>>
>> > RF interference and other related distortion
>> > issues also greatly impact channel S/N performance
>> > and tend to be overlooked. The attached URL is for
>> > one of a series of articles written on the subject
>> > by the highly regarded engineer, Charles Rhodes. I
>> > believe you will find this and his other relates
>> > articles of interest.
>> >
>> > =
>> http://www.tvtechnology.com/features/digital_tv/f_DTV_interference.shtml
>>
>> Thanks, Dale. Excellent article.
>>
>> I thought that dual conversion tuners were designed
>> specifically to combat intermodulation (and
>> cross-modulation?). Which is why I was surprised to
>> see a couple of recent press releases, where the
>> manufacturers were touting their single-conversion
>> chips as being something great. Maybe great, but
>> mainly for cost reasons, no?
>>
>> I guess you have to watch out for those more
>> obscure performance measures, such as this third
>> order intercept power. Maybe that's one reason
>> the LG 5th gen does so well, even with all those
>> analog stations still on the air.
>>
>> Bert
>>
>>
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