[opendtv] Re: Latest S/N test

Rhodes' current column is now on the website

http://www.tvtechnology.com/features/digital_tv/Features_Rhodes-01.19.05.shtml

Ron

Dale Kelly wrote:

>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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