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 >>> >>> >>> >>> ---------------------------------------------------------------------- You can UNSUBSCRIBE from the OpenDTV list in two ways: - Using the UNSUBSCRIBE command in your user configuration settings at FreeLists.org - By sending a message to: opendtv-request@xxxxxxxxxxxxx with the word unsubscribe in the subject line.