[opendtv] Re: Latest S/N test
- From: "Allen Le Roy Limberg" <allimberg@xxxxxxxxxxxx>
- To: <opendtv@xxxxxxxxxxxxx>
- Date: Thu, 27 Jan 2005 17:06:24 -0500
Dale asks: Is not preselectivity needed in a receiver designed to perform
acceptably in the current and future RF environments?
RF selectivity is very desirable in a DTV receiver. However, if an adjacent
signal has much greater power than the selected signal at the site of the
receiver, the generation of third-order intermodulation (IM3) in the first
mixer will still be a problem. Practically speaking, it is impossible to
make an electrically tuned RF tracking filter as selective as an IF filter.
Wideband AGC on the RF amplifier and first mixer uses an AGC detector after
the first mixer. The first mixer bandwidth is wider than that of the
ensuing IF amplifier, but it is constrained by the bandwidths of the RF
tracking filter and the mixer output filtering. The strong remnant of a
very strong adjacent signal reduces RF amplifier gain, so the mixer is not
driven so far into overload that unacceptably strong IM3 is generated. The
gain for selected signal is reduced, which reduces SNR slightly. This
reduction is better tolerated than IM3, a point that is treated in detail in
the Bendov and Patel paper. Gain for selected signal is recovered in the IF
amplifier, which is separately AGC'd by the narrowband AGC. The bandwidth
of this AGC is defined by the IF amplifier bandwidth.
There are ways to suppress unwanted phase modulation in the VSB IF signal,
which arises from noise-induced phase modulation of the local oscillator(s).
The first LO is the problem, since later LO's can be crystal stabilized.
With frequency synthesizers used as first LO's, phase modulation is usually
caused by jitter in the frequency divider networks used for scaling up
frequency from a crystal stabilized oscillator. U. S. patents Nos. 6,687,313
and 6,771,707A describe the conversion of VSB IF signal to DSB AM signal so
as to cancel quadrature AM sidebands (phase modulation), for example. But
this increases receiver cost.
----- Original Message -----
From: "Dale Kelly" <dalekelly@xxxxxxxxxxx>
To: <opendtv@xxxxxxxxxxxxx>
Sent: Thursday, January 27, 2005 1:51 PM
Subject: [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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> >
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