Al Limberg wrote: > 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. Interesting. So it seems you're saying you need both a tunable RF bandpass filter and the wide AGC. > 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. Wow! Finally we have an excuse for using VSB. The reason ATSC went to VSB is to improve performance in the presence of noise created by IM or other causes. Needed to use AM, so that noise-induced phase jitter in the first oscillator could be cancelled, which meant using VSB as the technique for compressing the bandwidth. Who knew? I knew that sooner or later an advantage of nAM-VSB would emerge. 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.