Re: XTB-II Options

[ddl@danlan.*com (Dan Lanciani)]

In article <xigeh.170378$Fi1.6598@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx>, "Jeff Volp" <JeffVolp@xxxxxxx> writes:

| I am familiar with their unit.  As you know, the TW523 returns data to the
| host controller during the last 11 cycles of each X10 command.  When
| enabled, that is when the XTB-II transmits repeated data to the powerline.
| So the boosted signal is transmitted bit-synchronous with the second copy of
| each normal X10 command.  I believe that is how most repeaters work.
|
| Because the 120KHz bursts are not phase-locked, this approach can result in
| destructive cancellation of that second copy on the original transmitter's
| circuit.  However, any receivers on that circuit should respond to the
| unaltered first copy from that transmitter.  Since the XTB-II would normally
| be driving at the electrical distribution panel, its output would override
| the original transmitter's signal on all other circuits.

This seems to be the unstated (except occasionally by me :) theory behind
most repeaters.  Sometimes I wonder, though, how probable it would be to
have a pathological case where the level heard by a particular receiver
from a transmitter is below the receiver's threshold, the level it would
hear from the repeater is above its threshold, but the difference between
the two signals is again below its threshold.  Operation could then depend
on the relative phase of the two free-running oscillators in the transmitter
and repeater, and that could lead to some head scratching--especially if
the levels shift in and out of the pathological ranges as other parts of
the network change.

Problems like this would presumably be more common with sub-optimal repeater
placement, but I don't think that's required.  If the repeater and transmitter
are electrically close (even if the repeater is at a service panel) while the
receiver is distant from both the transmitter and repeater (and/or is on a
high-attenuation circuit) I think you might see such an effect.

It's fairly tricky to set up good tests to observe X10 carrier interference
problems and I'll bet that the vendors don't do much in this area.  I suspect
that it's also fairly easy to fool yourself as I once did when I thought I
could get multiple RF->powerline transceivers to coexist.  With two
transmitters and a receiver plugged into a power strip you may well never see
a problem (I didn't.) simply because the transmitter levels are high and the
unit-to-unit variations are enough that the difference in level is still above
the receiver's threshold.

				Dan Lanciani
				ddl@danlan.*com