Metzler's 'Laws' of signals...

           Many fine circuits have been abandoned or ignored because of
                'components' that never appeared on the schematic.

        1) Any conductor that carries alternating current is considered to be 
                a transmission line.  Any energy that fails to appear at the
                far end went elsewhere.  Signals escape by way of capacitance,
                mutual inductance, common resistances (ground loops), or by
                radiating as RF. Its a bad idea to just hope the missing stuff
                turned into heat!  This includes power supplies, which must be 
                assumed to be carrying nasty stuff until proven clean...

        2) Reciprocity:  if stuff can leak out, stuff can leak in!

        3) If the conductor is << 1/8 wavelength (at the highest excitation
                frequency), time delays MAY be unimportant. In digital work,
                excitation frequencies (edge rates) are way higher than clock 
                frequencies.  In analog work, distortion products are way
                higher than signal frequency excitations.  Is the line still
                short?

        4) If there's a known resistance in range, try to match to it unless 
                there's a very good reason not to.  Even a simple series 
                terminator at the source end can help.  If you get lucky and 
                condition 5 is met, the line can be ignored.... maybe.

        5) ALL lines have return paths associated with them.  If you don't
                control them, Murphy will, in which case return will likely be 
                by way of another of your signal lines.  Return is by way of
                the lowest impedance, NOT the lowest resistance path, even at
                'audio' frequencies.  The smallest area loop will carry the 
                signal current.  DC powered amplifiers of ALL kinds work by
                shunting current between 2 or more 'power rails', which become
                the actual return points.  Have you tied them together?  Where,
                and with what?  Only a perfect transformer can keep these 
                currents off your line.  This includes logic gates.

        6) Capacitors have inductance, lots of it. Resistance too.  Know how 
                much if you can...  People who make capacitors don't like
                inductance and resistance and don't readily admit to having
                any!

        7) Inductors have capacitance, lots of it. Resistance too.  Know how
                much if you can...  People who make inductors don't like 
                capacitance and resistance and don't readily admit to having
                any!

        8) Resistors have capacitance, lots of it. Inductance too.  Know how
                much if you can...  People who make resistors don't like
                capacitance and inductance and don't readily admit to having
                any!

        9) Conductors are usually decent inductors.  Their capacitance may be
                due to lousy dielectrics.  Make sure yours is good enough.
                This includes ANY insulator between signal and return.

        10) ALL mismatched lines (most lines in general) are resonant somewhere
                in the spectrum.  If they're not resonant they're matched.
                PERIOD!  Sometimes one can get away with matching them only
                at high frequencies (snubbing).  Find or control 'Z' and 
                the frequency (length) rather than blindly trying out a slew 
                of resistor and capacitor values.  Never assume that where
                they're resonant isn't hurting your signal in some way.

        11) If something isn't working right and the voltages don't tell you
                why, start looking at the currents.


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