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[TowerTalk] Metzler's Laws of Signals

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Subject: [TowerTalk] Metzler's Laws of Signals
From: (
Date: Tue, 30 Jun 1998 09:07:13 EDT
Even though not on the subject of towers, but related to some recent threads,
I thought this might be of interest.  I've never seen these
ideas expressed in quite this manner before.

73, Don, WA6EE


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.  It's 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 also 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 current off of your lines.  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 dialectics.  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

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

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