K1TTT Technical Reference

 

 

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

way.

 

11. If something isn't working right and the voltages don't tell you

why, start looking at the currents.