Re: [SI-LIST] : Trace Impedance Selection

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From: [email protected]
Date: Thu May 11 2000 - 18:39:53 PDT

To Vinu, [email protected], and others on this thread:

Many of us have (in this thread and related ones) provided only limited
answers to questions because it takes substantial time to generate an
all-encompassing dissertation. However, we may spend MORE time by not being
complete in the first place (I'm often guilty of this).

Vinu asked, "Keeping traces close to the ref. planes", "reducing loop area",
are these not the same as saying lowering the characteristic impedance of the
line reduces

Someone else seemed to think raising the Zo to lower the trace current would
also reduce emissions. Still someone else expected the higher currents of
lower impedance lines would increase emissions.

The short answer to Vinu is YES. I'll use a prior example I submitted in a
comment to Mary on this same subject to help clarify the relationships of
different parameters, and to address the other speculations.

Surface traces (microstrip) create a loop antenna whose plane is vertical.
The radiation from that loop is linearly proportional to the effective area
and the magnitude of the frequency components of the current flowing through
the trace. The field concentrating effect of any dielectric with Er > 1
makes the trace appear electrically closer to the reference plane, which is
equivalent to making the effective area of the loop smaller (which will lower
radiation) and will simultaneously lower the trace impedance. The trace
impedance varies in a logarithmic fashion with the height, but the effective
area varies linearly with the height. Now, for a constant magnitude of
signal voltage applied to a line of 69 ohms (which I used in my example), we
will obtain a specific level of radiation. While maintaining the same trace
width, if we increase the trace height (dielectric thickness) by a factor of
four, we get a trace impedance of 119 ohms. We have not quite doubled the
Zo, but we have increased the area of the loop by four times. For the same
applied signal voltage, we reduce the trace current by less than two to one
(which by itself would also reduce the radiation proportionately). However,
the quadrupled loop area (by itself) would increase the radiation by four to
one. The net result is an increased level of radiated emissions for a higher
trace impedance, even though the trace current is reduced. This result is
easily verified by lab tests.

In some/many cases, a controlled impedance line must be created while still
keeping radiation low. Trace width could be reduced while thinning the
dielectric to maintain the desired Zo. In this case, the area and the
radiated emissions will vary directly with the height (dielectric thickness).
 The lower limit on dielectric thickness would be dictated by the
manufacturing limits for the ever thinning trace width, or vice versa,
whichever hit a practical limit first.

Although not all radiating structures are as simple as the loop configuration
noted above, they all must either directly conduct current or allow
displacement current to flow, AND (for more efficient radiation... the
undesired result) they must accommodate the transition from relatively low
onboard impedances to the impedance of free space (120*Pi). One gentleman
who had RF experience correctly noted that antennas are more like
transformers (I would quote him, but I have deleted most of the prior
correspondence). Larry Smith also recalled the "near exponential" horn
coaxial cable demonstration which is just another technique for increasing
the impedance of the driving source to better match the intrinsic impedance
of space.

Part of the rationale presented above is why, in many of my earlier comments,
I keep "simplistically" comparing the impedance match of the transmitting
structure (of whatever physical configuration) to 377 ohms. The radiated
energy can be envisioned to be the energy transfer from a low impedance
transmission line to the "load" impedance of 120*Pi. The higher the line
impedance, the more efficient the transfer (into radiated emissions). This
is a mental aid to help make real-time design decisions only, but it is
helpful to me. The noted exception above of the thinning trace with thinning
dielectric seems to conflict with the correlation of radiation to trace (or
structure) impedance, but only if one doesn't understand the whole picture.
For the practical, high-density manufacturing case of 5-mil traces with 5-mil
spaces, the dielectric thickness is the only parameter easily manipulated.

Although not too extensive a rationale, I hope the above clears the confusion
a bit.


Michael L. Conn
Owner/Principal Consultant
Mikon Consulting

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