I'm not disagreeing with anyone, but it is important to re-enforce how
things really behave and what actually causes problems.
> Most RF noise escapes from equipment as current flowing on wiring,
> either inside the box due to inadequate shielding and/or poor circuit
> layout, and on external wiring due to improper termination of cable
> shields (the Pin One Problem).
Absolutely. That is true enough on HF bands for low power gear, although
inadequate case joint contact can set the case up for "eddy currents" that
couple to other poorly enclosed things if high power is involved. High power
amplifiers and high power switch mode supplies are examples where currents
in a case can be high enough to couple from inadequate case bonding.
I'd worry more about cases, as a general rule, for VHF and higher. The
exception (for HF receivers) is how RF connectors mount and wire. Connector
shield ground loops can be a huge issue, that's why plastic cases and thin
traces for grounds on circuit boards are so bad. An open box harly causes
near the problems of poor shield path routing, and shield path routing is
often pretty poor.
The important thing to remember is it takes two things to cause it problem.
It has to leak out of the source, and it has to leak into the receiver. The
first step is always making the receiver system reasonably tight, and that
almost never involves a ground or a power line filter on the radio. In days
past, when line cords ran inside receivers with point-to-point wiring, line
cord ingress was often significant. Today, it is a non-issue. I'm not aware
of an exception, but perhaps someone knows of some receiver that is an
exception.
For the modern receiver, what we do to the antenna cable and related system
dominates ingress by far.
>The part of that current on external
> cables, and thus the noise radiated by that current, can be greatly
> reduced by ferrite chokes TUNED to the frequency(ies) of the noise. The
> resonance of a ferrite choke is quite broad, with Q typically on the
> order of 0.4 - 1, and a single turn through most ferrite cores typically
> resonates around 150 MHz.
It really comes down to an issue of impedance ratios, and it always comes
back to differential between things leaving that device. It might often be
common mode from the perspective of all conductors in one multiconductor
cable, but it is always differential excitation of that particular cable
against some other electrical path.
There is always something "differential" going on, and the impedance at the
choke insertion point always determines the effectiveness of any choke.
Consider a floating device, like a monitor, loop antenna, or laptop. If
every cable leaving that device is at the same potential as the case, there
will be very little common mode radiation because a noise source inside has
nothing to "push against".
If I have a monitor radiating noise on cables, and if I throw 5,000 ohms of
common mode impedance on a lead, it might make no difference at all. As a
matter of fact, if that impedance is inductive, it might make things worse!
(This is why air-wound choke baluns, or high-Q bead baluns, can actually
INCREASE feedline radiation even while they have a very high reactance.)
We have to picture the system in our heads, and fix what is really broken.
I like to relate the story of a CATV system in an apartment complex that was
built over the radial field of a AM broadcast station. The CATV company
installed double-shielded and triple-shielded cables, and finally gave up.
I had a crew disconnect all the fancy cable and special triaxial connectors,
and remove all the filters and traps on lines and on TV sets. We installed
regular CATV cable that closely followed power line wiring. Everywhere along
the system, the new cable's shield bonded to electrical safety grounds. In
the apartments, the shield grounded to the outlet boxes, which we tied to
the safety ground. We had no special grounds or anything.
This totally cleaned up hundreds of TV's and stereo's, and we wound up with
a huge box of surplus TVI filters. Only a few sets remained a problem,
requiring filtering or modification.
Our gear is no different, all we have to do is keep in mind how things
really work and wire accordingly. :-)
73 Tom
_______________________________________________
UR RST IS ... ... ..9 QSB QSB - hw? BK
|