In message <3.0.3.32.19980102070501.00759ae8@molsun.ophth.uiowa.edu>,
"Adam B. Kanis" <adam-kanis@uiowa.edu> writes
>Hi all,
>
>Please help me understand this:
>
>In several different circuits that have a segment where a signal is carried
>in a shielded cable, both rf and af, i've seen instructions call for
>grounding the shield at one end, but not the other. In other circuits, the
>shield was put to ground at each end. I did what was told, but left
>feeling empty - i don't like "cookbook" procedures without understanding
>the basis.
>
>I don't remember seeing this thread on the list before, but if i missed it,
>sri.
Ground loops are the reason for this. They don't usually cause problems
at RF, so RF cables are usually grounded at both ends. However, at AF,
with low-level signals, ground loops can introduce noise into a circuit,
like mains hum, and cause instability. The problem is basically due to
the potential difference between the two points where the shield is
grounded. To avoid ground loops, all grounds are often connected to a
common "star point".
See The Art of Electronics for a fuller explanation.
Leon
-- Leon Heller: leon@lfheller.demon.co.uk http://www.lfheller.demon.co.uk Amateur Radio Callsign G1HSM Tel: +44 (0) 118 947 1424 See http://www.lfheller.demon.co.uk/dds.htm for details of my AD9850 DDS system - schematic and software.
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