Jon has many good points on Greg's problem. After the fact, what one
presumes as square waves is not often the case since edges are never
perfectly square. I have seen so called linear RF amps produce -30 dBc and
-10 dBc second and third order products in my time.
EMI is a real problem when excessive logic speed is used when maybe not
necessary. This puts a jolt on a ground plane that everyone must ride like a
surf board. There is no such thing as ground because any ground plane has
inductance and resistance. Grounds are always relative to me. There is much
to be said about sectioning ground planes to steer high ground currents.
Anyone who has worked with data acquisition will agree. The other rule of RF
thumb is keep distances short and decoupling is crucial. Henry Ott's book on
Noise Reduction Techniques Electronic Systems is the best reference I have
ever found to working around EMI concerns.
Jon's suggestion of killing the buss smell for a problem is a great idea.
Like Jon, I do not know what the clock source is. If there is access to a
high band width scope or spectrum analyzer a sniffer coil can be used as a
beagle (hunter). Shielding is an important issue on times that should not be
overlooked.
Give Henry's book a read and look suspiciously around high current areas. If
anyone wishes to have the ISBN number they can email me off line, if someone
does not post it first.
Fabe
Fabian Hartery
Guigne International
Paradise, Newfoundland
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