Here's what Part 15 says about testing computer motherboards:
§ 15.32 Test procedures for CPU
boards and computer power supplies.
Power supplies and CPU boards used
with personal computers and for which
separate authorizations are required to
be obtained shall be tested as follows:
(a) CPU boards shall be tested as follows:
(1) Testing for radiated emissions
shall be performed with the CPU board
installed in a typical enclosure but
with the enclosureâ??s cover removed so
that the internal circuitry is exposed
at the top and on at least two sides.
Additional components, including a
power supply, peripheral devices, and
subassemblies, shall be added, as needed,
to result in a complete personal
computer system. If the oscillator and
the microprocessor circuits are contained
on separate circuit boards, both
boards, typical of the combination that
would normally be employed, must be
used in the test. Testing shall be in accordance
with the procedures specified
in § 15.31.
(i) Under these test conditions, the
system under test shall not exceed the
radiated emission limits specified in
§ 15.109 by more than 6 dB. Emissions
greater than 6 dB that can be identified
and documented to originate from a
component(s) other than the CPU
board being tested, may be dismissed.
(ii) Unless the test in paragraph
(a)(1)(i) of this section demonstrates
compliance with the limits in § 15.109, a
second test shall be performed using
the same configuration described above
but with the cover installed on the enclosure.
Testing shall be in accordance
with the procedures specified in § 15.31.
Under these test conditions, the system
under test shall not exceed the radiated
emission limits specified in
§ 15.109.
(2) In lieu of the procedure in (a)(1) of
this section, CPU boards may be tested
to demonstrate compliance with the
limits in § 15.109 using a specified enclosure
with the cover installed. Testing
for radiated emissions shall be performed
with the CPU board installed in
a typical system configuration. Additional
components, including a power
supply, peripheral devices, and subassemblies,
shall be added, as needed,
to result in a complete personal computer
system. If the oscillator and the
microprocessor circuits are contained
on separate circuit boards, both boards,
typical of the combination that would
normally be employed, must be used in
the test. Testing shall be in accordance
with the procedures specified in § 15.31.
Under this procedure, CPU boards that
comply with the limits in § 15.109 must
be marketed together with the specific
enclosure used for the test.
Given that manufacturers of computers like to say they pass at .1 dB below the
limit (got THAT T shirt!), I suspect the 6 dB option is less attractive than
using a case good for 10 or 20 dB and building a less expensive board.
Without knowing how the original test was done we can't tell whether it's
"legal" to change the cabinet.
Cortland
KA5S
-----Original Message-----
>[snip]
>don't mention approvals for cases per se, but the approval rules for CPU
>boards implies the case is expected provide at least 6dB shielding
>effectiveness. But there's nothing I see in the regulations that requires
>the cases to meet any performance test of their own. With only 6dB being
>expected of them anyway, the cases really aren't that big an issue for
>rules enforcement. The shop is probably in the clear as long as they use
>tested CPU boards, power supplies, and peripherals, and apply the correct
>markings to the unit ("Assembled from tested components..."). The idea is
>that machines built from tested parts will pass the FCC emissions test as a
>whole--but reality is another matter, with clock and bus speeds higher for
>each new processor generation.
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