[time-nuts] Frequency counter recommendation
Dave M
dgminala at mediacombb.net
Tue Dec 21 17:53:46 UTC 2010
I'd like to input a few suggestions regarding layout and construction,
namely, the PCB material and the form factor.
I suggest that any circuitry processing signals over about 2-3 GHz be
constructed on a PTFE or PTFE/ceramic PCB material, such as Rogers RT/Duroid
(www.rogerscorp.com/). Arlon (arlon-med.com) also markets similar PCB
material. The lower frequency circuitry can be made on FR4 or equivalent.
As to the modularity, I suggest that modules be made so that they are
stackable, but not attached to each other directly, but through a 24, 30 or
40-pin (or whatever width is necessary) ribbon cable on the rear. The cable
would have IDC card-edge connectors spaced so that they could attach to all
modules. Similar to the arrangement that older PCs had to interconnect
floppy and hard disk drives. The modules would be stacked on top of each
other, using aluminum or steel threaded spacers. That way, no card cage,
intermodule interface system or back plane would be needed. Physical size
could be determined at PCB layout time. Build all modules to be stacked,
using the smallest size that would accommodate all modules. Any RF In/out
connectors or displays could be brought out on the front edge of the
modules, and extended to the front of whatever chassis the user chooses by
short extension wires/cables.
As to the production issue, if these boards are envisioned to be distributed
in kit form, I suggest that consideration be given to the possibility of
having a fab house fix the small, high pin count parts to the PCB and leave
the larger, easier to solder parts for the kit builder to assemble. On the
other hand, it is probably just as cheap to have the whole board stuffed and
soldered than only a few ICs. That's a decision to be made by the PCB
manager.
I agree with the power supply approach that all supplies would be user
supplied, with a STRONG recommendation that all supplies be linear. SMPS
supplies could be used at the users' risk.
Thanks for listening,
David
dgminala at mediacombb dot net
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