Jeff Seeger wrote:
Hi, all -
I humbly submit that things change somewhat when an RF
design includes etched components, such as the inductors
that I think were mentioned.
Rather than a nominal impedance, which is adequate for
moderate speed digital signalling, IMHO this case needs
a bit more invasive specification. Etched components
are too subject to the variations that will still all
add up to the correct impedance, they are usually set
to values of a component of impedance. For better ref-
erence, try to look for a "Smith Chart" to see how the
multiple components of impedance converge. HP has one
online somewhere in a very good tutorial but I lack
the link at the moment, sorry!
It's a loop, really: Contact the fabricator of choice
and determine their optimimum values for the desired
impedance. Then use these values for determining the
inductors, being sure to understand where/how the etch
factors are being adjusted. Then echo the fabricator's
values, including laminate vendor and type #'s on the
print.
Good luck,
Jeff and all,
I agree concerning etched component requirements, as all those other design and
process dependent. Certainly you agree most qualified PCB fabricators provide
mostly the same etch capabilities, characteristics, and tolerances. This is wherein
the truth lies. If one can really prove holding 5% over another's 10%, and at the
same time provide acceptable price and delivery, go for it.
I must ask how, out of possible ignorance, one fabricates boards any differently
considering certain industry wide process parameters and management capabilities. I
cannot, in good faith, place in the hands of the board fabricator alone "their
optimum values for the "desired" (to me this must be specified) impedance." I only
(as an engineer and designer) can ask of them their optimum values for etch and all
other process capabilities needed to meet contract and drawing requirements. Then,
before placing an order, I would ask how - and for them to show proof thereof. Following
this, I would give them an order with specified requirements clearly indicated on the
master drawing (in graphics and notes) exactly what I want.
Never would I tell a board supplier how to process my designs. Neither would I ask a
board supplier how to design my designs. Concurrently, we would reach a solution either
through design or process modifications - or some of both.
If it becomes necessary to seek breakthrough, so be it. For now and most, evolution is
acceptable and required. In the good old days (these are them) customer, or mission,
requirements came first. This means finding a way to design something that can be built
using current technology, or find someone capable of inventing new technology - no matter
how less or more preferred.
Respectfully,
Earl Moon
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