As I interpret your message, you are finding many boards which do not meet
your controlled impedance requirements of 50 ohms +- 10%.  And when you
measure individual components, values are within +- 10% of spec.

Since you can get the board to work by component replacement perhaps you
just need to start with a tighter spec on incoming parts?

As to why the passive components must be replaced, the most obvious
question is whether the design was analyzed for the case of all components
on a transmission line being at the low or high end of the tolerance range?
For example a resistor and a capacitor  on a transmission line, which are
both low but in spec relative to nominal, could produce an out-of-spec
impedance depending on the design checking.

Alternatively, you might check that the RF boards are being made/delivered
with correct transmission line properties.  I suggest using a TDR
scope/probe and measuring the reflection coefficients for a 50 ohm setup to
estimate the transmission line impedance.  TDR might also be useful to
isolate if there are unexpected discontinuities in the transmission line
due to unanticipated coupling.





Ken Patel <[log in to unmask]> on 08/26/99 04:50:55 PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond
      to Ken Patel <[log in to unmask]>


To:   [log in to unmask]
cc:    (bcc: John C. Schultz/US-Corporate/3M/US)
Subject:  [TN] Any connection: Impedance Control value & RF Board




Experts,
We have couple of RF boards which are relatively new to us. Many times, we
have to replace passive components to make it work. Hence, I have this
hypothetical question for you. Is it due to not having correct impedance
value even though measured value on coupon falls within 50+/-10 ohms%? Do
you think we may to have tighten our current impedance specification
(50+/-10 ohms%) if there is a relationship?

Being a mechanical kind of guy this question may sound stupid! Hope to hear
from many of you.

re,
ken patel
______________________________________________________
Ken Patel                       Phone:  (408) 490-6804
1708 McCarthy Blvd.             Fax:    (408) 490-6859
Milpitas, CA 95035              Beeper: (888) 769-1808

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