[SI-LIST] Re: Conversion from dB to ohm
- From: Ray Anderson <Raymond.Anderson@xxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Fri, 25 Jul 2003 13:10:05 -0700 (PDT)
>
>Hi! The formula for conversion from ohm to dB is :
>Ztrans=25*10^(|S21|/20).
>So if S21=-60dB then Ztrans=25000, what is this value
>in i.e. units is in ohm but do I need to divide it
>with some factor to get in ohms.
>I am trying to convert and it is not corelating.
>Any inputs appreciated.
>Thanks
>Bob
>
I don't think you want the absolute value of S21, but the
signed value of S21. (there is a difference between -60 dB and
60 dB)
The expression 10^(dB/20) converts from dB to a dimensionless
S21 value. (The dB value is relative to a straight thru calibration
value which would be 0 dB)
The transfer impedance is then (S21 * Zo) which will then be
in ohms.
Note that for this expression to be valid, the impedance being measured
must be much, much less than 25 (which is the parallel combo of the
generator impedance and the load impedance).
Example:
Assume a pair of planes with a decoupling cap between the power plane
and the reference plane.
Probing the plane at two separate locations (say A and B) with a
VNA gives a measured value of -60 dB (compared to a straight thru
calibration connection) at the resonant frequency of the decap.
then:
S21 = 10^(-60dB/20) = .001
Z21 = S21 * 25 ohms
Z21 = .001 * 25 (25 milliohms) which in this case is the
ESR of the power distribution system.
See Istvan Novak's web site for a copy of a nice Designcon 2000 paper
covering this topic:
http://home.att.net/~istvan.novak/papers/dc2000_rev.pdf
-Ray
Sun Microsystems Inc.
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