[SI-LIST] Re: PCB Power Supply Impedance
- From: "Bart Bouma" <bart.bouma@xxxxxxxxx>
- To: clandrum@xxxxxxxxx
- Date: Wed, 7 Jan 2004 10:06:14 +0100
Hi Chris,
The calculation indeed is not difficult.
In fact it is a (complex-) voltage divider: consisting of the DUT and the
system impedance.
I assume you measured in a 50 Ohm system.
Since source and load are connected in parallel as seen by the DUT, the
system impedance seen by the DUT = 25 Ohm.
This 25 Ohm is now our reference, i.e. insertion loss is the ratio in dB
of the DUT's impedance to this 25 Ohm system-impedance.
The voltage divider: Vout/Vin = Zdut / (Zdut + 25) .
Ins Loss in dB : 20Log ( Zdut / (Zdut + 25) ).
(this is the readout of the VNA: S21 in dB).
Following simplified equation is valid as long as the measured impedance
is much lower than 25 Ohms
Ins Loss in dB : 20Log ( Zdut / 25 ).
Now the other way around:
Zdut much lower than 25 Ohms:
------------------------------------------------------------
Zdut [Ohm] = 25 * 10^(ins.loss/20)
__________________________________
To be complete:
the exact equation (to be used when Zdut is not much lower than 25 Ohms)
is:
--------------------------------------------------------------------------------------------
Zdut = 25 * [ 10^(ins.loss/20) / (1 - 10^(ins.loss/20)) ]
_____________________________________________________
best regards, Bart Bouma
www.yageo.com
Chris Landrum x311 <clandrum@xxxxxxxxx>
06-01-04 23:12
Sent by: si-list-bounce@xxxxxxxxxxxxx
Please respond to clandrum
To: "'si-list@xxxxxxxxxxxxx'" <si-list@xxxxxxxxxxxxx>
cc:
Subject: [SI-LIST] PCB Power Supply Impedance
Category:
Hello all,
I am attempting to measure the impedance curve of the power planes of a
test
board I have built. I am using a spectrum analyzer to accomplish this and
have obtained a nice capture of dB vs. Freq. I am having trouble relating
the dB axis to impedance (Ohms). Can anyone shed some light as to how I
would go about converting the dB to mOhms?
I'm sure this is an easy one, but it is escaping me.
Thanks.
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