[SI-LIST] Re: stupid VNA / TDR / TDT Measurement / Setup question
- From: steve weir <weirsp@xxxxxxxxxx>
- To: hermann.ruckerbauer@xxxxxxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Fri, 14 Nov 2003 01:34:32 -0800
Hermann,
If you test this board in isolation, you need to ask yourself what you are
trying to determine from that test. A passive board like that does not
exist in isolation.
Your situation sounds like a good demonstration for one of Chris Cheng's
pet peeves about the pitfalls of assuming that all planes are
interchangeable. Whether one plane is adequate as a reference for signals
that really refer to another depends on the impedance profile of the planes
over the signal frequency range.
Somewhere in your system power and GND connect back together, just as your
loads close the DC path to ground somewhere. The impedance profile of that
external power network and the intervening interconnect back to your board
is going to show-up in parallel with the roughly 82pF / sq in of plane
capacitance on your board. If that impedance profile is unacceptable, you
have two choices:
1) Fix the impedance profile, or
2) Alter the load to tolerate the impedance.
Regards,
Steve.
At 09:57 AM 11/14/2003 +0100, hermann.ruckerbauer@xxxxxxxxxxxx wrote:
>Hello all,
>
>I do have a stupid question, maybe you can enlighten me ....
>
>Assuming a 6 layer board (Sig1 / GND / Sig2 / / Sig3 / Power / Sig4) with
>some single ended ustrip and stripline transmissionlines teststructures on
>all 4 signal layers. There are no passive components on the PCB (e. g. no C)
>My probes do have a signal and a GND pin. All teststructures does have a
>signal pad and a GND pad for contacting with the probe.
>Therefor I'm fine with Sig1 and Sig2! both are nice GND referenced,
>therefore I do not expect a problem with the measurements there!
>
>But Signal 3 and 4 are Power referenced. And so far there is not too much AC
>short between PWR and GND plane. The distance between this planes is > 30
>mil, therefore I would expect the most AC short coming from the Vias. And
>now I'm not really sure what happens, if I'm doing a VNA / TDR / TDT
>measurement on Sig3 and Sig4, which are Power referenced, as my measurement
>ground is connected to the GND plane on the board.....
>
>Can the results tell me something, or can I forget the results completly ???
>Is there a difference for VNA / TDR or TDT Measurments in this case ?
>
>
>thanks
>
>
>Hermann
>
>
>
>
>
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