# RE: [SI-LIST] : Multi-drop backplane trace, with Z0=50 ohms - but my TDR says 40 ohms (fwd)

From: Michael Nudelman ([email protected])
Date: Tue Feb 13 2001 - 11:48:29 PST

The formula for equally distributed cap load is:

Z(trace-with-caps)=Zo * sqrt (C-trace/(C-trace + C-load))

-----Original Message-----
From: Salvador Aguinaga jr [mailto:[email protected]]
Sent: Tuesday, February 13, 2001 02:34
To: [email protected]
Subject: [SI-LIST] : Multi-drop backplane trace, with Z0=50 ohms - but
my TDR says 40 ohms (fwd)

---------- Forwarded message ----------
Date: Mon, 12 Feb 2001 18:51:14 -0600 (CST)
From: Salvador Aguinaga jr <[email protected]>
To: [email protected]
Subject: Multi-drop backplane trace, with Z0=50 ohms - but my TDR says 40
ohms

Hello All,

A backplane trace that was designed to have a characteristic impedance of
50 ohms registers at 40 ohms when I measure it with a TDR scope. Clearly
it's the capacitance of the equally spaced vias (or pin holes) that is
lowering the
impedance of the trace. Or, is it that the trace between two slots is so
short that the TDR cannot resolve the actual impedance of the trace from
slot to slot?

Taking a step back, the impedance coupons on this board say that the trace
with x width should be 50 ohms. However, this impedance test coupon only
has vias (or pin holes) at each end of the trace. So, naturally the
impedance registers at 50 ohms. But, as I mentioned before, on a real
application the trace is going to have pin holes at every slot.

If I absolutely have to have a 50 ohm trace, should I increase the
impedance of the traces between slots to compensate for the added
capacitance of the via? Or should one reduce the parasitic capacitance of
the via, by reasonably increasing the anti-pads on internal layers? And,
no I can't make the backlane any thinner!

Please, let me know what your thoughts are on this, especially if you'd
ran into similar situation.

Thanks a lot!

-- Sal

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