[SI-LIST] Re: TDR and line losses
- From: "Kyung Suk (Dan) Oh" <doh@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Mon, 24 Nov 2003 09:34:16 -0800
Hi, I would like to add one comment to this issue.
The conductor loss definitely contributes to this upward creep but
there is also an additional physics which contributes to this upward
creep and this one is often forgotten and I would like to clarify them.
The initial impedance level should be corresponding to the lossless
characteristic impedance. After initial impedance level there are
two mechanisms which make the impedance profile to creep upward.
The first one is resistive loss as others pointed out and
the second one is the internal inductance which increases
the characteristic impedance at low frequencies.
It is important to first understand that the upward creep is NOT due
to the reflected wave along the transmission line but it is the
reflected wave of the initial edge at the beginning of the
transmission line.
Mathematically, it is the convolution between the input edge
and the characteristic impedance only and not related with
the propagation constant.
Physically, this reflected wave does not contain any reflection
along the line (assuming it is uniform) until the reflection from
the other end comes back.
At the very beginning, the input edge actually sees the
characteristic impedance at the very high frequency which is
the impedance based on L over C, say Zc_inf.
And the later response sees the characteristic impedance
at lower frequencies. At these lower frequencies the characteristic
impedance is larger than Zc_inf due to the resistance term AND
the internal inductance term.
As you make the line longer, you would see the increasing in the
impedance profile which can be mistakenly thought as due to the
increase in the loss. As this creeping is not due to the "loss"
mechanism along the transmission line, but it is due to the change
in the characteristic impedance due to loss; hence, it is not
depending on the line length.
If you increase the line length to fairly large this creep will
eventually saturate to the characteristic impedance at dc which
would be finite if there is any dc conductance loss. Otherwise it
will continue to grow as the characteristic impedance becomes infinite
at dc without dc conductance.
In reality, the characteristic impedance measurement shows a finite
value at low impedance so the upward creep should be saturate beyond
a certain length.
"The bottom line is that if your characteristic impedance varies
significantly from dc to high frequency, the upward creep will be
there (assuming the impedance changes from high to low as the
frequency increases)"
I have attached the simulated TDR response using Hspice w/
the following three characteristic impedances to demonstrate
the impact of the internal
inductance:
case 1: sqrt(L/C)
case 2: sqrt((Ro+Rs*sqrt(f)+jwL)/(jwC))
case 3: sqrt((Ro+Rs*sqrt(f)(1+j)+jwL/(jwC))
Regards,
-Kyung Suk (Dan) Oh
Dima Smolyansky wrote:
> Suresh,
>
> The upward slope of the TDR trace is indicative of losses. However, the
> losses will need to be quite substantial for the upward "creep" to be
> clearly visible. In other words; your transmission trace (TDT) will show
> even fairly small losses through rise time amplitude degradation; however,
> when you begin to see the "creep" in the reflection (TDR), that will show up
> as large rise time and amplitude degradation in TDT.
>
> Also, Howard Johnson did an article once, where he played with skin effect
> and dielectric loss, and showed how they affect different portion of the TDT
> waveform. You can do the same in IConnect's lossy line model by varying the
> skin effect and dielectric loss parameters independently, and evaluating
> their effect on the TDT (or TDR) waveform.
>
> Thanks,
>
> -Dima
>
> ----- Original Message -----
> From: "Suresh Subramaniam" <Suresh.Subramaniam@xxxxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Sent: Friday, November 21, 2003 2:00 PM
> Subject: [SI-LIST] TDR and line losses
>
>
>
>>If I TDR (rise time 26ps) a long lossy transmission line, should I expect
>>the impedance profile to gradually creep up to a higher value (assuming I
>>start out with a 50 Ohm impedance?). In other words, how does the TDR
>>take into account the effect of losses?
>>
>>Thanks
>>Suresh
>>
>>
>>
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-- Binary/unsupported file stripped by Ecartis --
-- Type: application/pdf
-- File: tdr_study.pdf
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