[SI-LIST] Re: Number of points for your VNA
- From: "FU, Yejun" <yfu@xxxxxxxxx>
- To: Scott McMorrow <scott@xxxxxxxxxxxxx>
- Date: Mon, 13 Jan 2014 09:32:14 -0500
Hello Scott:
I think it only takes about 2 rounds trip to dissipate not 4
rounds, which is more easy to understand as from point view of phrase aliasing.
To avoid it, you have to make sure your phase rotating is less than 180 degrees
between your two adjacent frequency points.
Phase Theta¾ta*L here, beta=2Pi/lambda,
lambda=c/[f*sqrt(dielectric constant)],
phase variation, delta Theta, must less than Pi (180).
Then, combine all above relations then you will have:
Delta f < 1/[2*L* propagation delay]
So it is 2L
Johnny
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
Sent: Tuesday, January 07, 2014 10:10 AM
To: FU, Yejun
Cc: lingyunwang@xxxxxxxxx; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Number of points for your VNA
The rule of thumb to absolutely avoid aliasing in the time domain is to assume
that residual energy in the interconnect takes 2 round trips to dissipate. for
30" of trace in a low loss dielectric propagation is approximately 160 ps per
inch. Two round trips would therefore be 30 X 4 X 160 ps = 19.2 ns of delay.
That corresponds to a frequency of 1/19.2ns = 52 MHz.
The maximum frequency domain step size should be no more than 50 MHz (a nice
round number). Since you have enough resolution on the VNA to go down to 10
MHz, and I happen to like step sizes that give sample points that are multiples
of 5 and/or 10, you can set your VNA to sweep from 10 MHz to 40 GHz in 10 MHz
steps, which would be 4000 points.
A reasonable setting for your VNA is to sweep from 10 MHz to 40 GHz with 4000
points. This will give you all the resolution that you need for any
interconnect analysis in your system.
On Tue, Jan 7, 2014 at 8:47 AM, FU, Yejun <yfu@xxxxxxxxx<mailto:yfu@xxxxxxxxx>>
wrote:
Hello Lyn:
It's a 30inchs high speed differential trace, I set the span
from 10MHZ to 40GHz, with number of point 160001(please don't ask me why). I
have long argument and talking with many people then I post my question to you
guys.
Johnny
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>
[mailto:si-list-bounce@xxxxxxxxxxxxx<mailto:si-list-bounce@xxxxxxxxxxxxx>] On
Behalf Of Lingyun Wang
Sent: Monday, January 06, 2014 12:20 PM
To: fuyejun@xxxxxxxxx<mailto:fuyejun@xxxxxxxxx>
Cc: si-list@xxxxxxxxxxxxx<mailto:si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: Number of points for your VNA
Hi Johnny,
Unquestionably, the larger the number of points is, the better the accuracy
will be. The penalty along with that is the unendurable calibration time,
restless waiting for each measurement being done, and the large data file if
you want to keep it a record.
What kind of measurement that you are working on? If it is for PDN, the log
scale will be recommended since the low frequency points really matters. If it
is for high speed with long length DUT, the linear scale at high frequency
range to guarantee no missing points.
There is a list of questions needed to be known before finding the best
sampling points: such as the length of DUT, frequency range to be investigated,
purpose of the measurement...
It is the trade off between the measurement efficiency (mostly the time
consumed in the each measurement) and the accuracy. If the DUT is less than
50inch, the log scale with 501pt/dec would be a safe zone for kind of
measurement.
Best,
Lyn
On Mon, Jan 6, 2014 at 7:17 AM, Yejun Fu
<fuyejun@xxxxxxxxx<mailto:fuyejun@xxxxxxxxx>> wrote:
> Hi all:
>
> I want to know how to define your VNA's # of points, some
> person told me it is depends on the your DUT's length, some times if
> you have fibers in your DUT you have to change your points value.
> Could any one help to clear this question?
>
> Thanks
>
> Johnny
>
>
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Teraspeed Consulting Group LLC
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