[SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz frequency
- From: "Loyer, Jeff" <jeff.loyer@xxxxxxxxx>
- To: <papanovember3@xxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Mon, 1 Dec 2003 08:40:58 -0800
If your T-line isn't exactly 50 ohms, you should expect reflections, and
their associated periodicity. For the short line, I think the period is
large enough that you're only seeing a single peak. If you carried the
simulation out far enough, I believe you'd see troughs and peaks. For
the long line, you're seeing both troughs and peaks.
My calculations show that, for the short T-line (~0.4"), the troughs
should have a period of about 781MHz (assuming 160ps/in Tp). For the
long T-line, that period should be 7.81GHz. Below is a simple Hspice
deck that demonstrates the effect on ideal T-lines. You can adjust Z0
for 35, 50, and 75 ohms and see the effect. The same effect appears on
lossy T-lines, with the peaks and troughs added to the loss effects.
Simulation of VNA of 1 & 10cm 75ohm trace
.option INGOLD=3D1=20
.param vhi=3D2
.param tr =3D35p
V_1_1cm N_1_1cm GND DC=3D0 AC 2 $ 0VDC, 2VAC (will get divided by 2 by
source R)
* 50ohm source R =20
R_SRC_1cm N_1_1cm N_port1_1cm R=3D50
* 35ohm, 1cm Transmission line
Tline_1cm N_port1_1cm 0 N_port2_1cm 0 Z0=3D35 TD=3D'160p * 39.37' =
L=3D.01
* Load Termination =3D 50ohms
R_TERM_1cm N_port2_1cm GND R=3D50
V_1_10cm N_1_10cm GND DC=3D0 AC 2 $ 0VDC, 2VAC (will get divided by 2 =
by
source R)
* 50ohm source R =20
R_SRC_10cm N_1_10cm N_port1_10cm R=3D50
* 35ohm, 10cm Transmission line
Tline_10cm N_port1_10cm 0 N_port2_10cm 0 Z0=3D35 TD=3D'160p * 39.37' =
L=3D.1
* Load Termination =3D 50ohms
R_TERM_10cm N_port2_10cm GND R=3D50
=20
.AC Lin 800 30K 10G
.print v(N_port1_1cm) v(N_port2_1cm) v(N_port1_10cm) v(N_port2_10cm)
.end
Jeff Loyer
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Papa November
Sent: Thursday, November 27, 2003 9:40 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: Dielectric constant for FR4 at above 1Ghz
frequency
Hi
I am a newbie to SI. I have started learning HFSS recently. Here is my=20
problem;
I tried to simulate a 2 trace transmission on a pcb. I created two
models.=20
One is 10 cm long and the other is 1 cm long. All other dimensions are=20
identical.
I am getting a S11 (dB) curve against frequency with bumps for the long
line=20
but a continually increasing curve for the short line. Why is this=20
difference?, Which one is more accurate? Can anyone suggest a way to
cascade=20
10 if the small model S parameters to compare it with the long line
0dB|
|
| **** ****
| * * * *
| * * * *
| * * * * *
| * * * * *
| * * * * *
| * * *
| *
|* This is for 10 cm line
|
___________________________________________________________(f)
0 10 GHz
0dB|
|
| **** ************
| *
| *
| *
| *
| *
|* This is for 1 cm line
|
___________________________________________________________(f)
0 10 GHz
Thanks in advance.
PN
_________________________________________________________________
Is there a gadget-lover on your gift list? MSN Shopping has lined up
some=20
good bets! http://shopping.msn.com
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