[SI-LIST] Re: cpw question?
- From: kaustubh bhate <kaustubhbhate@xxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 13 Aug 2003 03:12:47 -0700 (PDT)
Hi,
I would like to thank everybody for their replies to
my posted questions.
I had a couple more follow-up questions.
1)What information is conveyed by the phase of the
s-parameters generated in hfss?
2)Also, hfss solves for all 3 modes of the cpw and
generates a large data-matrix.So when looking for the
s-parameters and characteristic impedance of the cpw,
do all the other modes, except the cpw mode, have to
be overlooked?
3)I have observed that when the substrate is
"dielectric" the characteristic impedance of the cpw
is rock-solid stable over a wide frequency range (0.8G
to 40G), whereas for a low resistivity substrate (low
resistivity silicon) the characteristic impedance
varies by about 30 ohms for the same above range.Is
there a way to design a cpw of characteristic
impedance of 50 ohm for such a lossy substrate even
over a narrow frequency range or is that only
possible for a particular frequency and not an entire
range?Increasing the oxide thickness on the silicon
substrate reduces the field interaction with the
substrate and hence the loss, so is this one possible
alternative.
I would appreciate all inputs.
Thanks in advance.
Kaustubh.
--- "McCoy, Bart O." <McCoy.Bart@xxxxxxxx> wrote:
> Hi! I put some responses below:
>
>
> > My question is "Is there a relation between the
> values
> > of alpha and s21? What do the values of alpha and
> s21
> > signify? How can I calculate the insertion loss in
> the
> > line.".
>
> Well, I'm no expert or anything, but I'll give this
> one a whirl.
>
> The electric field solution to Maxwell's equations
> for a forward
> propagating wave in a lossy media (TEM) shows:
>
> Ex = Em * exp(-alpha*z)*cos(w* - beta*z + Phase)
>
> x = x direction
> z = z direction (direction of propagation)
>
> Em is the initial magnitude of the E-field applied
> to your
> conductor (e.g. cpw). As you said, alpha is simply
> the real
> part of gamma and is the attenuation constant.
> Forget s-parameters
> for the moment. Alpha tells you only one thing:
> the rate of
> exponential decay of the e-field as the wave
> propagates in the
> z-direction. That equation also holds true for
> the backward
> propagating wave.
>
> Now move on to S-parameters. Insertion loss is
> defined
> with the assumption of the output of your
> transmission structure
> properly terminated (Gonzalez, "Microwave Transistor
> Amplifiers:
> Analysis and Design" 2nd Ed. p. 24). Actually, the
> s-parameters
> are defined in terms of voltage ratios-- not power
> as you mentioned.
> Just be careful in recognizing the difference,
> especially in
> unmatched conditions.
>
> Anyhow, IF you terminate the output of your cpw
> with a conjugate matched impedance, there will be no
> reflected
> power and no backward traveling wave. In that
> case,
> S21, the ratio of the voltages of the output and
> input with
> perfect termination (the definition of S21) will be
> precisely the same as the ratio of Ex/Em in the
> equation above.
>
> Thus, in the perfectly terminated case, S21 (dB)
> should be equal to
> 20*log10[ exp(-alpha * z) ].
>
>
>
> > By definition insertion loss is "power delivered
> to
> > matched load divided by power applied at input",
> but
> > since the cpw does not have a characteristic 50
> ohm
> > impedance, will the value of s21 expressed in db
> equal
> > to insertion loss, when I measure this structure
> on a
> > network analyzer.
>
> If you want the value to equal what you measure on a
> network
> analyzer, make sure your ports are 50 ohms. In the
> case
> of wave ports-- simply renormalize your ports to 50
> ohms,
> since by default, waveports always end up being a
> conjugate
> match port. The cpw won't be 50 ohms in real life
> either, so if
> your ports (virtual and real ones) are both 50 ohms,
> you should
> get the same answer in simulation or in the lab..
>
> If you simulated with lumped gap sources (I'm
> guessing not for cpw-
> but just in case), and those lumped gaps are
> something other than
> 50 ohms to make your system match better, then you
> will have to resimulate
> with 50 ohm lumped gap sources since you can not
> renormalize lumped-gaps.
> BUT, a resim with different lumped gap impedances is
> MUCH faster since HFSS
> won't have to remesh (if that's all you change).
>
> Hope that helps.
>
> - Bart
>
>
>
>
> -----Original Message-----
> From: kaustubh bhate
> [mailto:kaustubhbhate@xxxxxxxxx]
> Sent: Tuesday, August 12, 2003 5:37 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] cpw question?
>
>
> Hi All,
> I am simulating a simple ungrounded cpw in hfss.The
> cpw is on a 0.5 micron thick oxide on a low
> resistivity silicon substrate(1.5-2 ohm-cm).The
> length
> of the cpw is 650 microns and it's configuration is
> 100/50/100.The cpw characteristic impedance is not
> 50
> ohms.(HFSS gives zpi = 12ohms). After simulation
> hfss
> gives results in terms of s-parameters and zo and
> gamma.
>
> I realize that the real part of gamma will give me
> the
> propogation loss, and as the cpw is on a lossy
> substrate I get a high value of alpha (526.5 np/m ie
> 4.573 db/mm at 5 ghz) which is understandable.
> I also get a value of s21 as -3.547 db at 5 Ghz.
>
> My question is "Is there a relation between the
> values
> of alpha and s21? What do the values of alpha and
> s21
> signify? How can I calculate the insertion loss in
> the
> line.".
>
> By definition insertion loss is "power delivered to
> matched load divided by power applied at input", but
> since the cpw does not have a characteristic 50 ohm
> impedance, will the value of s21 expressed in db
> equal
> to insertion loss, when I measure this structure on
> a
> network analyzer.
>
> Thanks in advance.
>
> Kaustubh.
>
> PS:I realize that some folks may think this email
> would be more appropriate for an AE of hfss.
>
>
> =====
>
>
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