[SI-LIST] Re: AW: AW: Re: AW: S-parameter model from Datasheet
- From: Scott McMorrow <scott@xxxxxxxxxxxxx>
- To: Curt McNamara <curt.mcnamara@xxxxxxxxxxx>
- Date: Wed, 7 Mar 2012 13:17:53 -0500
Curt
yep, pretty much. Have you taken a look at the insertion loss and return
loss plots in the specification for the relay? If you look at the curves
you'll find that the really "interesting" stuff in the curves is out of
band. Any transformation of band-limited magnitude-only s-parameters with
Hilbert transforms is necessarily a one-to-many operation. you have to
guess at what is out of band. Unfortunately for signalling, what is out of
band at the 3rd and 5th harmonic may be important. And when you cascade an
element that shows out-of-band high frequency return loss peaking (i.e. - a
set of resonant discontinuities) with other other discontinuities, you can
create lower frequency resonances, which are not predicted by the
band-limited s-parameters.
I've been down this road before with the modeling and simulation of
inexpensive RF relays in the same frequency range of this part. I ended up
having to perform 3D EM modeling in order to model the interconnect channel
correctly, with multiple relays cascaded. I was dealing with a 75 ohm
relay, that in reality had better return loss at 50 ohm than it had at 75
ohm. My client had enough clout to push a design change that improved
return loss.
Remember, anything outside of the upper frequency limit of your
s-parameters is just a guess when it comes into filling in the "blanks" in
phase with a causality enforcing method.
regards,
Scott
On Wed, Mar 7, 2012 at 12:38 PM, Curt McNamara <curt.mcnamara@xxxxxxxxxxx>wrote:
> I am a little puzzled by your note. You seem to be saying the fixturing
> plays a large part in measurement accuracy. I get that.
>
> Assuming the vendor did a good job with fixturing, then the published plot
> should represent the relay and not the fixturing.
>
> However you say that the published plot is unlikely to be good, therefore
> a realistic model can't be made from it.
>
> Do I have parts of that right?
>
> Curt
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of Scott McMorrow
> Sent: Wednesday, March 07, 2012 11:28 AM
> To: Gert.Havermann@xxxxxxxxxxx
> Cc: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: AW: AW: Re: AW: S-parameter model from Datasheet
>
> Theoretically Ihsan is correct. However, as a practical matter in most
> structures energy is lost to crosstalk and radiation (especially for an RF
> relay). This distorts both the IL and RL curves. Launch variation for the
> measurement structure limits the practical bounds of return loss
> measurement accuracy, as does impedance variation of the lead-in trace to
> the structure under measurement. As a result, you are lucky if the actual
> accuracy of the return loss measurement is better than -20 dB, and return
> loss peaking (from whence we derive the phase information) is often
> dominated by launch transition discontinuities. Having done quite a few
> measurements of different types of traces, cables, and structures to
> excruciating detail, it is highly doubtful that a reliable model of this RF
> relay can be made purely from the plots shown in the manufacturer's data
> sheet, without knowledge of how they were obtained, and the uncertainties.
>
>
> --
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> Teraspeed(r) is the registered service mark of Teraspeed Consulting Group
> LLC
>
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--
Scott McMorrow
Teraspeed Consulting Group LLC
121 North River Drive
Narragansett, RI 02882
(401) 284-1827 Business
(401) 284-1840 Fax
http://www.teraspeed.com
Teraspeed® is the registered service mark of
Teraspeed Consulting Group LLC
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