[SI-LIST] Re: SDD21 and SDD11
- From: <Wolfgang.Maichen@xxxxxxxxxxxx>
- To: <johnlinc@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 30 Nov 2011 13:53:15 +0000
Hello John,
>> Can I say to meet S21 shall be OK for signal transmission?
Well, not necessarily. E.g. if your transmitter is not well matched to the line
impedance then the reflected signal sill get re-reflected at the transmitter
side and eventually end up at the receiver side, where it gets superimposed on
the desired data signal. If this reflection arrives at the same time as a
subsequent data transition you get an increase in timing jitter.
Take my example with S21=-0.5dB, S11=-9.6dB means the reflected signal has
about 30% of the incident amplitude. If the driver is strongly mismatched then
you get almost as much at the receiver!
Wolfgang
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of john lin
Sent: Wednesday, November 30, 2011 11:16 AM
To: si-list
Subject: [SI-LIST] Re: SDD21 and SDD11
Hi Wolfgang,
Thank you for shedding your light on my questions.
The spec I talk about is IB, running QDR and FDR.
Masks are given for insertion loss and return loss.
Can I say to meet S21 shall be OK for signal transmission?
S11 is something related to EMI control, mentioned by Daniel Lai in his
mail.
The masks are given in different purposes. S21 is necessary, but not S11.
or, are both "MUST"?
Thanks,
John Lin
2011/11/30 <Wolfgang.Maichen@xxxxxxxxxxxx>
> Hello John,
>
> keep in mind that for a loss-less transmission path (i.e. no energy
> absorbed, so anything that does not come out at the other end - S21 - must
> get reflected back - S11) energy conservation tells us that
>
> S11^2 + S21^2 = 1
>
> Or in other words, S11 = sqrt(1 - S11^2). Note that the formula takes the
> numbers as attenuation factors, not dB!
>
> S21 = -0.5dB = 0.944 gives a horrible S11 = 0.33 = -9.6dB.
> Even almost perfect S21 = -0.1dB = 0.989 still gives a relatively poor S11
> = 0.15 = -16dB.
>
> Regards,
>
> Wolfgang
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
> On Behalf Of steve weir
> Sent: Wednesday, November 30, 2011 4:36 AM
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: SDD21 and SDD11
>
> You can have passable Sx21 and still crummy Sx11. That is fairly
> common. You need to hunt down what is going on. It could be your
> measurement, or it could be you have a real problem with SDD11.
>
> Steve.
> On 11/29/2011 7:13 PM, john lin wrote:
> > Hi SI experts,
> > Could you please help to comment about I have a good SDD21, insertion
> > loss, but not a good SDD11, return loss?
> > Shall low insertion loss have a low return loss?
> > The total power shall include transmitted and reflected powers.
> >
> > From my recent S4P extraction for a differential pair about 3 inch in
> > length, I can see
> > SDD21 does meet spec. However, the SDD11 violates the spec, less than
> > -10dB.
> > will it be OK? Is the SDD21 our main concern?
> >
> > Thank you for your helps in advance.
> >
> > Best Regards,
> > John
> >
> >
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>
> --
> Steve Weir
> IPBLOX, LLC
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