[SI-LIST] Re: Ringing in Return Loss data
- From: steve weir <weirsi@xxxxxxxxxx>
- To: qazi <qazi@xxxxxxxxxx>
- Date: Tue, 26 Aug 2008 17:30:13 -0700
Is the return loss inadequate or are you just curious as to why the
ripple? How much ringing are we talking about? At what frequencies?
This is all in simulation?
Steve.
qazi wrote:
> Thank you guys. Yes I do not have any mismatch in routing other than that
> the phase matching of the P and N of the pairs are slightly out of sync. It
> is on the order of .03" and that is the only mismatch I see in the design.
> Istvan, do you have any reference document for the effect that you
> mentioned? I would like to read this in little bit details.
>
> -Qazi
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
> Behalf Of istvan Novak
> Sent: Monday, August 25, 2008 7:35 PM
> To: 'qazi'
> Cc: Jimmy; si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Re: Ringing in Return Loss data
>
> Qazi,
>
> Just to clarify: if this is Return Loss and not Insertion Loss, the
> periodic ripple (ringing) is inevitable.
> Actually it is inevitable in both, just in Insertion Loss it is hardly
> visible if the trace is close enough
> to the reference impedance. Return Loss, on the other hand,
> periodically drops to zero (minus
> infinite on the dB scale), wherever the frequency corresponds to
> multiples of half wavelength
> over the trace length. At even multiples of quarter-wave resonance the
> Return Loss reaches its
> maximum. Unless the trace is completely ideal, we can not get a
> frequency independent characteristic
> impedance, so even if the trace's characteristic impedance is close to
> the reference impedance, the
> nulls at multiples of the half-wavelength resonance make the remaining
> small mismatch very obvious.
> The Insertion Loss, on the other hand, will 'flatten out' to the naked
> eye if the characteristic impedance
> comes to within a couple of percent of the reference impedance.
>
> Regards,
>
> Istvan Novak
> SUN Microsystems
>
> Jimmy wrote:
>
>> As Steve stated, you have a mismatch somewhere in your routing. Suggest
>>
> you
>
>> look at layer changes and number of VIAs in each of the diff pair to see
>>
> if
>
>> both are the same. If they do change layers, check how that changes
>> regarding your planes and there distance from them. Check the driver
>> strength so see if these are being overdriven and you are having ring back
>> from undershoot/overshoot. Lastly, if you can find nothing wrong question
>>
> if
>
>> all the input into the simulator is correct.
>>
>> Omega
>>
>> -----Original Message-----
>> From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
>>
> On
>
>> Behalf Of qazi
>> Sent: Monday, August 25, 2008 8:20 PM
>> To: si-list@xxxxxxxxxxxxx
>> Subject: [SI-LIST] Ringing in Return Loss data
>>
>> Gurus,
>> I can explain ringings on a TDR data but can anyone tell me why I am
>>
> seeing
>
>> this on a Return Loss data for a layout that is geometrically correctly
>> done?
>> This is a simulation result based on a .brd file in allegro and I was
>> wondering if I was doing something wrong here as far as modeling the
>>
> device
>
>> or the SMAs. This board uses Fr-4 and the traces are around 4.5 inches
>>
> long
>
>> diff pairs. I know the impedances are kept constant (same geometry)
>> throughout the whole path and return path is ideal.
>> Rgds,
>> Qazi
>>
>>
>>
>>
>>
>
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