[SI-LIST]: Differential Signal S Parameters

Hi All,
I was extracting 4 port S parameters for differential signal on FR4 PCB
trace referenced to ground with connector and vias in the trace path.

S21 is having resonance at 10GHz where the S parameter file is single ended
version, while same S21 does not have resonance at 10GHz when the single
ended version is converted to mixed mode S-parameters using simulation
tools.

The resonance is expected due to discontinuity in the path but both P & N
of the differential signal is having same discontinuity, hence both P & N
are seeing same amount of insertion loss. Single ended impedance is 50 ohms
while differential impedance is 90 ohms.

So was wondering what is causing a differential signal not have resonance
compared to single ended signal leaving apart the mathematics used in
conversion to mixed mode :-)

Regards
Vinod A H


ah.vinod 5 years 2 months 22 days

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Answered byah.vinod 5 years 2 months 20 days
Thanks David, Ken & Jia Gongxian.
As mentioned rightly by David, It was due to mode conversion & fact that
this behavior is related to the even and odd modes propagate at different
speeds due to the non-homogenous nature of the microstrip interconnect.
Also connector too was adding up to the effect.

Regards
Vinod A H


On Wed, Jul 1, 2015 at 8:37 AM, vinod ah wrote:

The trace configuration is as below.

Top layer Microstrip 3 inch --> Blind Via to Inner layer 3 --> Stripline
3rd layer 3 inch --> Blind via to Top layer --> Top layer Microstrip 1
inch --> Connector.

Layer 2 is gnd while layer 4 is signal layer. Via separation between P & N
is 75 mils. Via differential impedance is 48 ohms.

Regards
Vinod A H



On Wed, Jul 1, 2015 at 3:49 AM, David Banas wrote:

Hi Ken,
I assume you meant to address me and not Vinod.

When I used the term "mode dispersion", I was referring to the different
propagation velocities of the even and odd modes of a differential
micro-strip pair, which is the physical structure I'm assuming Vinod is
discussing.

For an excellent treatment on how this dispersion results in far end
cross-talk, please, refer to section 11.11 of Eric Bogatin's book, "Signal
Integrity - Simplified".

Thanks,
-db


On Tue, Jun 30, 2015 at 1:07 PM, Ken Cantrell
wrote:

Vinod,
There are only two dispersive modes for a Microstrip. The first is when
v_g
(group velocity), is less than the v_p (phase velocity), called normal
dispersion. The second is when v_g is greater than v_p, called
anomalous
dispersion.
Under either scenario, I am not clear how this effects (enhances)
crosstalk.
Can you elaborate?

Thanks,
Ken

-----
Answered byah.vinod 5 years 2 months 21 days
The trace configuration is as below.
Top layer Microstrip 3 inch --> Blind Via to Inner layer 3 --> Stripline
3rd layer 3 inch --> Blind via to Top layer --> Top layer Microstrip 1
inch --> Connector.

Layer 2 is gnd while layer 4 is signal layer. Via separation between P & N
is 75 mils. Via differential impedance is 48 ohms.

Regards
Vinod A H


On Wed, Jul 1, 2015 at 3:49 AM, David Banas wrote:

Hi Ken,
I assume you meant to address me and not Vinod.

When I used the term "mode dispersion", I was referring to the different
propagation velocities of the even and odd modes of a differential
micro-strip pair, which is the physical structure I'm assuming Vinod is
discussing.

For an excellent treatment on how this dispersion results in far end
cross-talk, please, refer to section 11.11 of Eric Bogatin's book, "Signal
Integrity - Simplified".

Thanks,
-db


On Tue, Jun 30, 2015 at 1:07 PM, Ken Cantrell
wrote:

Vinod,
There are only two dispersive modes for a Microstrip. The first is when
v_g
(group velocity), is less than the v_p (phase velocity), called normal
dispersion. The second is when v_g is greater than v_p, called anomalous
dispersion.
Under either scenario, I am not clear how this effects (enhances)
crosstalk.
Can you elaborate?

Thanks,
Ken

-----
Answered bycapn.freako 5 years 2 months 22 days
Hi Ken,
I assume you meant to address me and not Vinod.

When I used the term "mode dispersion", I was referring to the different
propagation velocities of the even and odd modes of a differential
micro-strip pair, which is the physical structure I'm assuming Vinod is
discussing.

For an excellent treatment on how this dispersion results in far end
cross-talk, please, refer to section 11.11 of Eric Bogatin's book, "Signal
Integrity - Simplified".

Thanks,
-db


On Tue, Jun 30, 2015 at 1:07 PM, Ken Cantrell wrote:

Vinod,
There are only two dispersive modes for a Microstrip. The first is when
v_g
(group velocity), is less than the v_p (phase velocity), called normal
dispersion. The second is when v_g is greater than v_p, called anomalous
dispersion.
Under either scenario, I am not clear how this effects (enhances)
crosstalk.
Can you elaborate?

Thanks,
Ken

-----
Answered bycapn.freako 5 years 2 months 22 days
If you’re not seeing it in the differential S-params. then it’s probably not 
resonance, but rather cross-talk due to mode dispersion.
(I’m guessing you’re using micro-strips, as opposed to strip-lines; is that
correct?)
-db

On Jun 30, 2015, at 4:36 AM, vinod ah wrote:

Hi All,
I was extracting 4 port S parameters for differential signal on FR4 PCB
trace referenced to ground with connector and vias in the trace path.

S21 is having resonance at 10GHz where the S parameter file is single ended
version, while same S21 does not have resonance at 10GHz when the single
ended version is converted to mixed mode S-parameters using simulation
tools.

The resonance is expected due to discontinuity in the path but both P & N
of the differential signal is having same discontinuity, hence both P & N
are seeing same amount of insertion loss. Single ended impedance is 50 ohms
while differential impedance is 90 ohms.

So was wondering what is causing a differential signal not have resonance
compared to single ended signal leaving apart the mathematics used in
conversion to mixed mode :-)

Regards
Vinod A H