Hello Experts, I need some help in interpreting the measurement results that I am observing on a test structures board. The stackup for the test board is as follows: Signal ---------------------- Layer One Nelco - 13 Ref. Plane ---------------------- Layer Two Signal ---------------------- Layer Three Ref. Plane ---------------------- Layer Four Core/Pre-preg Ref. Plane ---------------------- Layer Five Nelco - 6 Signal ---------------------- Layer Six Nelco - 13 Ref. Plane ----------------------- Layer Seven Rogers 4350 Signal ----------------------- Layer Eight Total Board Thickness - 85 mils All the SMAs are located only on the top layer of the board. On each of the signal layers (Layer 1, 3, 6, 8), I have 2 differential pairs (25", 40") along with other test structures. The traces are routed in a loosely coupled fashion to hit 100 ohm differential impedance. I have taken the TDR measurements and they are meeting the impedance specification within a 10% tolerance. The traces are routed with wide trace widths to minimize conductor losses. The signal vias are back drilled on layer 3 & 6 to remove the stubs. I have done a four port VNA measurement (single ended) on the same traces and the insertion loss results (S21, S12, S34, S43) do not make sense to me. I would like to understand as to what is the cause of this behavior a) The insertion loss profile for the stripline traces is as expected a) Loss increases as a function of frequency & length. b) S21 profile for Layer 3 is better than Layer 6 b) The insertion loss profile for the microstrip traces show some kind of resonance behavior. a) There is a steep falloff in the S21 profile for the 40" microstrip trace on layer one at frequency less than 2GHz (1.9GHz). I see another steep falloff at 6GHz b) There is a steep falloff in the S21 profile for the 40" microstrip trace on layer eight at frequency less than 2GHz (1.74GHz). I see another steep falloff at 4.78GHz. c) The profile for 25" trace is similar to the 40" trace though the frequency at which the falloff happens is different. In both the cases the trace on layer one behaves slightly better than layer eight. I was hoping to see the best S21 profile for the differential pair on layer 8 followed by the diff pair on layer 1, layer 3 and layer 6 respectively. My question to the group is as follows: Why are the microstrip traces seeing a huge falloff in the S21 profile at specific frequencies? Is this an expected behavior for all the traces routed as microstrip? Is this behavior due to the difference in wave propagation due to non-homogenous system (Air/dielectric) material? If we route the diff. pair as an embedded microstrip instead of a true microstrip (air/dielectric), can we avoid this issue? I would really appreciate all your inputs on this issue. Thanks, Ravi. Confidentiality Notice. This message may contain information that is confidential or otherwise protected from disclosure. If you are not the intended recipient, you are hereby notified that any use, disclosure, dissemination, distribution, or copying of this message, or any attachments, is strictly prohibited. If you have received this message in error, please advise the sender by reply e-mail, and delete the message and any attachments. Thank you. ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List technical documents are available at: http://www.si-list.net List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu