[SI-LIST] Reasons for the steep falloff in the observed S21 Profile for Microstrip Traces.

  • From: "Ravindra Gali" <rgali@xxxxxxxxxx>
  • To: <si-list@xxxxxxxxxxxxx>
  • Date: Mon, 14 Jan 2008 15:56:51 -0800

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.

 



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