[SI-LIST] Re: Reasons for the steep falloff in the observed S21 Profile for Microstrip Traces.
- From: steve weir <weirsi@xxxxxxxxxx>
- To: rgali@xxxxxxxxxx
- Date: Mon, 14 Jan 2008 16:49:47 -0800
Ravindra I would attack resonances. Under 2GHz is kind of a low
frequency, especially if you are backdrilling. It makes me think you
have sizeable structural resonances. How big is the board, and how are
the reference planes bonded together?
Steve
Ravindra Gali wrote:
> 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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