[SI-LIST] Re: Inserion loss of fr4 , 50 ohm trace .

From: Hassan O. Ali <hassan@xxxxxxxx>
To: "si-list@xxxxxxxxxxxxx" <si-list@xxxxxxxxxxxxx>
Date: Sun, 24 Nov 2013 01:45:09 -0500
discrepancy between measurement and simulation. That huge discrepancy
seems to be caused by several factors. No single factor can explain
such a huge discrepancy.
 From the Shlepnev/McMorrow paper (and various cited references),
ENIG plating should contribute less than 1dB/inch of additional loss
even in the worst case at the problem frequencies. You may also need
to double-check your insertion loss curve if you're seeing similar
resonant behavior at around 2.7GHz. If you do, certainly ENIG could
be an issue.
 Incorrect dissipation factor (dielectric loss tangent) of FR-4
suggested by Istvan is another very probable cause. 0.035 sounds to
be a good number to use and easily may double your loss. There are
also substantial material property variations depending on the
supplier.
 Scott pointed out another important consideration - excitation and
the measurement mode. Coplanar waveguide mode may not be the mode
that you're simulating with. Confirm the mode in HFSS through Port
Field Display. Also make sure that you've chosen "Solve Inside" at
least for the line trace and coplanar grounds.
 I assume your HFSS model includes a mated coaxial connector. If not,
that's another source of additional unaccounted loss. And depending on
the type of coaxial connector you're using and the way it was
installed on the test board, you can have other secondary effects.
For example, observe if there is an air gap between the connector
body and the PCB pad it's soldered to. Or check if the connector
ground is well connected to the PCB ground. Even an air gap of less
than 1 mil can have significant insertion loss effects especially at
higher frequencies. 
 Metal surface roughness is also a secondary effect for the
frequencies of your interest. For normal PCB fab processes I don't
think there is much surface roughness induced loss at such low
frequencies of your interest.
 As a general comment, for FR-4, it's not always true that a wider
trace could give you a lower loss. You may need to re-consider your
choice of dielectric thickness and trace width to achieve a lower
loss solution for your line impedance target.
 Best regards,
 Hassan.
 On Sat 11/23/13 10:50 AM , Scott McMorrow scott@xxxxxxxxxxxxx sent:
 Steve 
 there is only one simulator that I know of that can simulate ENIG 
 correctly. It is Simberian Simbeor, where Yuriy Shlepnev introduced
the 
 necessary Landau-Lifshits Ferromagnetic dispersion model based on
our 
 research. 
 Nickel models that I have seen in other simulators uses only
magnetic 
 permeability, which will miss the dispersive resonance between 2 and
3.5 
 GHz, centered around 2.7 GHz. 
 Scott 
 Scott 
 On Sat, Nov 23, 2013 at 11:26 AM, steve weir  wrote: 
 > The demon nickel impact should have shown up in your simulation. 
Didn't 
 > you include the Ni and Au of the ENIG surface finish in your
model? 
 > 
 > Steve. 
 > On 11/23/2013 8:05 AM, sunil bharadwaz wrote: 
 > > Thanks Scott !! Yes , ENIG was used . I'am not suspecting the
CPW , as 
 > the 50 ohms 
 > > is showing correctly when tested with VNA . 
 > > Regards 
 > > Sunil. 
 > > 
 > > 
 > > 
 > > 
 > > 
 > > On , sunil bharadwaz  wrote: 
 > > 
 > > Thanks Bala !! I will try that . Yes , the cable losses were
already 
 > factored in & there are no Via's 
 > > on the RF line . 
 > > 
 > > Regards 
 > > Sunil 
 > > 
 > > 
 > > 
 > > 
 > > 
 > > On Saturday, November 23, 2013 7:15 PM, bala  
 > wrote: 
 > > 
 > > Sunil, 
 > > I would recommend to consider copper roughness and double check
your via 
 > and coplanar modeling in your simulation.I believe you considered
the loss 
 > of your measurement cable.or simply generate s-parameter model
from VNA, 
 >  check symmetric and passivity ,put that model in your sim setup
and see 
 > whats the loss.You shall definitely  get an idea. 
 > > Regards 
 > > bala 
 > > On 23 Nov 2013 18:33, "sunil bharadwaz"  
 > wrote: 
 > > 
 > > Hi friends , 
 > >> We are designing a dual band receiver ( 2.4 Ghz & 5.5 Ghz ) on
a Fr4 
 > substrate . 
 > >> 
 > >> The length of the 50 ohm RF trace ( CPW - 15 mil trace width &
18 mil 
 > spacing to ground ) , 
 > >> is approximately 5 cms .We have simulated this 5 cms long CPW
trace on 
 > HFSS using DK of 
 > >> 
 > >> 4.4 & DF  of 0.018 . The loss reported was about 1.6 dB . 
 > >> 
 > >> We have received the actual board , which shows about 6 dB loss
for the 
 > same trace (actual measurement on VNA) 
 > >> 
 > >> Measured the 50 ohm trace with TDR . It shows about 53 to 54
ohms . The 
 > fab report is also 
 > >> in sync with the measurement on the impedance front .The test
coupon ( 
 > from fab  )also shows 
 > >> the right impedance . 
 > >> 
 > >> Why are we seeing such a huge insertion loss ? The DF of 0.018
seems to 
 > be correct . 
 > >> I have double checked this number , with the fab . 
 > >> 
 > >> Can some one ( pls ) explain the inconsistency between the
simulated & 
 > actual results  ? 
 > >> 
 > >> Best Regards 
 > >> Sunil.Bh 
 > >>
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 > -- 
 > Steve Weir 
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 --  
 Scott McMorrow 
 Teraspeed Consulting Group LLC 
 16 Stormy Brook Rd 
 Falmouth, ME 04105 
 (401) 284-1827 Business 
 http://www.teraspeed.com 
 Teraspeedï¿½ is the registered service mark of 
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[SI-LIST] Re: Inserion loss of fr4 , 50 ohm trace .

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