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

  • From: Istvan Novak <istvan.novak@xxxxxxxxxxx>
  • To: sunil_bharadwaz@xxxxxxxxx, SI LIST <si-list@xxxxxxxxxxxxx>
  • Date: Sat, 23 Nov 2013 11:01:03 -0500

Hi Sunil,

You did not mention what frequency you had the 1.6dB simulated and 6dB 
measured loss...

Though a 15-mil trace looks wide enough that copper roughness may not be 
the biggest problem here, keep in mind that PCB manufacturers may use 
fairly rough foil on the outside to promote adhesion.

Lastly, when you say that DF of 0.018 was double checked with the fab: 
what do you mean by that?  Keep in mind that FR4 is just a broad 
category of laminates, and the worst-case DF at 1GHz could be as high as 
0.035.

Regards,

Istvan Novak
Oracle



On 11/23/2013 7:59 AM, 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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