[SI-LIST] Re: 50 ohm - RF Impedance Traces

From: "Tom Dagostino" <tom@xxxxxxxxxxxxxxxxx>
To: "'Hakim Sellaoui'" <hakim.sellaoui@xxxxxxxxxxxxxxx>
Date: Thu, 22 Dec 2016 09:34:15 -0800

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Hakim

=20

Let=E2=80=99s say your bad launch has a deep suck-out at 6 GHz and the =
information you are looking for is at 6 GHz. How much math will you =
have to throw at it to recover the data? Poor launches always degrade =
signal to noise ratios. It limits how much you can de-embed.

=20

Tom Dagostino

 =20

971-279-5325

<mailto:tom@xxxxxxxxxxxxxxxxx> tom@xxxxxxxxxxxxxxxxx=20

=20

LogoAddress

=20

From: Hakim Sellaoui [mailto:hakim.sellaoui@xxxxxxxxxxxxxxx]=20
Sent: Thursday, December 22, 2016 2:56 AM
To: tom@xxxxxxxxxxxxxxxxx; Gert.Havermann@xxxxxxxxxxx
Cc: 'si-list'; 'sunil bharadwaz'
Subject: RE: [SI-LIST] Re: 50 ohm - RF Impedance Traces

=20

Hi Tom,

=20

Say you do have a bad launch, but you=E2=80=99re able to de-embed the =
fixture/SMAs through whatever de-embedding technique, would the bad =
launch still have an effect?

=20

Can I get a copy of that pdf too?

=20

Thanks,

=20

-=20

Hakim Sellaoui

Associate Hardware Engineer

DisplayLink (UK) Limited

=20

Tel: +44 (0)1223 443920

Fax: +44 (0)1223 443921

E: hakim.sellaoui@xxxxxxxxxxxxxxx

W: http://www.displaylink.com/

=20

140 Science Park, Milton Road,

Cambridge, CB4 0GF,

United Kingdom.

Registered in England No. 04811048

=20

DisplayLink Corp.

A US incorporated company.

480 S. California Avenue Ste 305, Palo Alto, CA 94306, USA.

=20

From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of Tom Dagostino
Sent: 20 December 2016 22:48
To: Gert.Havermann@xxxxxxxxxxx
Cc: 'si-list' <si-list@xxxxxxxxxxxxx>; 'sunil bharadwaz' =
<sunil_bharadwaz@xxxxxxxxx>
Subject: [SI-LIST] Re: 50 ohm - RF Impedance Traces

=20

Sunil
It does not take much of a discontinuity in a launch to make a good =
trace have poor return loss. See the enclosed pdf "tale of three =
launches". I compare three different launches, a very good Ohmish =
discontinuity launch; one of several Ohms and a hand soldered poor =
launch. The discontinuity at each end affects both the return loss and =
the insertion loss. The three different launches have widely different =
unable bandwidths on the identical trace, from less than 2 GHz to =
greater than 20 GHz.

The two poor launches looked inductive. I've also looked at other SMA =
launches that were capacitive. The same results apply.

The best launch was one designed by Scott McMorrow. The second best was =
an early attempt by Molex for their connector. The worst example was a =
SMA connector soldered to Scott's design. The large loop formed a large =
inductor.

Some say a rule of thumb for evaluating measurement systems is to look =
at the difference between the insertion loss and return loss. If you =
have at least 10-15 dB difference you can use the system out to that =
limit.=20

I doubt that the attachment will get through the list. If you are =
interested in this drop me a note.

Regards,

Tom Dagostino
971-279-5325
tom@xxxxxxxxxxxxxxxxx=20

Teraspeed Labs
9999 SW Wilshire Street
Suite 102
Portland, OR 97225

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of Havermann, Gert
Sent: Tuesday, December 20, 2016 2:01 AM
To: dmarc-noreply@xxxxxxxxxxxxx; tom@xxxxxxxxxxxxxxxxx
Cc: si-list
Subject: [SI-LIST] Re: 50 ohm - RF Impedance Traces

I think that the topology is the problem. Coplanar waveguides are not =
easy to control, especially when the clearance to GND is too tight or =
too large, and 18 mil is really large. Just imagine that the electrical =
field will be active in an area at least 3 times the GND- distance all =
around your trace. When there are other traces/circuits underneath, or =
some mechanical parts (table, skrews, housings....) then they all =
influence the Impedance.
You also need to take into account that in reality you will not see an =
perfect open in the smith chart just because the trace has an open end =
somewhere. You need to take the trace inductance and capacitance into =
account, and electrical length also changes the "dot" smith chart. The =
Laminate itself is also frequency dependent so it will be almost =
impossible to create an perfect open with a long trace.

Please define " the launch does not look bad either". What is the TDR =
Risetime and what kind of impedance deviations do you measure? For 6GHz =
you should test with at least 100ps Risetime, and a decent launch should =
then vary by less than 5 Ohm. A good launch will vary in the 1ohm range. =
If you plan on even higher frequencies, then the same applies for 30ps =
risetime. And that=E2=80=99s just some rule of thumb without knowing all =
the details it is hard to really give a recommendation right on the =
spot.

BR
Gert

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On Behalf Of sunil bharadwaz
Sent: Tuesday, December 20, 2016 8:38 AM
To: tom@xxxxxxxxxxxxxxxxx; dmarc-noreply@xxxxxxxxxxxxx
Cc: si-list <si-list@xxxxxxxxxxxxx>
Subject: [SI-LIST] Re: 50 ohm - RF Impedance Traces

Hello Tom ,
Thanks much for your response .Thanks much to all ,who have replied to =
this thread.
The launch is a proper SMA connector . The final board is going to carry =
this RF connector . On the TDR , the launch does not look bad either =
.However , one observation is that , the larger the 50 ohm traces , =
worse is thereturn loss @ 5 Ghz & above .

I'am checking @ Test coupon as well as the actual board .On the actual =
board ,the RF trace has a SMA connector on one side & the other end is =
open .On the VNA , when we check the S11 @ Smith chart , ideal =
Transmission line should have a dot to the right extreme indicating that =
the line is open .However , i see all kinds of shapes @ frequency.

Can we get the solder mask done in normal way or do we have to take any =
special care in getting the solder mask done on the RF traces ?
We generally use Co planar Waveguide traces (with ground ) , for 50 ohms =
lines. Typically , theRF trace widths are 15 mil @ 18 mil distance from =
ground running on either sides . We did extractthe S-parameters before =
sending the Layout for Fab . Fab of course did send us the impedance =
report (measured on TDR ).

RegardsSunil.B
From: Tom Dagostino <tom@xxxxxxxxxxxxxxxxx>
To: dmarc-noreply@xxxxxxxxxxxxx
Cc: si-list <si-list@xxxxxxxxxxxxx>
Sent: Sunday, December 11, 2016 11:47 PM
Subject: [SI-LIST] Re: 50 ohm - RF Impedance Traces

How good is your launch into your test trace? If it has a large =
discontinuity it may be masking the true impedance. Try looking at the =
trace with a TDR display and see what the trace impedance is.
Tom Dagostino
Teraspeed LabsOn Dec 11, 2016 8:17 AM, sunil bharadwaz =
<dmarc-noreply@xxxxxxxxxxxxx> wrote:

Hello Group ,
We seem to be running into issues while getting PCB's fabricated with =

50 ohm lines.Our Freq band of interest is 2 Ghz to 6 Ghz.Tried checking =
these traces on a VNA.The return loss looks decent @ 2 Ghz , but seldom =
looks good at 5Ghz & above .

The fabricator does send test coupons & impedance reports @ TDR .This =

does notcover the freq response .

This is not issue with any single fab . This seems to be a general =

problem , where return loss isnot flat across freq band .

Wanted to check if there are ways to tackle this issue .
Thanks much.

RegardsSunil.B

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<html xmlns:v=3D"urn:schemas-microsoft-com:vml" =
xmlns:o=3D"urn:schemas-microsoft-com:office:office" =
xmlns:w=3D"urn:schemas-microsoft-com:office:word" =
xmlns:m=3D"http://schemas.microsoft.com/office/2004/12/omml" =
xmlns=3D"http://www.w3.org/TR/REC-html40";><head><meta =
http-equiv=3DContent-Type content=3D"text/html; charset=3Dutf-8"><meta =
name=3DGenerator content=3D"Microsoft Word 14 (filtered medium)"><style></style></head><body lang=3DEN-US link=3Dblue =
vlink=3Dpurple><div class=3DWordSection1>Hakim<o:p></o:p><o:p> </o:p>Let=E2=80=99s say your bad launch has a deep suck-out at 6 GHz and =
the information you are looking for is at 6 GHz.=C2=A0 How much math =
will you have to throw at it to recover the data?=C2=A0 Poor launches =
always degrade signal to noise ratios.=C2=A0 It limits how much you can =
de-embed.<o:p></o:p><o:p> </o:p><div>Tom =
Dagostino<o:p></o:p>=C2=A0=C2=A0=C2=A0 <o:p></o:p>971-279-5325<o:p></o:p><a href=3D"mailto:tom@xxxxxxxxxxxxxxxxx";>tom@xxxxxxxxxxxxxxxxx</a> =
<o:p></o:p><o:p> </o:p><img border=3D0 width=3D246 height=3D85 id=3D"Picture_x0020_1" =
src=3D"cid:image001.png@01D25C36.81379AA0" =
alt=3DLogoAddress><o:p></o:p></div><o:p> </o:p><div><div =
style=3D'border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in =
0in 0in'>From:=
 =
Hakim Sellaoui [mailto:hakim.sellaoui@xxxxxxxxxxxxxxx] ; Sent: =
Thursday, December 22, 2016 2:56 AM To: tom@xxxxxxxxxxxxxxxxx; =
Gert.Havermann@xxxxxxxxxxx Cc: 'si-list'; 'sunil =
bharadwaz' Subject: RE: [SI-LIST] Re: 50 ohm - RF Impedance =
Traces<o:p></o:p></div></div><o:p> </o:p>Hi Tom,<o:p></o:p><o:p> </o:p>Say you do have a bad launch, but you=E2=80=99re able to de-embed the =
fixture/SMAs through whatever de-embedding technique, would the bad =
launch still have an effect?<o:p></o:p><o:p> </o:p>Can I get a copy of that pdf too?<o:p></o:p><o:p> </o:p>Thanks,<o:p></o:p><o:p> </o:p>- <o:p></o:p>Hakim Sellaoui<o:p></o:p>Associate Hardware Engineer<o:p></o:p>DisplayLink (UK) Limited<o:p></o:p><o:p> </o:p>Tel: +44 (0)1223 443920<o:p></o:p>Fax: +44 (0)1223 443921<o:p></o:p>E: <a =
href=3D"mailto:hakim.sellaoui@xxxxxxxxxxxxxxx";>hakim.sellaoui@displaylink=
.com</a><o:p></o:p>W: <a =
href=3D"http://www.displaylink.com/";>http://www.displaylink.com/</a><o:p>=
</o:p><o:p> </o:p>140 Science Park, Milton Road,<o:p></o:p>Cambridge, CB4 0GF,<o:p></o:p>United Kingdom.<o:p></o:p>Registered in England No. 04811048<o:p></o:p><o:p> </o:p>DisplayLink Corp.<o:p></o:p>A US incorporated company.<o:p></o:p>480 S. California Avenue Ste 305, Palo Alto, CA 94306, =
USA.<o:p></o:p><a =
name=3D"_MailEndCompose"></a><o:p> </o:p><div><div =
style=3D'border:none;border-top:solid #E1E1E1 1.0pt;padding:3.0pt 0in =
0in 0in'>From:</span=

<span =

style=3D'font-size:11.0pt;font-family:"Calibri","sans-serif"'> <a =
href=3D"mailto:si-list-bounce@xxxxxxxxxxxxx";>si-list-bounce@xxxxxxxxxxxxx=
</a> [<a =
href=3D"mailto:si-list-bounce@xxxxxxxxxxxxx";>mailto:si-list-bounce@freeli=
sts.org</a>] On Behalf Of Tom Dagostino Sent: 20 =
December 2016 22:48 To: <a =
href=3D"mailto:Gert.Havermann@xxxxxxxxxxx";>Gert.Havermann@xxxxxxxxxxx</a>=
 Cc: 'si-list' <<a =
href=3D"mailto:si-list@xxxxxxxxxxxxx";>si-list@xxxxxxxxxxxxx</a>>; =
'sunil bharadwaz' <<a =
href=3D"mailto:sunil_bharadwaz@xxxxxxxxx";>sunil_bharadwaz@xxxxxxxxx</a>&g=
t; Subject: [SI-LIST] Re: 50 ohm - RF Impedance =
Traces<o:p></o:p></div></div><o:p> </o:p>Sunil It does not =
take much of a discontinuity in a launch to make a good trace have poor =
return loss. See the enclosed pdf "tale of three launches". I =
compare three different launches, a very good Ohmish discontinuity =
launch; one of several Ohms and a hand soldered poor launch. The =
discontinuity at each end affects both the return loss and the insertion =
loss. The three different launches have widely different unable =
bandwidths on the identical trace, from less than 2 GHz to greater than =
20 GHz. The two poor launches looked inductive. I've also looked =
at other SMA launches that were capacitive. The same results =
apply. The best launch was one designed by Scott McMorrow. The =
second best was an early attempt by Molex for their connector. The worst =
example was a SMA connector soldered to Scott's design. The large loop =
formed a large inductor. Some say a rule of thumb for evaluating =
measurement systems is to look at the difference between the insertion =
loss and return loss. If you have at least 10-15 dB difference you can =
use the system out to that limit. I doubt that the attachment =
will get through the list. If you are interested in this drop me a =
note. Regards, Tom Dagostino 971-279-5325 <a =
href=3D"mailto:tom@xxxxxxxxxxxxxxxxx";>tom@xxxxxxxxxxxxxxxxx</a> =
 Teraspeed Labs 9999 SW Wilshire Street Suite =
102 Portland, OR 97225 -----Original Message----- From: =
<a =
href=3D"mailto:si-list-bounce@xxxxxxxxxxxxx";>si-list-bounce@xxxxxxxxxxxxx=
</a> [<a =
href=3D"mailto:si-list-bounce@xxxxxxxxxxxxx";>mailto:si-list-bounce@freeli=
sts.org</a>] On Behalf Of Havermann, Gert Sent: Tuesday, December 20, =
2016 2:01 AM To: <a =
href=3D"mailto:dmarc-noreply@xxxxxxxxxxxxx";>dmarc-noreply@xxxxxxxxxxxxx</=
a>; <a =
href=3D"mailto:tom@xxxxxxxxxxxxxxxxx";>tom@xxxxxxxxxxxxxxxxx</a> Cc: =
si-list Subject: [SI-LIST] Re: 50 ohm - RF Impedance Traces I =
think that the topology is the problem. Coplanar waveguides are not easy =
to control, especially when the clearance to GND is too tight or too =
large, and 18 mil is really large. Just imagine that the electrical =
field will be active in an area at least 3 times the GND- distance all =
around your trace. When there are other traces/circuits underneath, or =
some mechanical parts (table, skrews, housings....) then they all =
influence the Impedance. You also need to take into account that in =
reality you will not see an perfect open in the smith chart just because =
the trace has an open end somewhere. You need to take the trace =
inductance and capacitance into account, and electrical length also =
changes the "dot" smith chart. The Laminate itself is also =
frequency dependent so it will be almost impossible to create an perfect =
open with a long trace. Please define " the launch does not =
look bad either". What is the TDR Risetime and what kind of =
impedance deviations do you measure? For 6GHz you should test with at =
least 100ps Risetime, and a decent launch should then vary by less than =
5 Ohm. A good launch will vary in the 1ohm range. If you plan on even =
higher frequencies, then the same applies for 30ps risetime. And =
that=E2=80=99s just some rule of thumb without knowing all the details =
it is hard to really give a recommendation right on the =
spot. BR Gert -----Original Message----- From: <a =
href=3D"mailto:si-list-bounce@xxxxxxxxxxxxx";>si-list-bounce@xxxxxxxxxxxxx=
</a> [<a =
href=3D"mailto:si-list-bounce@xxxxxxxxxxxxx";>mailto:si-list-bounce@freeli=
sts.org</a>] On Behalf Of sunil bharadwaz Sent: Tuesday, December 20, =
2016 8:38 AM To: <a =
href=3D"mailto:tom@xxxxxxxxxxxxxxxxx";>tom@xxxxxxxxxxxxxxxxx</a>; <a =
href=3D"mailto:dmarc-noreply@xxxxxxxxxxxxx";>dmarc-noreply@xxxxxxxxxxxxx</=
a> Cc: si-list <<a =
href=3D"mailto:si-list@xxxxxxxxxxxxx";>si-list@xxxxxxxxxxxxx</a>> Su=
bject: [SI-LIST] Re: 50 ohm - RF Impedance Traces Hello Tom =
, Thanks much for your response .Thanks much to all ,who have replied =
to this thread. The launch is a proper SMA connector . The final =
board is going to carry this RF connector . On the TDR , the launch does =
not look bad either .However , one observation is that , the larger the =
50 ohm traces , worse is thereturn loss @ 5 Ghz & above =
. I'am checking @ Test coupon as well as the actual board .On the =
actual board ,the RF trace has a SMA connector on one side & the =
other end is open .On the VNA , when we check the S11 @ Smith chart , =
ideal Transmission line should have a dot to the right extreme =
indicating that the line is open .However , i see all kinds of shapes @ =
frequency. Can we get the solder mask done in normal way or do we =
have to take any special care in getting the solder mask done on the RF =
traces ? We generally use Co planar Waveguide traces (with ground ) , =
for 50 ohms lines. Typically , theRF trace widths are 15 mil @ 18 mil =
distance from ground running on either sides . We did extractthe =
S-parameters before sending the Layout for Fab . Fab of course did send =
us the impedance report (measured on TDR =
). RegardsSunil.B From: Tom Dagostino <<a =
href=3D"mailto:tom@xxxxxxxxxxxxxxxxx";>tom@xxxxxxxxxxxxxxxxx</a>> To=
: <a =
href=3D"mailto:dmarc-noreply@xxxxxxxxxxxxx";>dmarc-noreply@xxxxxxxxxxxxx</=
a> Cc: si-list <<a =
href=3D"mailto:si-list@xxxxxxxxxxxxx";>si-list@xxxxxxxxxxxxx</a>> Se=
nt: Sunday, December 11, 2016 11:47 PM Subject: [SI-LIST] Re: 50 ohm =
- RF Impedance Traces How good is your launch into your test =
trace? If it has a large discontinuity it may be masking the true =
impedance. Try looking at the trace with a TDR display and see what the =
trace impedance is. Tom Dagostino Teraspeed LabsOn Dec 11, 2016 =
8:17 AM, sunil bharadwaz <<a =
href=3D"mailto:dmarc-noreply@xxxxxxxxxxxxx";>dmarc-noreply@xxxxxxxxxxxxx</=
a>> wrote: > > Hello Group , > We seem to be =
running into issues while getting PCB's fabricated with 50 ohm lines.Our =
Freq band of interest is 2 Ghz to 6 Ghz.Tried checking these traces on a =
VNA.The return loss looks decent @ 2 Ghz , but seldom looks good at 5Ghz =
& above . > The fabricator does send test coupons & =
impedance reports @ TDR .This does notcover the freq response . > =
This is not issue with any single fab . This seems to be a general =
problem , where return loss isnot flat across freq band . > Wanted =
to check if there are ways to tackle this issue . > Thanks =
much. > > =
RegardsSunil.B > > > > > > =
------------------------------------------------------------------ &gt=
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References:
- [SI-LIST] Re: 50 ohm - RF Impedance Traces
  - From: Tom Dagostino
- [SI-LIST] Re: 50 ohm - RF Impedance Traces
  - From: sunil bharadwaz
- [SI-LIST] Re: 50 ohm - RF Impedance Traces
  - From: Havermann, Gert
- [SI-LIST] Re: 50 ohm - RF Impedance Traces
  - From: Tom Dagostino
- [SI-LIST] Re: 50 ohm - RF Impedance Traces
  - From: Hakim Sellaoui

[SI-LIST] Re: 50 ohm - RF Impedance Traces

Other related posts: