[SI-LIST] Re: Microstrip Loss Correlation

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Hello Mark,

I have attached a copy of the insertion loss plot, but basically you can 
see that there is a small rise, than a
cliff after the 2.4Ghz.  The gnd plane under the trace is a full plane. 
The board is shaped like a stop sign
that is nearly 3"X3", so there is plenty of gnd under the trace.  The area 
where the gnd is cut away is only 
just under the top layer pad for the signal conductor of the sma.  The pad 
itself is 40 mils wide, but the GND
underneath is 50 mils wide.  I have attached a .bmp for reference.




Thanks,
Chris




"Mark Burford" <Mark_Burford@xxxxxxxxxxx>
10/21/2005 12:31 PM

 
        To:     Christopher Crowley/Corporate/FSC@Fairchild, 
<si-list@xxxxxxxxxxxxx>
        cc: 
        Subject:        RE: [SI-LIST] Microstrip Loss Correlation


Is this loss a continuing attenuation i.e. does loss increase past 2.4GHz 
or are you experiencing a 'notch' type attenuation at the frequency you 
stated where past 2.4GHz less loss is seen?
You are right in saying that the superior quality material should give 
less attenuation at this frequency but then it all depends on rise time of 
you signal, trace width (a wider trace will increase dielectric loss per 
length), around this frequency dielectric losses should be dominating skin 
depth (conductor) losses so perhaps reducing the line width if poss may 
reduce your attenuation. Obvioulsy reducing trace length would help too.
 
How wide is the ground plane under the trace? Is it at least 2.5 - 3 times 
the trace width where you have cut it away (not sure exactly what you have 
done so couldn;t comment further) because this would affect the return 
current if the return plane was not continuous at about 3 times the trace 
width. Also this would cause reflections and affect your measurement.
 
I hope these simple questions and reminders are of use to you.
 
Regards
Mark
 
-- 
Mark R Burford M.Eng M.I.E.E.
EngD Research Engineer 
School of Electronics and Computer Science
University of Southampton

________________________________

From: si-list-bounce@xxxxxxxxxxxxx on behalf of 
Christopher.Crowley@xxxxxxxxxxxxxxxxx
Sent: Fri 21.10.2005 16:53
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Microstrip Loss Correlation



Hello All,
I am working on measuring the bandwidth of a couple fixtures and am
surprised
by the results.  I am hoping that someone may be able to help shed some
light on
the subject.  I have two fixtures that are identical other than the
materials.  One board
is made of Isola's FR406 and the other is Arlon 25N.  The only minor
difference is
that the stackup may have been changed slightly to accommodate for 50
ohm's on
12 mil trace widths.  Four layer board, 2.8" total length traces, routed
only on the
top layer microstrip.

I have used edge launch sma's to interconnect to the network analyzer.
Johnson
Components 142-0701-801 PN.  The issue is that when making an S21
measurement
across the 2.8" trace I see both boards have a 3db loss at approximately
2.4GHz.
Since I would obviously expect the Arlon 25N board to perform much further
out in
frequency one thing I am suspect of is the sma interconnect.  I know that
the sma
footprint should be optimized and we have tried to do that by cutting away
some
GND under the signal conductor of the sma.  It seems like it is still
causing an issue
though.  Does anyone have any footprint or other recommendations?

Thank you,
Chris



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