FreeLists / si-list / [SI-LIST] Re: SMA launch

[RMELLISON@xxxxxxx]

Bob,
the circular plastic (delrin) washer was small enough to fit inside the four 
SMA ground posts.  The only pin that went through the washer hole was the 
center pin.  I used this because the SMA appeared capacitive.  Raising the 
SMA center pin off of the via added a little series inductance.  The result 
on the TDR was that the signal started to rise above 50 ohms before it took a 
capacitive dip.  When the rise equals the dip, you have achieved the best 
possible match (somewhere I read that this is called a doublet).    If you 
have access to a TDR with a very fast risetime, I would suggest trying the 
concept by not soldering the SMA to an existing card, and moving the 
connector in and out of the card while viewing the TDR waveform.  If the 
holes are not too sloppy, you should be able to see the difference easily.
It was around '89 when I did the board compensation stuff, and I can't 
remember what the stackups were.   I don't remember if the unused vias were 
removed or not, and I don't know how many layers I was dealing with.  My 
suggestion to the person on the SI List overcompensated his situation, and he 
wound up with his SMA launch looking inductive.
Referring to your second paragraph, you misunderstood my suggestion, which 
was to remove the copper (increase the antipad) all the way out to where the 
antipad almost touched the inside corners of the four ground pins.  The 
ground pins were soldered on
all sides to the plane, as usual.  This makes for a very large antipad, and I 
suspect it was in the days before we started removing the unused vias for 
signal integrity purposes.  
Richard Ellison 
214-544-1920 bus
214-544-1924 fax


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