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From: Gerhard Hoffmann <dk4xp@arcor.de>
Newsgroups: sci.electronics.design
Subject: Re: RF Connector Type ID
Date: Sun, 9 Jun 2024 22:38:50 +0200
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Am 09.06.24 um 17:03 schrieb john larkin:

> 
> Our boards are mostly 0.062" thick, and I like layer 2 to be a solid
> ground plane. Some internal layers are power planes. If you solder an
> edge-launch SMA or SMB connector to layer 1, the layer 2 ground plane
> is too close to the center pin of the connector, so that bit of the
> signal path is too low impedance. So one does creative shaped cutouts
> in the various layers to keep the impedance uniform and the
> reflections down. We did e-m simulations (with ATLC) and test boards
> to get that right.
> 
> The cheaper connectors have a giant round center pin, which requires
> attention.
> 
> https://www.dropbox.com/scl/fi/vfi56wy5hkwva51bx77z6/SS_SMA_Edge.JPG?rlkey=nbmbyb0f54uslcpc0nh7rs21p&raw=1
> 
> That pin is around 100 ohms in free space!
> 
> There are more expensive conectors with a tiny flat pin that
> theoretically matches a layer 1 trace width directly, but I haven't
> found them to be worth it on FR4 boards and 30 ps edges.
> 
> https://www.dropbox.com/scl/fi/rptmh5f4umscqqpegatw6/Mueller_SMA.JPG?rlkey=1eonuljqekvrszyb9tce5s1hr&raw=1

That giant center pad gives away the advantage that you could have
from the costly SMA connector.

< 
https://www.flickr.com/photos/137684711@N07/51843174578/in/datetaken/    >

This is nearly as bad as the unpopulated SMA at the right end of the 
line. The left SMA is one with a thick center pin. Unsuitable on this
cheap JLCPCB 4 layer board. you even see the amount of solder.

This is the next try:   (4 TI LM2594/95 synthesizers + test stuctures)
< 
https://www.flickr.com/photos/137684711@N07/51844498620/in/datetaken/     >
red = top, green = bottom, inner layers cut out.

setup on tdr refl. + transmission:
< 
https://www.flickr.com/photos/137684711@N07/53780177186/in/dateposted-public/ 
    >

result:
< 
https://www.flickr.com/photos/137684711@N07/53780597885/in/dateposted-public/ 
      >

The SMAs are at division 7 and 8. Very slightly overcompensated,
it could also mean that I need some more GND vias.

The microstrip is slightly more than 50 Ohms. JLCPCB's calculator
says 11.5 mil for 50, it took only 10 to see if I can easily escape
a BGA.  The funny bends are not really punished.

The 10 cm of FR4 do not demolish the rise time. Not bad for
€1.40 per 4 layer board.

regards,
Gerhard