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From: Cursitor Doom <cd999666@notformail.com>
Newsgroups: sci.electronics.design
Subject: Re: RF Connector Type ID
Date: Sun, 9 Jun 2024 16:41:15 -0000 (UTC)
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On Sun, 09 Jun 2024 08:03:18 -0700, john larkin wrote:

> On Sun, 9 Jun 2024 10:19:52 -0000 (UTC), Cursitor Doom
> <cd999666@notformail.com> wrote:
> 
>>On Sat, 08 Jun 2024 17:06:18 -0700, john larkin wrote:
>>
>>> On Sat, 8 Jun 2024 22:54:07 +0100, John R Walliker
>>> <jrwalliker@gmail.com> wrote:
>>> 
>>>>On 08/06/2024 22:47, John R Walliker wrote:
>>>>> On 08/06/2024 20:43, john larkin wrote:
>>>>>> On Sat, 8 Jun 2024 18:10:59 -0000 (UTC), Cursitor Doom
>>>>>> <cd999666@notformail.com> wrote:
>>>>>>
>>>>>>> Gentlemen,
>>>>>>>
>>>>>>> Can anyone identify these connectors? HP use 'em an awful lot for
>>>>>>> interconnecting the boards of their analyzers to route 50 ohm RF
>>>>>>> signals around the various inside sections.
>>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> CD.
>>>>>>>
>>>>>>> https://disk.yandex.com/i/LQ1ytGUQCf7OTw
>>>>>>
>>>>>> Probably SMBs.
>>>>> 
>>>>> +1
>>>>> 
>>>>> John
>>>>> 
>>>>Most of the connectors in the image look like SMB which is a "click
>>>>on"
>>>>push fit connector and is cylindrical at the end.  A few which have a
>>>>hex profile at the end may be SMC which is a threaded connector.
>>>>SMA is threaded and a bit larger, but has the best high frequency
>>>>performance of them all.
>>>>
>>>>John
>>> 
>>> We like SMBs because that are quick to mate and un-mate without tools,
>>> so can be mounted very close together. They work fine to 6 or 8 GHz,
>>> about what you can do with discretes on FR4.
>>> 
>>> Shining Star makes nice cheap edge-launch SMAs and SMBs. A proper pad
>>> stack keeps things 50 ohms.
>>
>>Could you possibly expand on that last sentence, please? I've never
>>encountered a 'pad stack' before.
> 
> A PCB's pad stack is a diagram of the number of conductive and
> insulating layers and their thickesses and composition. It must be
> specified when you buy a board.
> 
> Sometimes you get to choose between a few canned stacks, and sometimes
> you diagram your own.
> 
> We mostly do 4 and 6-layer boards, with some 2-layer and the occasional
> 8 or even 10 copper layers. Never single layer.
> 
> 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

Thanks, John; I'm somewhat less ignorant now than I was!
Seriously though, it's clear the reason I've not encountered these is due 
to the fact I just experiment with single layer boards which don't need 
them, from what you implicitly state. Life must get really "interesting" 
with RF in multi-layer boards. ;-)