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NNTP-Posting-Date: Sun, 09 Jun 2024 15:03:19 +0000
From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: RF Connector Type ID
Date: Sun, 09 Jun 2024 08:03:18 -0700
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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