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From: john larkin <jl@650pot.com>
Newsgroups: sci.electronics.design
Subject: Re: RF Connector Type ID
Date: Sun, 09 Jun 2024 10:25:32 -0700
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On Sun, 9 Jun 2024 16:41:15 -0000 (UTC), Cursitor Doom
<cd999666@notformail.com> wrote:

>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. ;-)

We do picosecond time-domain stuff, which resembles RF except that is
broadband, all the way down to DC, so we can't tweak to tune things
into some narrow RF band.

I expect that it's impossible to do really fast stuff, 30 GHz-ish,
with parts soldered to FR4 PC boards.

I just disassembled a National Instruments PXIe RF module (with great
difficulty) and it's really bizarre. And it's only 6 GHz!

I might post some pics.