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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 17:31:54 -0000 (UTC)
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On Sun, 09 Jun 2024 10:25:32 -0700, john larkin wrote:

> 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.

Please do! I struggle with parasitics at even modest frequencies so it's 
always fascinating to see how the pros overcome these barriers.