RF Connector Type ID

Some variant of sma?

Probably SMBs.

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.

You need to manage the transition between 50R transmission line in the cable and the 50R trace in the board. Traces buried in the board can be non-dispersive, and you can work out how wide the trace has to be and how far from ground planes that carry the return current to get you 50R.

A connector on the edge of the board needs special care. The pad stack is a collection of metalised areas in the edge of the board that provide a more-or-less seamless transition.

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.

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.

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.

Gentlemen,

The previous one I queried was indeed an SMB; many thanks to those who correctly identified it. Now, does anyone know anything about these ones? Ignore the N-type part of it, I'm referring to the rest of the item in the photo. They too are 50 ohm and snap-on; very much like SMB but much smaller. Any insight would be most welcome.

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

RF gets more understandable when you realize that electrical signals really propagate as electromagnetic fields *between* conductors, rather than as currents and voltages.

Jeroen Belleman

Yes please!

Jeroen Belleman

U.FL.

Cheers

Phil Hobbs

Am 09.06.24 um 17:03 schrieb john larkin:

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

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) <

red = top, green = bottom, inner layers cut out.

setup on tdr refl. + transmission: <

result: <

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

That pic was the connector on a PCB that was intended for the big-pin part. A skinny trace would be appropriate for the Mueller SMA, but in real life the cheaper big-pin part TDRs about the same.

We tweak the pad stack to compensate for the big pad. That works pretty well.

Big improvement.

Short traces on cheap boards are pretty good.

Here it is:

As noted, this is bizarre. And obviously very expensive to build. What's with those machined cover plates?

But cheap on ebay! Maybe nobody wants this.

Aha! Thanks, Phil.