Coaxial wiring

Coax cables soldered to the master oscillator

While dealing with an annoying problem with the analog digital converter, I thought I’d spend some time writing about the coaxial wiring.  There are two types of wiring that I’m using in my analyzer.

Direct-solder method

The first is the simplest and cheapest.  On the connections between boards that won’t change, I solder the coax directly onto the PCB.  With the semi-rigid coax that we use it’s pretty easy because the outer jacket is much like a solder-soaked copper braid (in fact, that might be exactly what it is).  I simply cut the circumference of the outer jacket through the inner dielectric (be careful not to nick the inner conductor).  Leave just enough inner conductor sticking out that you can solder it to the other side of the board.

Testing the direct solder method

At one point, I tried cutting the outer jacket into 4 “wings” that might provide more mechanical support to the solder joint, as you can see in the above image and the top image on the left.  I ended up deciding that it was a complete pain to do this, and it probably wasn’t necessary.  Since then, I’ve been simply cutting it flat and soldering it on there.  I use plenty of solder flux around the base of the solder joint to ensure that everything heats up as fast as possible.  This is so I can get in, get the job done, and let it start to cool before heating the dielectric too much.

SMA Connector method

The second method for making inter-board connections is to install SMA (or other UHF-appropriate) connectors on each endpoint and buy, make, or steal a cable to bridge them.  In my case, I bought the cable-mount connectors for the same cable I’m using in the direct-solder method, and I’ll make cables where appropriate.

Cutting the ground lugs

The first thing I did was source some (relatively!) inexpensive board-launch SMA connectors to modify.  These connectors are end-launch for a .063″ PCB and have a .031″ center pin, which is about as small as you can find.  This is desirable because it requires removing less material from the PCBs.  I mounted my Dremel into a drill-press mount and installed the abrasive cut-off wheel.  I set the height to as close as I could to the required cut-point for a smooth surface.  I err’d on the side of cutting off too little, and smoothed it off later.

Before and after cutting off ground lugs

Here is the result before and after cutting off the ground lugs.  I tried to make the surface on the sides as smooth as I could, but I don’t think it matters too much.  It is important not to disturb the center pin, nicks and cuts can affect the signal path, including changing the impedance and causing reflections.

.033″ carbide bit

I had to enlarge the holes in the PCB to accommodate the center pin of the SMA connectors, as it’s a bit larger than the center conductor of the coax.  I was able to find a .033″ high-speed carbide bit at digi-key.

Connector waiting to be soldered

This is what it looks like when the connector is sitting in it’s socket.  I didn’t solder this one because I wasn’t going to waste a connector.  After seeing this connector type in another page, Scotty mentioned that the gap easily seen in this photo can cause a significant leakage problem at UHF.  In response, I’ve filled these gaps with solder.  In larger gaps, such as the one shown, I used a little solder wick.  When the gap was narrow, I just made a solder fillet.  In the future, I’ll probably continue to cut the legs off in the same way, so I can inspect after soldering the pin.  I’ll just cover the gap with solder.

SMA Connector soldered in place

There is an example of a fully-soldered connector, including the fillet used to cap the open ends.

SMA Jumper construction

SMA Plug kit contents

Now, we need to make jumpers to bridge between these SMA connectors.  You can save about 50% the cost of pre-made cables by making them yourself.  From what I’ve seen, pre-made cables are about $20/ea.  The connectors are usually less than $3/ea and the cable cost is usually negligible, adding up to less $10.  It is a little unbelievable how expensive this stuff is.  Anyway, whining aside, here’s the process I used to make the jumpers.  The spectrum analyzer kit (from Cash Olson) contains a little more than 3′ of RG-405 semi-rigid cable.  I ordered a few Amphenol solder-on SMA plugs to match (Digi-key link).  The assembly instructions are decent, but there’s a little missing from them in my opinion.  Hopefully this helps.

Trim center conductor to 3mm or 2.5mm

I cut the center conductor to 3mm, even though the instructions specify 2.5mm.  Even with it cut to 3mm, it fits just fine in the center pin.  This way, there is a little more stability to the solder joint and it’s easier to do by hand. Important!! In the picture below I hadn’t installed the teflon washer.  I had to unsolder the center pin and replace it.  Don’t make this mistake.

Center pin soldered on (leave .4mm space between pin and dielectric)

I use a little bit of solder flux to help the solder wick into the gap between the pin and center conductor.  Try to avoid getting solder onto the outside of the pin, though there probably will be some.  It’s not pictured in this image, but the teflon washer goes between the center pin and the jacket.  This helps prevent electrical shorts, and makes the assembly more rigid and easier to build. (I’ve said this twice because it’s that important.)

Insert the cable until center pin just short of flush

I know this image seems like it’s skipping ahead, but this photo is to show where the center pin sits.  Look at a professionally-made cable for reference.  The pin is almost as long as the edge of the front of the connector, but not quite.  If you set the spacing from the center pin and the dielectric correctly, the cut edge should be flush with the shoulder of the connector in the inside of the housing.

Outer jacket soldered to connector

With the cable inserted to the right depth, solder it to the housing.  Solder flux is again called for, this time to aid in the thermal transfer from the iron to the connector body and cable.  Again, speedy thermal transfer helps keep the dielectric from heating up too much.  It’s like cooking, except that you don’t want to cook the inside ;).

Insulator installed

Finally, press-fit the insulator.  For me, it’s an extremely tight fit.  I don’t have a creative technique to do it, but to start, I press it in with the tip of some needle-nose pliers.  When once it’s in most the way, I screw it onto a SMA jack that I don’t care much about.  I have no idea whether this is a good idea or not.  If you have a better way, or an opinion about whether this was is appropriate leave a comment.

  1. #1 by Jerry Barnett on April 8, 2011 - 5:27 pm

    I use a small nut driver (XceLite 3/32″) to push the plastic insert into the connector. An alternative is to use a small wooden dowel or brass rod with a hole drilled to clear the center pin.
    Also, before soldering the connector housing to the outer sleeve of the cable, screw the connector to be assembled onto a spare female SMA connector. Then, push the two connectors together and solder the connector and cable shield. That way, the center pin protrusion will be properly set. This works for lack of the proper tool found in a commercial assembly kit.

    Jerry

  2. #2 by hpux735 on April 9, 2011 - 6:39 am

    Great advice, thanks Jerry!

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