Lately, the pace of things to do has been picking up quite a bit for me. Along with trying to make progress on the Zigbee project, I'm also helping the Tokyo Hackerspace set up their website and preparing their boards for production. It's quite a big time suck because I need to source a bunch of parts for those boards. The good thing is that alot of the parts can be reused across the boards I'm making as well, which is the way I designed it. However I didn't realize that sourcing components was such a big pain-in-the-sphincter. There's a balance that's required because the hackerspace boards can't be too expensive and so I'm buying direct from the manufacturers for a lot of the less important components like connectors. DigiKey charges an arm and a leg for connectors and buttons, whereas you can pick them up from the manufacturers for a fraction of the price. The problem is that you need to buy in rather large quantities which is why I'm trying to share parts between boards.
On the FreakZ project, I needed to pick up a calibration kit for the network analyzer so that I could make accurate measurements. I was hoping I could get away with the calibration kit I had, but it unfortunately had Type-N connectors. Those connectors are big-ass RF connectors that are probably used if you want to set up a radio station. I was hoping I could just pick up some Type-N to SMA converters, but after discussing with some RF people, that turned out to be a no-go. The calibration would have been useless unless I put the same Type-N connectors on the board. If you can imagine the connectors on a typical garden hose connected to a 1.5 inch board whose main component is a 5mm x 5mm QFN IC, you can see the quandary that it posed.
So it turned out that I had to pony up some cash, about $2000 to be exact, to buy a used 3.5mm 50-ohm SMA calibration kit. Both my wallet and my ass are still hurting from it. Well, anything goes to get this project out the door I guess. It's definitely a learning experience. One of the scary things is that it's dangerous to buy the SMA calibration kits used because you don't know if the previous owner knew how to properly care for them. If you twist the connectors when you connect them to the analyzer, you can strip the gold plating and the fragile internal leads. That would turn a shiny $2000 machined calibration kit into this year's Christmas ornaments.
Anyways, I tested out the calibration kit and was able to calibrate my analyzer correctly for 2.4 GHz. The next step is to test the return loss of the boards I made. I actually made an additional radio board with the AT86RF230, but with a more flexible structure on the RF line. One of the reasons why I'm going through so much trouble on these boards is because I'm trying to make a 2-layer radio board. Normal boards are 4-layers and you can control the characteristic impedance to 50 ohms through the PCB manufacturer. Thus, you can say I want the boards so that 10-mil lines will be 50 ohms and they'll use a dielectric thickness that allows the board to match this impedance.
For a 2-layer board, you're pretty much stuck at 59-mil or 31-mil dielectrics which means that you're going to need a big, ol' fat trace to get down to 50 ohms. At 59-mil dielectric, you'd need something like a 110-mil trace to get close to 50 ohms. At 31-mils, you're board is going to be flimsy. 110-mil traces aren't very feasible, especially since the max trace size that can fit between the legs of an SMA is about 70 mils. The alternative is to use a sensible trace width (I used 70 mils) and put an impledance matching circuit on the line. I'm hoping that I can make the trace look like 50-ohms at 2.4 GHz by tweaking the components. My goal is to get a return loss value of < -15dB which translates into > 97% power transmission.
If you followed what I just said, you must be fairly well versed in RF design. Anyhoo, gonna be running some experiments this week to see how things go.
In the meantime, you're looking at one broke-ass mofo...looks like I'll be eating at home for a while...