Proof I'm not an RF engineer

Back in 2013 I did a project with a Nordic Semiconductor nRF24LE1 chip. This is an 8051 compatible microprocessor with a built-in 2.4 GHz wireless transceiver. Conscious of how poor my knowledge of RF design is I followed the sample layout of the RF section exactly for the antenna. And by "exactly", I mean I overlaid the sample Gerber file onto my PCB layout and used that to position the components and route tracks. Without the proper tools to tune an inverted-F antenna like on the original, I used a commercial 2.45 GHz chip antenna with a 50 ohm impedance.

Everything seemed to work exactly as designed except the RF section. I could not get the board to talk to a commercial board using the same nRF24LE1 chip. Clearly I'd botched something.

I called a friend who is an Extra-class amateur radio operator and asked his advice. He reviewed my choice of passive components in the RF section and had me change out a couple which he deemed inappropriate for use at 2.45 GHz. This made them work when talking to each other, but they still wouldn't talk to the commercial boards and had significantly reduced range. Without the test equipment necessary to look at a 2.45 GHz signal, the project got shelved.

KiCad schematic and board

Over the weekend I finished up the schematic for the nRF24LE1 programmer. Here's a screenshot:

First KiCad project

A couple of years ago I built a set of boards using the Nordic Semiconductor nRF24LE1 radio chip. These boards are designed to fit snugly into a Compact Flash card case. For programming and debugging it plugs into a standard PCB edge connector. Here's a photo of an incomplete board (this one was damaged during assembly).

When I started prototyping this project I used a nRF24LE1 breakout board plugged into a solderless breadboard. To program it I used an FTDI FT232R breakout board plugged into the same breadboard and a custom Linux program. Later, when I had completed radio boards, I used a commercially-available cable that has a 2x13 PCB edge connector on one end and a pin header socket on the other to connect the radio board to the solderless breadboard. The photo below shows both the breakout board and the radio board plugged in at the same time, which of course I wouldn't do in real life.

This worked well during initial development and testing, but the breadboard jumper wires tend to come loose with use (two loose wires are visible in this pic). It's also occupying one of my breadboards for which I have other uses. Since there is actually very little circuitry required to support the FT232R I decided a dedicated programmer board would make an excellent first KiCad project.

Disappearing parts

It might seem like I'm a bit paranoid about single-source parts like the NXP BSS83. When I decided to actually build the discrete-part 4004 CPU, one of the first things I did was order enough of all the parts I'd need to build the whole project, plus extras for goofs. Since then I've received several courtesy notices from DigiKey about parts becoming obsolete, but all of them would be easily replaceable if I'd needed them. But I already have everything except the PCBs.

I have another project going which involves very low power RF circuits. It's another one that I started a couple years ago and work on in fits and starts. My plan was to build 5 or 6 of these little devices, and I carefully acquired what I thought would be the critical bits. I'd had 6 PCBs made and assembled two. This evening I decided to assemble another three for a larger-scale test. The only thing I don't have on hand are the #6-32 nylon screws that hold the PCB to the plastic case, so I went over to my local big-box hardware store where I'd bought them before. They no longer carry them. They didn't sell enough of them to waste the shelf space on them.

Why nylon? Metal has a way of detuning tuned electronic circuits so I don't want metal screws that close to the antenna.

This is not a major catastrophe. I can get suitable screws elsewhere. The big-box store will even let me order them in reasonable quantity and deliver them for pickup at the store at no cost -- with a 10-day lead time. But I wanted them tonight.

The moral of the story is that it's a really good idea to buy everything when you start, because you never know what will disappear or become harder to get.