I built the kit according to the very well written (German) description. Since it describes tests during the various stages it is very easy to verify the correct working of the unit during the building process. In the end, it worked almost instantly.
The biggest challenge was the software. I tried a lot of different SDR programs, but my favorite is HDSDR. I really dislike PowerSDR, which is promoted by Bernd. First it seems it will only work with one sound card, which you have to switch over between transmit and receive, secondly the way you have to tune is not to my liking. This is of course a matter of taste, but I like to click on the screen when I see a signal, and it doesn’t work like that in PowerSDR.
I then had to think about how I would like finalize the unit. Building the kit is one thing, but making it a complete working transceiver is another. My wishes are the following:
- It should be plug and play, with one cable between the computer and the transceiver
- It must contain an internal sound card
- Everything must be enclosed in one cabinet
- It must have a ‘decent’ power amplifier, with low pass filters
- Low pass filters must be auto-selected, i.e. no need for an extra selector switch
- Power output and SWR must be indicated in some way
- Power amplifier must be equipped with a temperature controlled cooling fan
he first two points are easy. I selected an external (USB) sound card and added a USB hub. Now it is just a matter of plugging it in, and both sound card and LIMA Sdr can communicate with the laptop I have reserved for this. The cabinet was the next challenge. In the end I decided to use a 19″ box I once used for a large power supply. This had been rusting away for years and I didn’t need it any more, so I decided to dismantle it and reuse it for the Lima. The front panel could not be re-used since it contained too many holes, but fortunately Okko, PH2CV helped me out with a new front plate.
The box is big enough to house the 12V power supply, the Lima SDR, Hub, sound card, power amplifier, low pass filter and additional electronic control circuit. I have a power supply for the PA (50V), but that doesn’t fit in the box. I think I can live with that. I rescued a cooling fan from an old computer, and mounted that on the box.
Then I had to think about the switching of the low pass filters, measuring the power and SWR, controlling the fan. I decided to use an Arduino micro controller (Arduino Micro) and a small TFT LCD screen. This small screen is big enough to give the necessary information in a graphic form. Since it is a full color unit, it can be used to show bar graphs with dynamic color, so that you instantly see if the temperature or the SWR gets too high, since then the bar graph will turn to red. It also shows the chosen band (low pass filter) as text on the top. Finally, it indicates if the unit is in receive or transmit mode. I tested most of the functions on a piece of breadboard, until it worked to my satisfaction. Next stage is making it more permanent on a experiment eurocard PCB. This will be mounted in a aluminum case mounted inside the box.