Inspired by a presentation by 9V1AL, Alex, I have ordered a classic Softrock RX kit from Tony Parks back in 2007. An awesome little kit with lots of SMD soldering yields a tiny little PCB which I built into a mint box (what else) and have since dragged to many countries. The classic softrock operates with a fixed frequency crystal oscillator around which 48Khz are audible, dependent on the abilities of the attached soundcard. It takes a lot of trying and experimenting but once I had calibrated it, I was surprised of it's accuracy in measuring RF levels.

Of course I wanted to replace the fixed crystal with a more versatile VFO so I could extend it's reception range and was very enthusiastic about the SI570 based concepts. Unfortunately adding a wider range of frequencies mandates the usage of a more fexible preselector which makes things more complicated and expensive.

LIMA is a DIY project designed by Bernd DL9WB for which he ships just the PCBs and, if desired, preprogrammed microcontrollers. The remaining components can be ordered by one of our local distributors as a package. The project is documented in a very very detailed fashion, almost comparable to the excellent work by Elecraft so building is a piece of cake! As opposed to the Softrock the only SMD chip that needs to be soldered is the SI570. Not that I do not enjoy SMD soldering but it makes things a bit easier if there is plenty of space to test the components just added to the circuit. The downside of that of course is that the PCB won't go into a mint tin anymore.

Read about the project on this website (I linked it through Google translate). If you speak German, here is the original.

Building takes a day or two. All inductors are constant value chokes with the exception of the broadband input transformer which is very easy to wind.

Two controllers sit on the board, one is the classic DG8SAQ AVR chip that allows the software to change the frequency of the SI570 through USB. The other is a PIC that senses the SI570's frequency and switches a bank of band filters accordingly.

Using prewound fixed value inductors for the band filters makes life very easy! The filter bank is switched with diodes, that saves a lot of space.

The circuit comes with a fairly accurate 20db preamp which can be switched externally.

The BF is bidirectional so when adding the optional transceiver board it will filter that signal too.

Integrates nicely with PowerSDR-IQ. Works like a breeze with DRM reception, fax, PSK, RTTY and whatnot.

Using prewound inductors appears to be a problem from a RF perspective as those are specified at 7.5 or 10 MHz but not at 30 MHz where they would display a different inductivity. The forum users have complained that at least two of the filters significantly inhibit the signal which I can confirm from my prototype. Luckily I do not use this band very often so this gives me time to improve the matter later on. Switching does not happen with an integrated high speed IC but with field effect transistors. Although this seems to work nicely, the entire circuit draws significantly more power when operated at high frequencies and needs cooling.

In my opinion there is no need for two controllers. The PIC (which switches the filter bank) was chosen as it comes with an internal prescaler but apart from that, a bigger AVR could have easily driven both the USB and the filter bank. Although the circuit layout is very neat and clearly separates all elements, there certainly is the risk of interference of the digital circuitry with the RF circuitry. In fact, I have seen artifacts which might be attributed to this effect. I will need to add shields one of these days.

The prescaler for the SI570 is manually switched with a jumper. Not that it needs to be moved every day but it would have been nicer to switch it via software. Same with the preamp. In RX only configuration it can only be switched with an external switch.

The 3 TO220 sized voltage stabilizers can get pretty hot, especially when operating at higher frequencies. I added a fan to cool them which works fine.

The project is still work in progress as I have not yet found a perfect setup. Will update this article when something new comes up!