Friday, May 22, 2015

Building a SoftRock Ensemble RXTX SDR


The last couple of months I have been listening a lot to the soldersmoke podcast. In case you do not know, it is two guys, Bill N2CQR and Pete N6QW babbling about building homebrew radios. I am very happy I found this podcast. It is highly inspiring, and by listening to the two experienced homebrewers I immediately wanted to melt some solder and build radios myself. Hence, I had to get myself a licence (got it last week: LB0MG) and start soldering radios.

As much as  I would like to be in the possession of a totally homebrew fundamentalist discrete component ham radio station (like Bill N2CQR), I concluded that I rather should start out with a kit. (I guess some homebrew fundamentalists threw up a bit now since I mentioned the word "kit").

Building the Softrock RXTX

Anyway, choosing a kit I might as well do it the modern way and build a SDR. I initially considered the PeaBerry SDR, but ended up with the Softrock Ensemble RXTX SDR Transceiver kit since I have the required sound card capabilities laying around after my previous synth projects.

The kit is nicely packaged and contains about 250 parts. Some through hole and some SMT.
I am pretty sure I will not do much contesting in any perceivable future so I choose to build the Softrock for the 17m, 20m, and 30m bands (two contest free bands).

There is an excellent build manual on the pages of WB5RVZ.  I addition I have had great use of some of the videos from W2AEW to understand the concepts of SDRs and to improve my toroid winding technique. He has some of the best engineering videos on the entire Youtube.


So far I have built the entire RX part of the kit and it has come along quite nicely. The SMT parts are rather easy to solder. Surprisingly enough, the through hole parts are a bit more challenging since there are very little space around the components, and the pads are really small and not gold plated. A lot of flux really helps.

The only mistake I did was that i lost the LT6231 SMT RX opamp (probably in the vacuum cleaner). Straight away I ordered two new chips from DigiKey and when they arrived at my door merely 36 hours later, I was so eager to get the RX working, that I accidentally soldered the small SMT chip in the wrong orientation. Hence I had to use the second one (glad I ordered two of them). But it worked! I got a small Image rejection problem, but traced it down to a setting in the sound card I used (Behringer UCA-202).

In the above picture you can notice the excellent indoor dipole "antenna" connected to the Softrock. The length of the dipole is about 5m, so It is not the best choice for receiving either of the bands. I guess some you antenna fundamentalists threw up watching this picture, but hey, the antenna sort of works. 

To please the homebrew fundamentalists out there, I used my 22 year old homebrew LM317 bench power supply to provide clean and pure 12V DC.

I connected the receiver to HDSDR, grabbing the I/Q signals. I have received SSB, RTTY, WSPR and JT65 on 17, 20, 30 and 40m bands. In the above picture you can see some digital signals popping in at 20m. I tried to use WSPR software directly on the I/Q signals from the radio but with little success. I had to output USB audio from HDSDR via a second sound card and then into WSPR.

From my perspective the RX test was a great success. Even with the indoor "antenna" I received WSPR on 30m from 60 different transmitters over a 24h period, and even across the pond. You might see LB0MG in the above map: Yeah, that's my Softrock sniffing RF from the ether, placed in an attic in Norway. 

I even got reports coming in from K1JT (yes, the Nobel laureate, Joe Taylor, that created WSPR) himself and got a bit starstruck. It is funny to think about the fact that he was transmitting his modest 5 Watts from New Jersey, and that my home soldered Softrock received his RF vibrations all 6000km away, even with a crappy piece of wire as antenna. The radio gods are undoubtedly on my side on this one.  

Now, I am eager to generate some RF myself and will get on finishing the TX part.

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