Continuing my exploration of SDR and the Softrock RX/TX Ensemble …

You can catch the beginning of those posts here.

The programmable oscillator on the softrock runs (in my case) at four times the desired mixer frequency. My unit is set to start up at 14.080 MHz, in the 20m band.

But - it would start up with the master oscillator running off frequency, which led to calibration being shifted by about 20 KHz on the band.

I’ve been using an application called usbsoftrock to access the firmware interface on the radio. Usbsoftrock is available packaged for Ubuntu as described here. I think that usbsoftrock is supposed to allow me to calibrate my radio so it will start on frequency every time. I grabbed the source for usbsoftrock, and read the README, and it looks like the proper steps are to run the calibrate command:

$ ./usbsoftrock -a calibrate
Version     : 15.12 fXTALL = 114.440115

Then as  I understand it, I can just set the crystal frequency in the radio eeprom like this:

$ ./usbsoftrock -a set xtall 114.440115
Version     : 15.12

This just resulted in the exact same behavior. Actually, when I ran the calibrate command, the frequency would change to being much closer to where it should be, but it was still a bit off. As soon as I would cycle power, it would return to the original value.

I decided to experiment with writing different crystal frequencies to the eeprom, and had the following results, measuring after cycling power:

Set xtall    |   Output
——————-|——————
 114.265     | 56.3253
 114.2745    | 56.3207
 114.275     | 56.3204
 114.2758    | 56.3200  <——-
 114.285     | 56.3155
 114.440     | 56.2390

Since I want it to start at 14.080 MHz, the correct value is four times that or 56.3200 MHz. Incidentally, I’ve been measuring at top hairpin lead of R13 using a frequency counter.

Having written the correct value, the radio starts up on frequency every time. It’s very close to being dead on, I can tell by listening to WWV on 10 MHz.

I’m not sure why the procedure I thought I should use with usbsoftrock didn’t work. I may investigate that more later. I’ve asked on the yahoo softrock group, and there’s a lot of expertise there. If I get an answer I’ll post an update here. Otherwise, at least here’s a trial and error method for you.

Once I got the radio into a case and out of the rats-nest of cabling that I was using for testing, and disconnected the counter, the signal quality improved drastically. Here’s a screenshot covering the 20m band from about 14.050 to 14.095 MHz. You can see some CW at the left, a bunch of psk31 traffic around 14.070, and rtty around 14.085. It was amazing to see the phone portion of the band, there’s some sort of contest going on this weekend.

My previous two posts are about the Softrock RXTX Ensemble board, and getting it built. After I got mine built, I had trouble finding software applications to use it. Applications are out there, it’s just that I couldn’t find them. I’d be really happy to have something like Flexradio’s PowerSDR available for Linux. That application is great and it’s open source, but it’s Windows only.

(I’m definitely open to advice here, from the Linux SDR people who find this)

I’ve ended up being able to use an application called “quisk” as a receiver, but it required some customizations. My work was all done on Ubuntu Lucid 10.04.

This is not a solution you can just install and run, so be warned. You’ll have to twiddle with it, especially with respect to how your sound devices are configured.

Also, in order for this to work, you will have to have previously installed usbsoftrock as described in my last post, and it will have to be on your default path. This is because quisk starts usbsoftrock in the background to control the SDR board.

When I went looking at quisk, I found an older version on the yahoo softrock group which had been modded to work with the softrock, but it didn’t work for me. I ended up grabbing the latest version of quisk (3.4.8) and copying and modifying some files from the one that was on the yahoo group files area.

Grab quisk from the link above, then also grab this tarball containing two files.

Put the file named quisk_hardware_vk6jbl.py in the directory with the rest of the quisk source. Copy the file named ai4qrdot.quisk_conf.py to $HOME/.quisk_conf.py

At a minimum, you will probably need to edit .quisk_conf.py to set your audio input and output devices. Quisk will receive audio on both stereo channels of the input (actually higher in range than you can hear, up to half the sampling rate). On my system, this is set like this:

name_of_sound_capt = “hw:0”

There’s a script in the quisk directory named portaudio.py, which will print information about sound devices - this may help you find the one you want. You’ll also need the correct input selected in the Ubuntu sound mixer, have it set to mic level, and adjust the volume there. Once you get this right, you’ll be able to see some noise (and hopefully signals) on the quisk display.

In order to hear the selected (tuned) output, you’ll have to have the output device set also. Now for me I was unable to listen on the speakers, which are the same device number as the input I’m using. I would get a split second of sound and then silence. I changed the output to be a pair of USB headphones I use, and that worked. For me, the USB headset device was selected like this:

name_of_sound_play = “hw:2”

A few other things to be aware of - If everything appears sort of ‘mirrored’ around the center of the display, i.e. you tune up in frequency and the signals you see shift up (right) instead of down, then you have the I and Q channels reversed, and need to swap them in these lines in the config file:

channel_i = 0

channel_q = 1

I hope that’s enough information to help. As I was doing it I didn’t really have in mind to document it, only to get something working.

It looks like quisk is capable of transmitting, using AM, SSB, or CW, but as far as I can tell it won’t handle PSK-31 and doesn’t make the audio available for other applications like fldigi. I could be wrong, I haven’t gotten into it very deeply. This may be possible using “jack” but I don’t know.

I’ve also only just discovered sdr-shell, and that looks like it does exactly what I want, with DttSP. In fact, I see that Bob McGwier was one of the starters of DttSP, which invokes full recursion.

In my last post, I described how I ended up finally starting with SDR. I’m experimenting with the Softrock RXTX Ensemble. This kit is a transceiver with 1 Watt output, definitely QRP. It features a USB interface which allows setting the frequency and keying the radio for SSB and PSK31 transmissions. It also has a key jack for CW, and connectors for everything, so you don’t have to hang wires from the board for connections.

Here are some links related to the board:

• WB5RVZ’s Build notes and information
• Ordering page (Tony Parks KB9YIG)
• Yahoo Group for Softrocks

A word of warning - These kits sell out very fast, and apparently Tony is having trouble getting components. One of the effects of the global recession we’re in is that there are shortages of electronic components, so I’m not sure what the availability will be.

The kit has surface mount components, and requires winding transformers and inductors on small toroids. If you’re not comfortable with this then try to find someone to help you with these parts. Mounting Surface Mount Devices (SMDs) without any special tools other than a very fine-pointed soldering iron and a good head magnifier is not that hard, but it really helps to get some pointers and watch someone else do it. If your vision and motor skills are average or better, then you can do it, and it will be a rewarding project.

The kit can be built for a number of “super bands” as described here. The kit comes with all components for all options, which means that 1) you have to pay attention to the instructions while you build it, and 2) you will have parts left over.

I built mine for the 20m/30m/40m option. This is because I’ll be traveling with it, and having both 20m and 40m gives me bands which are open during day and night, respectively (at least as it stands now in the current solar cycle). I also like 30m, so this is a good set of bands for me.

I built it in exactly the order recommended in the build notes, but didn’t actually test it until it was finished. This isn’t recommended, as the various tests in the build notes are helpful in isolating problems before they get compounded. So this is a case of “do as I say, not as I do”, unless you are confident that you can troubleshoot your way out of problems of your own making. I did have one short caused by sloppy lead clipping, which didn’t cause any damage.

The thing that caused me the most troubleshooting time is that the audio signal coming out of the board is labeled “Line In”, and the signal into the board is labeled “Line Out”. Presumably this labeling is for what they should connect to on your computer sound card but I missed that, despite the fact that it is correctly marked on the schematic and block diagrams. 

An aside: The kit includes parts and instructions for a low-pass filter which must be used on the TX output “if transmitting on 30m”. Does anyone know why this would only apply to 30m? Are there stricter requirements for harmonic emissions on 30m? I’ve been meaning to look this up but while I’m writing this maybe I can crowd-source the answer.

One reason I skipped the build tests is that I hadn’t found a good application for Linux that would allow me to control the oscillator on the board using the USB port. There is a ton of information on the softrock yahoo group, in files and in the message archive, but the majority of what’s there relates to using windows applications. I did actually try to use a windows 7 machine to connect to the board, but after losing about 4 hours to erratic behavior caused by conflicts in USB drivers and the application, I gave up. It was a reminder of all the things that I don’t like about windows.

I’m an Ubuntu Linux user (and developer), so I decided to just figure out how to make this all work there, and document it. Everything documented here is for the Ubuntu Lucid 10.04 release.

The one thing you need early in the build is a way to control the oscillator on the board via the USB port. This lets to make sure that it works, and check the quadrature signals to the mixers. On the softrock yahoo group, I found an application named usbsoftrock, which does this. In writing this post, I’ve discovered that usbsoftrock has been packaged for Ubuntu and is available in a PPA by Jonathan, AF6YF (now N6JU). He has some notes here. I’ve not tried this, but hopefully it will work for you and save you the trouble of building it.

Usbsoftrock expects the softrock (USB) device to appear as /dev/softrock. In order to make this happen, you are going to have to add a udev rule. I found a rules file by G3VBV somewhere in my searching of the web. I’m sorry I can’t link to the original because I can’t find it, but I’ve put a copy here. copy that file to /etc/udev/rules.d/88-softrock.rules. You’ll have to be super-user. Make sure it’s owned by root and has permissions 644:

$ sudo chown root:root /etc/udev/rules.d/88-softrock.rules

$ sudo chmod 644 /etc/udev/rules.d/88-softrock.rules

Now restart udev to pick up the new rule:

$ sudo service udev restart

Now, read the 88-softrock.rules file and follow the instructions for adding a softrock group and adding your username to it. Now when you plug in the softrock board, the device /dev/softrock should be automatically created, and you should be able to access it.

Usbsoftrock has an interactive mode that lets you change the frequency through a curses text interface, which is helpful for testing. To invoke it use “usbsoftrock -a interactive”.

That should be enough software to get you through the build. 

Next, how I was able to actually receive signals using the softrock and Linux software.

I’m pretty easily distracted by shiny things. That’s one reason that things get stalled in my project queue. If an opportunity or project of interest comes along, I’ll indulge that and devote some time to it.

I’ve been interested in SDR (Software Defined Radio) for a while. I studied signal processing at university, and I once wrote a prototype FSK modem as a work project. I’ve been following developments, hanging around in the FlexRadio booth at hamfests, and thinking about getting involved “some day”, when I could get in at a reasonable cost (meaning under $100).

I’ve also been wishing for a small QRP (low power) tranceiver that I could use for CW and PSK-31 while traveling. I travel light, so it needs to be small. I have a KX-1 that works for CW, and that size is about right.

A few weeks ago I met Bob McGwier, N4HY on Facebook, because he noticed that we had some interesting mutual friends. Bob has done some really cool things, and is an expert on SDR, so I asked him whether he could recommend a small, cheap introductory SDR rig that was a transceiver. He was very helpful and recommended the Softrock RX/TX Ensemble, designed by Tony Parks, KB9YIG. Tony has brought low-cost SDR kits to a lot of people, enabling a lot of amateurs to get into SDR experimentation. There is a really excellent set of builder information for the kit by WB5RVZ, which helps make the kit accessible to more experimenters.

I dropped an email to Tony, and got an immediate response. So I was already having an amazing week, having connected with two rock stars of the SDR world. Tony told me he had an order in prep, and if I sent him payment via paypal, he’d have a kit for me. I did, and a few days later, the kit arrived.

Now, as of today, I already have the rig running as a receiver, but I’m stopping to document some of how I got here, especially since a lot of the documentation and software applications available are for Windows, and I used Linux.

I’m going to break this up into a few posts for readability. Next, some information about the Softrock RXTX Ensemble.