Calibrating the softrock RX/TX Ensemble on Ubuntu

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.

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About Steve

I'm Steve Conklin, AI4QR I'm employed by Canonical, Inc as a Linux Kernel Engineer. Interests include Linux, open source software and hardware, electronics and music, and amateur radio.
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