My SDR Configuration


I spent some time with Linrad to make it work with the soft66 SDR radio. Others may like to know what I ended up with:


Expert?                    "Y"
Fontscale?                 "1"
MIT-SHM?                   "YES"
Processors?                2
Specify Screen Size?       "Y"
Enter width:               "1024"
Enter height:              "768"
Normal or Realtime?        "3" (realtime)
Sure about this?           "Y"
Autostart?                 "B" (this is CW)

"A" for Rx input setup      "A"
Hardware for input?         "A"  (soundcard)
Use portaudio for rx input? "N"
Press Y to keep realtime    "Y"
Select rx input card        "03" (This is /dev/dsp3, which is my Cirrus card)
Open as RDWR?               "Y"
Sure?                       "Y"
Enter sample rate           "48000"
Select radio interface      "2"
Select receiver hardware    "0" (This is the "undefined" interface

"B" for Rx output setup      "B"
Use portaudio for rx output? "N"
Select rx output card        "00" (This is /dev/dsp0, which is my NVidia on-board card)
Open as RDWR?                "Y"

When the main window appears, on the left side there will be a box in which to type the center frequency. Here I type “14.060” or whatever the center frequency is. The display on top will change to show the current frequency. I left click any signal to listen to it.

Some of the settings in my config came with “Are you sure?” counter questions. The author of the package warns that some of them (in particular the “realtime” option) – may lock up the computer. I played devil-may-care, because I found the non-realtime mode to be very sluggish. We take our chances, I guess. My setup on FreeBSD has not “locked up” to this point. Opening anything RDWR (read-write) is more risky than RD (read only). So, there’s my devil-may-care thinking about a FreeBSD system that I probably re-install at least twice a month anyway. Others may want to be more careful.

Using the SDR software, I notice a number of spurs in the waterfall. Really, they’re not too objectionable, and the SDR software allows their removal if the AGC option is enabled. I just select them and press “E”. Bandwidth is controlled by the up/down keys, and frequency by the left/right keys, but it’s just as easy to click and drag things. The vertical line (shown at the left of the waterfall) in the figure, is the tuning bar which can be dragged back and forth across the band (much more quickly than the mechanical dial on my old Omni!). I can envision a modification of the SDR software that would allow for the automated scanning of bands for WxFax signals, and automatically generated prints!

linradsrn1 Figure 2: The tuning bar is shown at the far left, where I am listening to a CW signal.

The board doesn’t look very complicated. I think that’s the beauty. The board measures 2 inches x 2 inches, so will fit nicely inside the tablet’s enclosure. The 7VDC-12VDC range power supply requirement is due to the on-board regulator and its headroom. The tablet already has the needed 5VDC from batteries, so I can bypass the regulator and just use the main tablet supply. In terms of the SDR board circuitry, there are four low profile integrated circuits (not very visible in the photos – sorry) that do the heavy hauling. For tests, I’ve applied a 12VDC power supply voltage, because the board’s venerable 7805 linear regulator provides 5VDC. There’s very little power dissipation on the board (good for my batteries), so the clunky old regulator would be just fine in the event I used the higher voltage. In addition to the ICs, there are about twenty capacitors, a diode, a trimmer pot, five crystals, a dozen or so inductors, and a similar number of resistors, etc. It’s an amazing product, considering the fairly low parts count. How does the kit producer include five crystals for the price, I wonder? Wow. Worth $20 any day of the week, especially when that includes the shipping!


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