TRANSMITTING A CLEAN AND STABLE CW SIGNAL #1
The first objective is to get the radio working to the point that when Tx is pressed in CW mode a clean and stable CW tone is transmitted.
I’m rapidly learning that with this SDR radio setup there is a strong need to have a conventional radio or a second radio on hand to check the signal that the rig is transmitting.
This leads to the proverbial chicken and egg situation, plus the potential financial considerations. Should I have bought a conventional rig and then thought about building an SDR? Now I’ve built an SDR I need a conventional rig available to test it against. It may have been easier and cheaper to build a 6 foot brick wall and paint a bullseye on it and just head butt it repeatedly!
Thankfully, a very generous Andy has leant me a Sangean 803A/Radio Shack DX-440 (http://www.eham.net/reviews/detail/2814) from his youth. It is a truly classic receiver, which unfortunately they don’t make any more and I wish I could lay my hands on one. It has the advantage of having a separate BFO which is needed for this exercise.
Using the DX-440 with my trusty Sound Blaster Live 24 and an old Dell Inspiron netbook I was gifted over New Year, I’ve got the means to test what’s coming out of the Softrock.
The netbook has been reinstalled with a copy of Linux Mint 17.1 Xfce 32-bit as the Windows 7 installation on it meant it took nearly 20 minutes to boot! Gotta love Microsoft! It now boots in just under 40 seconds and comes with a whole host of useful radio based applications thanks to the repositories available. Our old friend Fldigi is installed and ready to go!
It’s a simple setup which allows the spectrum to be examined easily.
The baseline settings on the radio PC are as follows –
Within the Windows sound settings ‘Recording’ Line In for the Asus Xonar DX audio device is set to 2ch 24bit 192kHz
Within the ‘Playback’ Speakers for the Asus Xonar DX audio device is set to 16bit 192kHz
Both are set to 100% volume at present. There is the potential to overdrive the radio which may be causing an issue but at the minute we’ll leave them as they are.
Within HDSDR Soundcard options –
RX Input (from Radio) – Xonar Line In
RX Output (to Speaker) – Realtek Speaker
TX Input (from microphone) – Line 2 VAC (I’m ignoring this at the moment as I haven’t plugged a microphone in and annoyingly you can’t select an audio device option when there’s no device plugged into it. There isn’t a non or not connected option, so you have to select something)
Within HDSDR Options –
TX Output (to Radio) – Xonar Speakers
Within HDSDR Options –
Options > Input Channel Mode for RX > I(Left) / Q (Right)
Options > Output Channel Mode for RX > AF to Left Channel only
The RX settings aren’t relevant at present as this a test of the Tx functionality
Within HDSDR TX Options – (only available when the program is running and you hit the TX button!)
Options > Input Channel Mode for TX > I(Left) / Q (Right) [No Mic!]
Options > Output Channel Mode for TX > not selectable but set at I(Left) / Q (Right)
Swap I and Q Channels for TX output > enabled
Now the logic of this test goes like this. Tune the DX-440 to 14087 kHz, tune HDSDR to 14088 kHz in CW and hit Tx with a dummy load attached
The CW signal which is produced is received on the DX-440 and appears in Fldigi on the waterfall.
Again after much fiddling and stupidity on my part we eventually got somewhere.
This is a snapshot sequence of keying on HDSDR
On the DX-440/Fldigi setup this is what is received
Unplugging the cable between the sound card and the DX-440 and listening in a reasonable quality CW tone can be heard. After a bit of playing around, bearing in mind I possess zero Morse key skills, I was able to key out a few letters sufficient enough for Fldigi to accurately decode them.
But it’s no time to for high 5’s and cigars! Look closely at the received signal in Fldigi. It bends and then becomes linear! That said, there are no spurious signals around my CW signal.
This is as far as it goes at the moment as I’ve run out of time, but we’ve accomplished something! We’ve got a radio that transmits a CW tone sufficient for a receiver to detect and decode it.
As to the quality of the signal, that still needs work. I have concerns about possibly over driving the radio and I need to have a long look at the Output Channel Calibration for TX settings within HDSDR. The bent signal is almost as if there’s a lag somewhere. These things need to be looked at again in a methodical fashion but it’s progress!