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Monday, August 10, 2020

Using SDR without SDR Hardware -- N2CQR Adjusts Analog BITX20 using Web SDR

I know what you are thinking:  Software Defined radio with the TOTAL elimination of hardware? WHERE IS BILL AND WHAT HAVE THEY DONE WITH HIM?  

Relax my friends.  All is well at SolderSmoke's East Coast HQ.  In fact, just yesterday I was making use of one of the systems described in this video.  Here's how: 

I was on 20 meters with my BITX 20 Hardware Defined (HDR) homebrew transceiver. Everything was going smoothly.  We have some sunspots now, so DX is once again possible.  I heard a loud U.S. station (that will remain anonymous) calling CQ, so I gave him a call.  

The trouble started right after he looked at my QRZ page. You see,  I have pictures of my homebrew rigs there.  These images sometimes trigger hostile reactions, especially from guys who have almost their entire stations INSIDE their computer boxes.  I also admit to occasionally making things worse by pointing out that it is possible to build a BITX20 for about five dollars U.S.   Some guys apparently don't like hearing about this. 

Anyway, the fellow I was talking to proceeded to give me a very blunt and harsh signal report:  "Well, I suppose your signal is strong enough, but your audio is TERRIBLE!"  Interpersonal relations pointer:  This is NOT a good way to alert a fellow ham to possible technical problems in the rig that he has built by hand, from scratch, in his home workshop.  Especially when the person delivering the harsh signal report is using a "rig" that was built by credit card in a robotic factory on the other side of the planet. 

So that QSO ended rather quickly. But I did want to follow-up in the comment about the audio.  And here is where I turned to Web SDR.   Mehmet NA5B has a really nice WEB SDR receive system in Washington DC, just about 8 miles east of me.  Before 20 meters opened, I called up Mehmet's SDR on my computer, tuned it to the frequency of my transceiver and watched the screen as I asked if the frequency was in use.  I then issued a couple of hopeless CQs, again watching the screen.    I could see in NA5B's waterfall that my signal was indeed seriously lacking in low frequency audio. 

Now it was time to turn to hardware.  Rig on the bench, 'scope and sig generators fired up,   I quickly determined that the problem most likely resulted from my placement of the  carrier oscillator frequency in relation to the homebrew USB crystal filter.  I had placed it about 300 Hz too low.  This resulted in a low AF frequency roll off not at the desired 300 HZ, but instead at around 600 Hz.  That would make the audio sound "tinny." So I moved the carrier oscillator up 300 Hz and went back to Mehmet's SDR receiver.  I could see that the lows were now at the right level.  Thanks Mehmet. 

One note about the audio coming out of the many SDR radios on the air:  When you look at the passbands in the Web SDR receivers you can see audio going almost all the way down to the frequency of the suppressed carrier.  With non-SDR rigs  you usually see a gap of around  300 Hz between the carrier freq and the start of the SSB signal.  This is often the result of our filter rigs having IF filter skirts -- you would place the carrier oscillator frequency a bit down the skirt -- this would help with opposite sideband suppression and all you would be losing would be the lows below 300 cycles, which weren't really necessary anyway.  I had placed the carrier oscillator too far down on the skirt.  

Of course, sometimes SDR rigs will also have a gap between the carrier freq an the start of the audio if the operator has set the passband this way, or if the microphone attenuates below 300 Hz.  But you see a lot of signals with audio filling almost the entire passband --some of the "Enhanced SSB" guys are running audio passbands that go as low as 50 Hz.  

Has anyone else noticed this "full passband" effect when looking at the waterfalls?  
Any other tips for using Web SDR for troubleshooting?  


  1. Wouldn't DOUBLE SIDEBAND be more fun?

  2. There will never be an SDR without hardware.

    Software needs hardware to operate.

  3. Michael: I think you have misunderstood. He was just talking about SDR systems that are accessible via the internet that do not require any additional hardware in the user's location. So I could set up an SDR system using an antenna (hardware here) and an RTL-SDR dongle (hardware here) and then connect it to SDR software in my computer. OR --- this is the important point that you may be missing -- I could just log onto a distant Web SDR receiver and listen to it -- WITHOUT ANY ADDITIONAL SDR HARDWARE AT MY LOCATION. See?

    Michael: What is your call sign -- I often want to send you something but don't have any contact info on you. Thanks, Bill

  4. Bill, I use my own SDR to adjust my BITX40 into a dummy load. And I use NA5B's web SDR to check my receive for week stations.

  5. I love web SDRs! After you heard my Michigan Mighty Mite - the long 8 miles between our QTH's - the next thing I did was hook it up to a keyer and send CQs out in to the ether. What a thrill when I heard my call on the color burst frequency on a web SDR in Maine! I've also used them to check sound quality on my Furlough 40.

    As far as the unfriendly ham on 20 - I have now happily joined the small club of folks that have put 100% homebrew rigs on the air - and received the astonishment of many and scorn of a few. On Sunday I was having a QSO with a ham in Ohio who couldn't quite get his head around the fact of a homebrew rig.....

    "5/9 and great signal OM. What are you running?"

    "The rig here is homebrew - QRP"

    "Wow good signal for QRP (he's running 1KW) Kit radio huh? I used to build kits."

    "No kit. 100% homebrew - built it myself."

    "Where did you order the board?"

    "No board, no printed circuit - just plain copper."

    "Oh, where is the schematic?"

    "No schematic - actually several really - and a bunch of scribbled notes in my notebook and WEb pages and emails from the designer (N6QW - thank you Pete)."

    (Increasingly agitated)
    "How can you THAT"?

    "Check out my QRZ page."

    "Oh .. I guess it sounds OK for a homebrew."



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