Acid Test

A project log for Careless WSPR

A desultorily executed Weak Signal Propagation Reporter beacon.

ziggurat29ziggurat29 08/23/2019 at 16:440 Comments


The project was tested against a 'real' radio and digital interface into a PC running the WSJT-X software.  It was verified to have modulated the WSPR signals correctly.


We're veritably at the end-of-the-road of implementing software for the WSPR beacon, save bug fixes or usability enhancements.  We need to verify if it works.  I don't want to actually put it on the air because it might not be working.

What I decided to do was use my HF radio (an IC-756PROiii) with the WSJT-X program from the creators of the digital mode.  However, the radio just down-converts signal to AF; there needs to be something else that gets in into the computer.  (I know that some of the newer radios have this interface built-in, but mine does not.)  The most common way of doing this is via the sound card.

I didn't want to tie up my regular sound card, but I had some years ago bought a cheap USB one.  They still make them, they are called '3D Sound Adapter'.  (I don't know why it is called '3D', because it just has a standard stereo plug.)  I had been planning to do this for a while now, so I did a little mini-project of building this adapter.  The gist is to adapt the 'sound card' via an attenuator to a pair of isolation transformers, which are then connected to the radio's 'Accessory' port.

I kludged this together:

The transformers can be had from Mouser for $2 each, or you can go for China for $0.40 each if you are willing to buy ten and wait (as I did).  Use eBay to search for this item:

    "Audio Transformers 600:600 Ohm Europe 1:1 EI14 Isolation Transformer"
and you'll probably find it.

The sound device can be found on eBay as well if you search this item:
    "USB 3d Audio Sound Card Microphone Headset Adapter"
These run about $2-4 depending.  The shell is easily separated so you can solder wires and not mess with the jack.

You tweak the trimmers to give a signal that doesn't cause clipping.  Incidentally, the 20 K pot I used for the Mic input is a log taper.  Both trimmers wound up being OK-ish in their mid positions, so not much tweaking was actually required.  I suspect that I could just measure and not use trimmers for a 'production' unit, but I'm not sure.  Maybe they should even be regular pots...

One other thing I also used was a Computer Aided Tuning (CAT) cable.  This allows the computer to set the dial frequency and do other stuff, like key the transmitter (this saved me some circuitry, I originally planned to make a VOX-like PTT feature, hence the larger perfboard than required).  If you search eBay for this item:
    "USB CI-V CAT Cable For ICOM CT-17 IC-275 IC-756Pro Shortwave Radio"
you should be able to find one for about $10.  It's nothing more than a USB-to-serial adapter, so if you have some of those lying around (and you should!) then you can whip one up with ease.  I don't know the details, but I'm sure the web abounds with info.  I prefer the FTDI parts, but my cable had a CH-340 and it works just fine.

Having stuck all that stuff together, and having configured WSJT-X to receive WSPR from folks around me, it was now time to try it out.  But what for an antenna?  Well, it turns out that I didn't need an antenna for this test.  My open board project spewed out enough stray radiation that my nearby radio was easily able to pick it up without a proper antenna.

The signal was decoded successfully.  I feel confident now that the software works, and the signal modulation is correct.  Now I have to go analog and get it on the air.  There are some things to consider, though:  is there enough power, and is the signal clean enough to be legal?  The first question is 'maybe', and the second is 'no'.  The output of the synthesizer is a square wave and as such has far too much harmonic content to be legal.


Filtering harmonics