WSPR-ing VXO

No sooner had I smoothed out the wrinkles in my WSPR audio generator, I decided to return to the original plan: generating WSPR signals with a voltage controlled crystal oscillator.

I had enjoyed success with my QRSS and S/MT Hell Beacon using a reverse biased Zener diode to trim the frequency of a crystal oscillator. Ever since building that system, I planned to use the same approach to generate the 4-level FSK signal at the heart of WSPR.

I had Gene Marcus, w3pm's PIC code running in the AF generator, so all the timing problems were solved for me. All I had to do was add the ability to send control codes to the ADC on my VXO system relating to the four FSK frequencies. This was achieved by inserting a subroutine call within each of Gene's "symbol send" code segments...


You can see these additional subroutine calls highlighted by the red lines.

The called subroutines are really just individual entry points to the same routine - one entry for each of the four frequencies...


The actual Data send to the MCP4922 analog to digital converter is performed over a three-wire SPI interface and the driving code is embarrassingly "brute-force" - but it works...


With these simple modifications to w3pm's code for the PIC 16F628A (which, conveniently, had four unused I/O pins in Gene's application), I could apply the three required SPI lines to VCO/mixer board, via the pins in the socket that originally housed the PIC 16F676 in my system...


I also used the existing Tx keying line in the k3bm code to mute my VXO-derived WSPR signal, as required.

The system worked first time, delivering a nice clean 50mW (17dBm) into 50 Ohms...


I didn't even need to change the control codes (mapping to the VXO control voltages) that set the FSK frequencies for WSPR - they're the same step sizes I used in generating the S/MT Hell signal, obviously close enough to the required frequency deviation of 1.43 Hz in WSPR.

Talking of my measured 17dBm, I wonder just how many of the WSPR signals one decodes are from transmitters really running 37 dBm. Certainly, the majority I spot seem to declare themselves as pumping out 5 Watts. On the one hand, they are likely to be received at such power levels (to my mind, 5W isn't a "weak signal" by any means). On the other hand, 37dBm seems to be the "default" setting for the Power in the "Station Settings" dialog of the WSPR PC application - perhaps many people don't change this setting.

This marks the end of my production of DSB WSPR signals. What would have been my "lower sideband" (when running on 30m) is down near the 80m band and it is well attenuated by the 10MHz bandpass in my beacon system, as evidenced by the clean sinusoid in the (poor) photo above.

Here's an idea of performance of the new system...


No spectacular dx, just good solid propagation into Europe.

What now? Well, there's obviously a chance to replace the breadboard with something a little more robust. But the latest number of RadCom has just landed on the doormat, in which Richard, g0vgx, describes a QRP PSK31 beacon for 30m. That could be fun!

...-.- de m0xpd