Latest situation of the WSPR-RX
A while ago I did build my first 30m WSPR-RX to participate in the WSPR-challenge. I must admit it is very addictive! It did not took long before I was building my second WSPR-RX. This time a little bigger since the first one was too much miniature. Also I wanted to add some tweaks.
WSPR is a very small band signal. Actually it only covers 200 Hz on (30m) 10,138.70 MHz. When you build an x-tal oscillator for this receiver, it has to be quite stable to be able to hold the WSPR-RX between its lines. And the original design drifts. Too much for me. At some point I could read the rooms temperature from the drift of the decoded WSPR-signals.
In the original PA2OHH-design of this receiver there is an Negative temperature coefficient thermistor (NTC) with a varicap to compensate for frequency drift. I think Remco PA3FYM has left these parts out for simplicity but I wanted to build it back in.
Voltage regulated fine tuning
Furthermore to put the oscillator on frequency is a little challenging by itself. Since the original design almost never hit the 1500Hz band middle of the WSPR-band right away, you have to tune a little to get it right. Great, but how do you tune an oscillator that has no tuning options?
So I did build (with tips and hints from Remco PA3FYM) a voltage regulated add-on. As it turns out, you can re-engineer an 7805-voltage regulator to a variable regulator pretty easy. I did use this concept to design a voltage regulator for the whole design. I’m able to set the voltage between 11 to 14 volts which is enough to set the oscillator somewhere between 1400 and 1600Hz.
In the meantime I am able to reach in the top 20 of the WSPR-challenge. Consequently to get into the top 10 seems very hard! I need all the (WSPR)-ears I can get. As a result I took everything I could find (including Roeland PA3MET‘s Sony ICF-SW7600G short wave radio) to create as many WSPR-receivers as I could in addition to the home made WSPR-RX’s. But it is still not enough to reach the top. I need to come up with another plan.