New QRP WSPR transmitter on the air

UPDATE Feb 26, 2009

I've added a couple of additional features to the QRP WSPR transmitter.

1. Completed the switched attenuator so that I can send at 23 dBM, 17 dBm and 10 dBm under control of the PIC.
2. Added GPS support (see below)

I use the GPS to get time of day information and also an accurate 1 pulse per second clock. This is used to update the time of day and to calibrate the symbol clock for WSPR (generated from the internal soft clock generated from the 32.768 Khz (nominal) crystal.

Being paranoid, I also added a thermal fuse to my "oven" so that power to the heater is removed (permanently!!) if the temperature inside the enclosure gets to 72C.

I got the TX on the air just after sunset local time today so missed the bulk of the day's propagation - that said, my first spot was from WA2YUN!

Look forward to getting spots - especially at the lower power levels!


With thanks to Gene W3PM and indirectly to Johan SM6LKM, I am now on the air with a homebrew WSPR transmitter running 0.5 watts (antenna = 160m inverted L with remote tuner) on 30m.

The transmitter is a version of the design on 9H1LO's web site with my own designed PIC controller keeping everything running. The PIC handles:

- Time of day (local 32.768 Khz clock)
- Transmitter on/off
- WSPR symbol selection for FSK
- "Oven" control for the crystal oscillator

The "oven" was inspired by an article I found on the RSGB web site:

I adapted the mark I design by bolting a high power resistor to the oscillator enclosure and then using a LM35 precision temperature sensor together with the PIC A/D converter to form a control loop in software. With some simple tweaking, I've got the oven stable to +/- 0.25 C which maintains the frequency to +/- 0.25 Hz with a cycle time of about 2 minutes due to the thermal inertia of the system. Not too bad for a guy in his garage!

I have yet to finish an outboard switched attenuator that will also be controlled by the PIC to be able to send WSPR at lower power levels - right now thinking about 500mw, 100mw and 50mw.

Building this and getting it operational has been a ton of fun!