UPDATES LIKELY: THIS IS AN EVOLVING PROJECT
See also TX only 10m WSPR beacon
For some time I had been considering a simple WSPR transceiver for 28MHz based on a low cost 14.060MHz GQRP club crystal. The idea was to use the crystal, pulled slightly up (to 14.0623MHz), in a Polyakov mixer based half frequency injection DC receiver and into a doubler and single balanced modulator on TX using a DSB transmitter. On transmit, half the power is wasted in the other sideband, but with QRP the interference is negligible.
First I breadboarded the RX part and then the TX part. Both work very well. The design uses very inexpensive parts (no NE602s or other ICs) and the PA device, currently a 2N3866, can probably be replaced with some paralleled up 2N3904 transistors costing just a few pence. The 2N3866 runs quite hot in class A and I want to change this biasing
in the final version but it has run for 30 hours continuously without problems.
Output power (before over-driving occurs) is around 200mW DSB, so about 100mW pep in the WSPR band.
CLICK IMAGE FOR A LARGER VERSION
me know if you spot any obvious errors (e.g. where I have put grossly
wrong values of components in) so I can update the schematic.
The 14.060MHz crystal is pulled very slightly HF to 14.0623MHz. This is doubled and used as the injection into the single balanced mixer. Audio is applied directly from the PC sound card with the audio level set using the Windows audio controls. The double sideband output from the TX mixer is linearly amplified in 2 stages of gain. Linearity is important in a DSB transmitter otherwise the signal will distort and splatter either side of
the carrier for tens of kHz if driven too hard. Adjust the drive so the
output from the PA is a clean, DSB signal free from obvious audio
distortion. This is best done by changing the driver transistor's emitter resistor. In a USB WSPR transmitter PA linearity is not important and class C stages can be used.
The same crystal oscillator is used to provide a half frequency injection signal into the Polyakov mixer (2 diodes back-to-back). RF from the antenna is applied to the source of a common gate amplifier. Although gain of this stage is low, it helps to keep any remaining LO from being radiated and helps eliminate common mode hum that can be picked up. The Polyakov mixer produces an audio output (this is a direct conversion receiver) which is amplified in the common base and common emitter stages. The audio output is connected to the mic input of the PC.
Results in the first couple of days could not be more pleasing as the table shows. Power out is around 50-100mW pep in these tests.
Unique WSPR spots Sept 8/9th with WISPY-10
Here are photos of the RX and TX breadboards:
WISPY-10 TX breadboard
Although I don't want to do this myself, the basic idea here would make a nice kit based on a simple PCB and low cost, easily available parts. All in, I doubt the components would cost more than £5 (7 euros or 8
dollars) even if all the parts were obtained new. To build "dead bug" style I recommend MePads from QRPme.com
. Simply glue these to a piece of copper laminate and you have a simple way of making a neat layout. The RX breadboard is a better example of this.
This brings the FUN back to amateur radio.