What do we need to receive
WSPR signals? First, realize that
WSPR is transmitted as a single-side band (SSB) signal so it cannot
be received with an ordinary AM
receiver. Some shortwave radios
have a BFO (beat frequency
oscillator) and could be used if
they possessed sufficient accuracy
and stability. Also since the signal
is in a very narrow band and
almost inaudible, manual tuning
would be difficult. Ham band
receivers are another option, but
more costly. So, it appears that
finding a low cost WSPR receiver
can be problematical. Is there
another approach? Yes! Let’s
design our own unique WSPR receiver from the ground
up. On the hardware side, we need to build a WSPR
receiver with high accuracy, stability, and sensitivity at low
cost. In addition, it should not require complicated
alignment or adjustment procedures involving special
equipment. Quite a task.
Recall that the function of the receiver is to convert
the WSPR radio signals to audio for the computer sound
card. A good candidate for these requirements would be
some type of fixed, direct conversion (DC) radio. DC
receivers convert radio signals to audio directly without
any intermediate stages. A block diagram of the proposed
radio is shown in Figure 4. Here is how it works.
RF signals from the antenna are bandpass filtered at
the WSPR receive frequency to prevent overloading the
mixer from strong out-of-band signals. A local oscillator
(LO) tuned to the WSPR transmit frequency is mixed with
the RF. Since the two frequencies are separated by 1,500
Hz, audio centered at 1,500 Hz is produced at the output
of the mixer. Finally, this audio is fed to the computer
sound card. WSPR software has a 200 Hz bandpass
centered on 1,500 Hz with narrow digital filters. So, if we
get the audio to the sound card there is a good chance it
will be decoded. Implementing the block diagram requires
some unique thinking. First, we do not want an adjustable
LO as this would require special equipment. Also, we do
not want special coils that need to be tuned because
measuring these would be difficult for many hobbyists.
Figure 5 shows the circuit conceived. It functions like the
Two changes need more explanation. First, the LO is
replaced with a programmed oscillator. This removes the
need to adjust an oscillator. Programmable oscillators are
essentially custom programmed crystal oscillators.
Secondly, the input bandpass filter is now a crystal tuned
to the WSPR frequency. Crystals provide a narrow bandpass function without using conventional tuned circuits.
However, special crystal frequencies are required. Crystals
FIGURE 6. WSPR breadboard.
for the 20 and 30 meter WSPR bands will be available as
noted later. Here is a description of the circuit in Figure 5.
From the antenna, the signal is passed through the crystal
filter to a FET amplifier. Next, it is fed to one input (pin 1)
of U2 — the SA612 mixer. The SA612 functions well in this
application since it has gain (unlike passive mixers) and is
protected from out-of-band signals by the crystal filter.
Programmable oscillator U1 — programmed to a specific
WSPR transmit frequency — feeds pin 6 of the mixer. The
result is a differential audio signal on pins 4 and 5 of the
mixer. This signal is further processed by U3 and
presented to the sound card via JP1 and J2. Select the
“line” or “microphone” jumper for your computer sound
The crystal filter may need peaking via the 20 pF
variable capacitor CV. Since this is a sensitive place in the
circuit, use care to avoid false readings. Adjust for
maximum audio at 1,500 Hz. One way is to look at the
audio spectrum with a free program like “Spectrum Lab,”
www.qsl.net/dl4yhf. A better method of alignment is
to receive a nearby signal generator set to the WSPR
receive frequency. Adjust CV for maximum while viewing
the 1,500 Hz audio spectrum peak.
The circuit is powered by five volts through an AC to
DC wall mount converter. Such converters can be
troublesome. Check its voltage under load before using.
Exceeding 5. 5 volts may damage some parts. Using a
computer USB port may work. We had good experience
powering from the USB ports of some older laptops, but
experience with other computers has produced strong
interference from the USB port. In these cases, use the
five volt wall mount converter.
For demonstration purposes, a breadboard of the 20
m receiver was built on a circuit strip shown in Figure 6.
This technique, generally speaking, is not a good method
for RF circuits but it worked here. A PCB is a better option
and will be available also noted later.
Figure 7 gives the Parts List for the project. Most
January 2012 47