Introduction.
WSPR (pronounced as ‘whisper’) stands for Weak Signal Propagation Report and
is invented by Nobel laureate Joe Taylor K1JT.

WSPR algorithms are able to look ca. 30 dB deep into the noise. The result is that with
very simple and low power equipment world wide communication is possible.

Having built a standalone WSPR beacon with an Arduino and AD9850, I thought
it was time to build a simple 30m WSPR receiver.

The following isn’t Nobel prize worthy nor state of the art, but can be considered as an example of how
lots of fun is realized with a minimalistic approach and simple components.

Minimalistic approach.
Google found some simple 30m receivers. However, as fas as I could ascertain the ’30m root design’
is from Onno PA2OHH using a subharmonic mixer, also known as the Polyakov or ‘Russian’ mixer.

A mixer is actually nothing more (or less) than switching a given signal, in this case the received 10 MHz signal,
at a certain rate. This rate is called the ‘local oscillator’ (LO). The (mathematical) result in an ideal world
is that the LO signal is nulled out and two side bands mirrored around the (disappeared) LO signal appear.

Although being not super ideal, the elegance of the Polyakov mixer is that switching occurs at
both the ‘positive’ and ‘negative’ peak of the LO (sine) signal, thus having two ‘switch opportunities’
during one sine period.

In practice this means the LO frequency can be halved, which is advantageous in a direct conversion receiver approach.

I happened to have two 5.0688 MHz block oscillators in my junkbox and went for a similar approach as Joachim PA1GSJ
but did not include the 10.140 MHz ‘band pass’ crystal as I didn’t found one in my junkbox.

It is stated that subharmonic mixers do not work properly with square wave forms because
there are no clear ‘switching points’ for the diodes. As I hadn’t a 5.0688 MHz ‘filter’ crystal available,
reshaping the square wave of the block oscillator was done with one RC-network.
The result is a ‘triangularish’ wave form with supposedly enough discernable thresholds for the diodes.

The receiver was build with junkbox material and the circuit diagram and prototype are depicted below
(click on images to enlarge in a new tab).


Figure 1. Circuit diagram                            Figure 2. One hour later ; -)

Circuit description.
The circuit diagram in fig. 1 is straight forward.
L1 provides some selectivity and a ‘Josti Kit’ microphone amplifier boosts the received
10 MHz signal, which it is fed to the Polyakov mixer, consisting of two anti parallel diodes.

I did not use a potmeter to ‘equalize’ the diodes. The output of the mixer is fed through a RC filter (2k15/2n7),
selecting the audio spectrum (f-3dB = 1/(2πRC). This is fed into another Josti Kit amplifier with
another RC low pass filter. The resulting audio level is more than enough for standard (internal) PC soundcards.

The exact LO frequency was determined with a GPS locked signal generator by determining
the zero beat frequency. With the oscillator block used this appeared to be 10.135.538 Hz.
Apparently this block oscillates somewhat lower than 5.0688 MHz, namely around 5.0678 MHz.

Inserting -47 dBm 10.140200 MHz into the receiver yielded, after tuning the input circuit (60 pF trimmer)
and 4k7 LO potmeter, a nice sine wave with ca. 1.6 Vpp on my oscilloscope, see below (click to enlarge).

Update: After my UK trip (read below) and while fiddling and trying to improve the receiver,
i.e. maximizing the audio level relative to the injected 10.140200 MHz (Pref = -47 dBm),
I discovered that the waveform of the LO signal is more important than I initially thought.

Increasing the 100 pF capacitor to 220 pF in the LO RC-network resulted in a ‘cleaner’ LO waveform
but with lower amplitude. This is not surprising as f-3dB of the RC-network is now around 2.2 MHz, thus
‘damping’ the 5 MHz signal significantly. An alternative could be two RC-networks, but I was too lazy for that ; -)

Replacing the mixer diodes (initially 1N4148, BAW62 etc etc) with low barrier diodes like BAT86′s resulted in a
‘smoother’ adjustment of the LO level with a sharper maximum in the audio response.
The audio level increased around 10% to ca. 1.8 Vpp.

I tried a common base amplifier behind the mixer as it is said that a Polyakov mixer needs low Z termination.
Besides I had less less audio (of course), I didn’t experience less receiver noise.
On top of this, the minimum discernable signal (MDS) was higher compared to the original setup.

I also tried KE3IJ’s ‘double’ Polyakov mixer. It indeed results in more audio, but during that
evening it was an ‘AM hallelujah’ with the BBC world service competing with other broadcasters who
would be stronger than the desired WSPR signals.

So, I went back to two antiparallel diodes trusting they are ‘balanced’ enough.
</update>

Receiving WSPR signals.
By default WSPR software looks into the 1400 – 1600 Hz part of the audio spectrum.
The user has to tune his SSB receiver to the ‘USB dial frequency’, which is 1500 Hz
lower than the actual transmitted 4-MFSK WSPR signal. On 30m 10.140.100 – 10.140.300 Hz
is allocated for WSPR. Thus the ‘USB dial frequency’ normally is 10.140.2 – 1.5 = 10.1387 MHz.

However, this receiver has no ‘USB dial’, it uses a single frequency LO, which happens to be
lower than 10.1387 MHz, in my case 10.135538 MHz. This ‘USB dial’ difference is 3162 Hz.

In WSPR 2.xx the I/Q-option (for SDRs) can be (mis)used to compensate for this frequency difference.
Just fill in 3162 as ‘Fiq’ in the appropriate settings window.

How does it work?
Summarizing: the receiver works flabbergastingly well!
Especially considering its simplicity and the fact you receive both sidebands.
I experienced no AM ‘bleed through’ and it takes around 5 minutes before the LO is stable.

During a short holiday near Rye (England, JO00JW) I tested this receiver using only 5m of thin wire
as antenna, fed through a window of our camper. The received unique 30m spots can be viewed here.
Perhaps you are among them? It should be noted that the RX environment was very quiet.
MUCH better than here in Holland with lots of PLC equipment and LED lights polluting the radio spectrum!

This experiment also demonstrates that a clean radio spectrum is of significant importance
and not specifically receiver/conversion gain. As long as you can hear antenna noise it’s OK.
If you hear more than antenna noise, like computers, plasma screens, LED lights, PLC etc. more expensive
(and better?) receivers have no added value.

I was pleasantly surprised repeatedly receiving my own 30m WSPR signals from Holland!
Output there is around 150 mW and the antenna is a 3m long curtain rail (with curtains ; -)

During the evening and night lots of US stations were logged with good signals, despite the small RX antenna.
The fact that I was located near the sea certainly may have helped.

Below some pictures of the setup in and around the camper (click to enlarge in new tabs).