In order to improve the SNR performance of my previous WSPR receivers/grabbers
I decided not to reinvent the wheel and used Onno’s PA2OHH design with some tweaks.
By Onno’s knowledge I was the first to use his SSB design. Why …. ?
It’s a direct conversion (DC) receiver with LO at half frequency using subharmonic or Polyakov
mixers. The unwanted lower sideband (LSB) is suppressed using the phase shift method.
Theoretically this should increase SNR with +3dB (<– a difference between a ‘yes’ or ‘no’ WSPR decode!)
Of course you also need +3dB more components in the detector ; -)
Fortunately this design uses common junkbox components.
To improve the audio response (read: selectivity) I deployed some filtering
after the ‘adder’ <– the two transistors adding the ‘I & Q signals’.
Although it is stated that additional filtering is not necessary for WSPR, I adhere the credo
rubbish in = rubbish out. Furthermore, the receiver is also used for QRSS reception.
Filtering is done by a band pass filter with Q = 10, Fc = 1500 Hz and G = 100 (at Fc),
followed by a gyrator also with Fc around 1500 Hz.
Why a gyrator? Well, it’s cool to say that you’ve a receiver with a gyrator ; -)
Seriously, the gyrator was also inserted with another feature in mind: it is designed as
emitter follower, having a low output impedance (Z). Low Z out reduces hum and/or noise.
A gyrator simulates a parallel LC circuit with f = 1 / (2π√LC) as depicted below.
Lots of (home brew) power supplies provide stable and accurate output voltages.
However, there is something very important to consider, namely noise.
E.g. very popular 78XX voltage regulators are not bad, but they are noisy.
A very simple and effective trick to reduce power supply noise and hum is to insert a
capacitance multiplier, consisting of only three components (see below).
The transistor ‘isolates’ the receiver from the power supply and its base
capacitor value (CFilter) is multiplied by its current amplification factor (hfe or β).
In this way very large capacitors can be created, resulting in effective noise damping.
Receiver circuit diagram
The circuit diagram of my receiver is shown below (left), as well as the ‘finished’ prototype (right).
(click on images to enlarge in a new tab. Big pictures! ; -)
Note: lots component tolerance is allowed, except for the phase networks.
E.g. 511Ω also may be 560 or 470Ω, 442K may be 390 or 470K, etc.
It depends how diverse and large your junkbox is : -)
Initially I used a NE5532 low noise dual opamp as adder, but for whatever reason it kept oscillating.
To save time I went back to the original design with two transistors.
Injecting -47 dBm (1mV) 10.140200 MHz into the receiver delivers a ca. 3 Vpp
beautiful non distorted 1500 Hz sine wave on the oscilloscope.
So, overall gain of the receiver @10.140200 MHz is 20*log(3/0.001) = ca. 70 dB
(assuming 50Ω RF in and 50Ω audio out).
Note: 10.140200 – 10.138700 = 1500 Hz and 10.140200 MHz is the middle of the WSPR band.
Injecting -47 dBm 10.137200 MHz (thus ‘-1500 Hz’, i.e. the lower sideband) after adjustment revealed a
noisy (estimated) 15 mVpp, making the overall lower side band suppression 20*log(3000/15) = ca. 46 dB (!)
Receiver circuit simplicity considered (and ‘standard’ junkbox components) this performance is remarkable !
Audio response / selectivity
With Audacity frequency response was recorded during night (left picture below).
In the spectrogram below (right), recorded around 2230 utc, sensitivity around 10140200 Hz is clearly visible.
(click on images to enlarge in new tabs)
As can be derived from the left picture, ‘audio power’ is around -6dB @1000 Hz and @2200 Hz.
I tried to find out whether the Audacity shows ‘voltage’ or ‘power’ dB units … with no success.
Anyway, the audio response is quite sharp and centered around 1500 Hz, the middle of the WSPR band.
Despite its simplicity, this receiver performs very well and competes easy with more complex
and expensive colleagues.