Remco.org

Seductive serendipity / Verleidende serendipiteit

October 31st, 2015

Experimental 30m QRSS grabber

A priori.
When operational, my experimental 30m QRSS grabber can be seen here. (<– click to open in new tab)

Introduction.
The receiver from my previous post can be used for other weak signal modes like QRSS.
QRSS is transmitting information at very low speeds. At the receiver side this information
can be integrated for long periods, increasing (weak) signal to noise ratio (SNR) significantly.

It’s a public secret that QRO (high power) guys use low power (QRP) techniques
to optimise their top (contest) stations. Besides having sufficient output power it is VERY
important you’re also able to receive low power.

The quest therefore is to improve the RX SNR of your (contest) station.
This is where WSPRP/QRSS comes in.

QRSS grabber?
In order to experiment with the super simple DSB subharmonic receiver I installed
a grabber. A grabber is a piece of software analyzing the audiospectrum using Fourier
transform techniques (FFT).
This allows you to visualize the weak signals because you can’t hear them.

Onno PA2OHH wrote LOPORA (LOw POwer RAdio) grabber software in Python.
After installing python 2.7 and some fiddling I managed to get it running.

First results.
While eagerly awaiting the first ‘lopshot’, results were disappointing. Besides some weak
WSPR signals I hardly couldn’t see anything with my ‘quick & dirty’ 30m antenna
consisting of 5m wire running through a window into the garden around 2m above ground.

Because I live in a very noisy environment, my first action was to minimise noise from
the receiving contraption itself. The receiver is connected to a SMPS (I know, not ideal)
and a computer. All their (ground!) connections were fed through 6 hole RFC’s.

Subsequently I decided to lengthen the antenna wire to around 7.5m (1/4λ on 30m)
and use my central heating system as counterpoise.

These simple actions resulted in a dramatic SNR improvement!

It almost resembled my first experience listening to Beverages on 160m. At first
glance you think your receiver is broke because you think you don’t hear anything.

Below is the difference between the 5m wire and the temporary 7.5m wire + counterpoise.
(click on image to enlarge in a new tab)

Believing the SNR algorithms of WSPR, SNR improvements were around +12 dB (!!)
Although the receiver now sounds very quiet (I still here my antenna btw), my amount of WSPR
spots increased flabbergastingly and am now spotting a new league of WSPRers.

I also had to fiddle with the audio level, FFT settings, contrast and brightness levels in LOPORA.
The result was opening of a new 30m QRSS world. When a (WSPR) signal appears very bright,
its SNR is mostly around -6 dB. Most signals are between -20 – 25 dB SNR, or even lower.

Future improvements.
My benchmark is the 30m grabber of Steen Erik LA5GOA. Click on this link and see why (of course it depends
on the time of the day, try between 11 – 18 UTC).

Apparently Steen Erik lives in a very quiet environment and must have a good take off, also due to the
nearby sea (salt water!). It’s almost incredible what he’s able to receive with his PA2OHH style
DC receiver (I reckon also with a subharmonic mixer) with ‘own adjustments’.
I mailed him and learned he uses the same setup as Joachim, or vice versa.
LA5GOA’s antenna is a dipole directed E/W.
Below is a picture of Steen Eriks receiver (click to enlarge in new tab).

Less receiver noise.
One of the first things I’ve to do is decreasing the intrinsic receiver noise. Thus, like Onno did,
surpress the unwanted lower sideband. Theoretically this results in +3dB SNR improvement.

Secondly, increase the selectivity using a 10.140 MHz crystal as band pass filter.
Onno measured 800 Hz crystal filter band width. Bandwidth of the usable audio now is around 7 kHz.
Narrowing this to 800 Hz theoretically increases SNR with 10log(7000/800) = ca. +9 dB.
Btw, my WSPR/QRSS audio is around 4.5 kHz due to the LO frequency of my receiver.

In other words, if these two measures are carried out another +10 – 12 dB SNR improvement is possible!

Increasing frequency stability.
The receiver now lies open on the living room table without measures to stabilize it.
E.g. it’s not temperature compensated and I notice around 2-3 Hz/°C frequency drift.

When one of my cats lies next to the receiver (for whatever reason she wants to) LO frequency goes up,
and when she leaves LO frequency goes down ; -)

Antenna improvement.
At this moment of writing 7.5m wire with the central heating system counterpoise is used.
I did not measure the antenna impedance, but from 40m I know that such antennas are noisy.
Also the radiation pattern is lousy due to its low height above ground. It’s looking up to the clouds (NVIS).

Building a (vertical) deltaloop introduces two assets: a) the antenna is a closed loop <– less noise,
b) the take off angle is relatively low <– less interference from (strong) nearby signals and good for DX.

This may result in at least additional +3dB, but more likely +6 dB SNR improvement.

Receiver QTH.
Last, but not least, try to look for a nearby quiet place to install the receiver. I live in a busy city
with lots of interference like Power Line Communications (PLC), LED-lights, plasma screens, etc.

When the receiver is installed in a quiet environment +15 dB SNR improvement will be a (very)
conservative estimation. It might be an idea to use Beverages (or BOGs) there, but a preamplifier
is inevitable then.

Let’s wait and see . . .?

Update: I made a DCTL antenna for 30m but it initially seemed no success.
That is, no SNR improvement was visible, only lower signal levels.

I discovered that the power supply of my sampling laptop generated some noise.
Replacing the power supply with another one resulted in less noise (around -6 ‘WSPR’ dB).

Antenna input coupling was changed to an isolated coupling loop, just like Onno did.
I still could discern antenna noise and had to increase the ‘audio input slider’ in Windows.

The microphone audio input is used and is now 100% (without ‘MIC boost’ or AGC),
resulting in around 4 dB ‘WSPR noise’.

Below the temporary input coupling loop is depicted. I experienced less audible noise when
the ‘hot side’ of the antenna (yellow clamp, green clamp is GND) is connected to the
‘cold side’ of  the coupling loop, i.e. that side which is more near to the cold side of the input coil.

The other way around more noise was audible. With my temporary antenna I tend to believe:
“Less audible noise = better SNR” ;  -)

Before these two modifications (power supply & coupling loop) I had +16 – 20 ‘WSPR’ dB noise with
100% MIC volume. Above on the right a ‘lopshot’ showing more and more signals.

If I may believe the WSPR SNR algorithm my SNR further improved with 12 – 16 dB so the
(sub) total SNR improvement since the grabber is up amounts 24 – 28 dB (!?)

That evening I was one of the few EU stations copying ‘early’ US stations and was ‘competing’
with some EU WSPR ‘big guns’ on receiving several US and Asian 30m stations.

Promising? Yes and no. For example, I discovered PI4THT (Twente WebSDR using a Miniwhip
antenna) was able to receive the same stations with sometimes 20 dB higher SNR’s (!!)

The next day I took some additional  measures:

1. Tried the experimental DCTL again.

2a. Connect both L + R channel of the soundcard to the receiver as I am not sure whether WSPR
and LOPORA sample ‘in stereo’. If this is the case, then on one channel only (audio) noise is present
which adds to the so called quantization noise of the soundcard.

2b. Isolate the audio path with a 1:1 audio transformer.

The results are visualized below (click on images to enlarge in new tabs).


Measure 1. Switch between wire and DCTL.  Measure 2. Audio -> mono and isolate audio with xfmr.

In the above left picture Hell Schreiber traces of GM4GKH IO77WL become visible with the DCTL.
Apparently the overall SNR of my receiving contraption was not good enough when I tried the DCTL
the first time?

Reconnecting the 7.5m wire with counterpoise delivers more signal but results in lower SNR,
to such an extend that GM4GKH’s Hell traces disappear.  These are the ‘highlighted’ parts in the spectrum,
in which the signal of my GPS locked signal generator @10.140000 MHz are also visible.
In this snapshot the reference signal may look wobbly,  however this is the receiver LO, not my signal generator!

The influence of 2a + 2b may be seen at first glance. The right picture above looks darker (click to enlarge).
Audio settings between the left and right picture were equal.

Judge for yourself, but I reckon the overall SNR improvement of 1. and 2. combined is at least around 6 dB.
I.e. from seeing nothing (no GM4GKH with 7.5m wire) into seeing something (+ 3dB)
and being able to identify it (another 3dB) = 6 dB.

It could be that the difference in radiation patterns of the wire + counterpoise vs. the DCTL are
responsible or perhaps propagation. However, after these two measures I’m able to see GW4GKH’s traces
and other signals still look ok.

Therefore I estimate the total SNR improvement since the grabber is up to 6 + 24 – 28 = +30 – 32 dB !!

Bear in mind this is still the DSB receiver, i.e. no 10.14 MHz filter crystal and no Weaver SSB demodulator.

Another interesting fact is that at this moment of writing I am six unique WSPR spots
ahead of my benchmark (LA5GOA/RX2) in the last 24 hours.
</update>

 

 

 

 

October 27th, 2015

Junkbox 30m WSPR receiver

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).

 

 

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