Using a PC with soundcard as a VLF receiver
By Wolfgang Buescher, DL4YHF.
This article describes how to use your PC as a receiver for narrow-band signals in the VLF radio spectrum. If you are more interested in the reception of VLF natural radio (which is a broadband audio application), there is another document for you.
Last modified: October 24, 2010.
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Check for the latest release of Spectrum Lab somewhere at
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The PC is used to convert the VLF signal into an audible signal
A PC with at least 266 MHz Pentium II, a soundcard (original Creative Labs Soudblaster 16 has been tested, others work too, but some soundcards don't..)
For a start, just connect some meters of wire as an antenna to the MIC input of the soundcard and try to listen to a local signal source (like a tube TV set with flyback transformer). If you can get the system (described below) to work, add a simple LC network parallel to the input as VLF preselector. This will also protect your soundcard if you connect a longer piece of wire. In my own tests I have found that the system is very sensitive even with only passive components (including 2 protection diodes on a 200 meter long wire).
If there are no strong LF-, MF-, and HF-transmitters close to the receiver's site, an active antenna like the one shown below may also work for VLF narrow band reception (even though it was designed as a broadband antenna; see VLF Natural Radio):
Note: The lower FET's source resistor should be tweaked
to have approximately half the supply voltage at its drain .
The neon lamp shall protect the FET against moderate statics,
but it's no protection against lightning. 4,7 nF should have been 4.7 nF (etc) -
old habits die hard .. it's the usual decimal separator in Germany.
All settings required for a receiver for 17.2kHz are contained in the user
profile "SaqRcvr1.usr" which is contained in Spectrum Lab's installation
archive (since V1.65).
(Why "SaqRcvr" ? SAQ is the name of
Grimeton Radio, a historic radio
station in Sweden. More infos are/were available at
www.alexander.n.se in Swedish language,
very impressive !).
After starting Spectrum Lab, go to the menu "Quick settings"..."User defined Menus"..."Import". Select the file "SaqRcvr1.usr", "GBRrcvr.usr" or " VlfStns.usr", then click "Ok, Load". This will install a new item in the Quick Settings menu which you can use later to recall these settings.
Now switch to SpecLab's "Circuit" window (Menu "View/Windows...Spectrum Lab Components"). If everything is ok with the soundcard, you see something like this :
(component window after loading the application "SAQ - Receiver #1" )
If one of the function blocks is colored red, the program has detected a malfunction- maybe the PC is too slow (it did work with a 266MHz-P2) or the soundcard is not supported, or the soundcard doesn't support the sampling rate (which is either 44100 or 48000 samples/second. Good cards & drivers support BOTH). If necessary, change the sampling rate in SL's audio settings .
Now connect a headphone to the soundcard's output. You should hear a weak 650-Hz signal. Otherwise you must find the correct audio settings using the control program for your soundcard (some info is in SpecLab's help system, but SpecLab cannot adjust the audio gain in the soundcard itself).
Note: This was more complicated in older versions. Now frequency
conversion and filtering is done inside the FFT-filter; so the oscillator
+ mixer/multiplier are not used in this setup any longer)
To modify the VLF "RX-frequency", move the red "VFO" marker on the frequency
scale of the spectrum plot or waterfall display in the main window. Move
the mouse across the frequency scale, and hold the left mouse button pressed
to move the marker.
Note: The 'connection' of the red
frequency marker ("diamond") for
the VFO frequency is a programmed feature of the "VLF-Receiver" profile.
How this works internally is explained
here in the SpecLab manual, but you
don't need to know it if you only want to use this feature.
The "SAQ receiver" application uses one of Spectrum Lab's DSP blackboxes as automatic gain control. This is the green square with the questionmark in the circuit diagram. If necessary, the AGC can be configured as explained here. But for most applications the default setting should be ok.
If the audio output is too weak, though the soundcard volume control is at maximum, you can add more gain by software. If you click at the amplifier symbol in the circuit diagram (triangle just right to DSP blackbox), a small dialog opens where you can enter the gain, or connect the gain control to the vertical slider on the left side of the component window shown above.
If you don't hear anything: To verify if a signal is getting in, activate the "Input Monitor" (click..). Increase the "Vmag" value of the oscilloscope until you can see a signal.
If you cannot hear any audio coming out of the soundcard, activate the "Signal Generator" and connect it to the sound output (click on the function block and then start the sine generator). If you still can't hear anything, check the Volume Control from the program "sndvol32.exe" which is part of Windows. To modify the "recording" settings (here: Sound IN), you must change the properties in the "Options" menu of sndvol32. (Sorry, Spectrum Lab cannot find the optimum settings itself. It can not access the soundcard's "Mixer" like other nice programs ;-).
Modifying the Filter characteristics
Depending on the bandwidth of the signal you are looking for, you may want
to use different audio filter characteristics. A 50 Hz filter is often a
good choice for medium-speed morse transmissions, while 500 Hz may be better
for digital transmissions like RTTY and PSK. There is a number of
"pre-configured" filters which you can load from disk files, but you can
also create your own filters:
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The file "VlfStns.usr" has the frequency markers set to some VLF transmitters which can/could be heard in Europe. This makes identifying them on the waterfall display easier. In addition, there is a 'radio station list' which also contains the frequencies of some 'known' VLF transmitters.
(spectrogram with radio station display)
Some of these stations are:
| Frequency (Hz) | Callsign | Station | Country | Mode, Remarks |
| 11905, 12500, 12649, 12695, 13281, 14881, 15625 (!) |
? | RSDN-20 ("Alpha") navigation system. See notes below table. |
RUS | Pulses ("beep,beep") all seven frequencies heard in Dec. 2003 |
| 15100 | ? | ? | ? | |
| 15625 | -- | Your neighbour's (tube-) TV | Continuous line sync signal, emitted by flyback transformer |
|
| 15980 | GBR | Navy Rugby | G | RTTY (MSK) 100 Bd heard in March 2003 went QRT in 2003 (*) |
| 16300 | VTX1 | Navy Vijayanarayanam | IND | CW, rare |
| 16400 | JXN | Navy Novik | NOR | RTTY (MSK) 200 Bd heard in March 2003 |
| 16800 | ? | RTTY | ||
| 17000 | VTX2 | Navy Vijayanarayanam | IND | CW, rare |
| 17200 | SAQ | Grimeton | S | CW, historical transmitter (Alexanderson alternator) 1-2 times per year |
| 18100 | RDL,.. | Navy Krasnodar + .. | RUS | RTTY (multi-tone FSK?) heard in March 2003 |
| 18200 | VTX3 | Navy Vijayanarayanam | IND | CW, RTTY, rare |
| 18300 | HWU | Navy Le Blanc | F | RTTY (MSK) 200 Bd heard in March 2003 |
| 19000 | GBZ(?) | Navy Criggion (?) | G | RTTY (MSK) not heard in 2003 yet |
| 19200 | ? | ? | ? | RTTY (MSK) heard in March 2003 |
| 19600 | GBZ(?) | Navy Criggion (?) | G | RTTY (MSK) 200 Bd heard in March 2003 |
| 20270 (+/-) | ICV(?) | Navy Tavolara, Sardinia | I | RTTY (MSK) heard in March 2003 |
| 20900 | HWU | Navy Le Blanc | F | RTTY (MSK) heard in March 2003 |
| 21100 | RDL | Navy, Krasnodar et al | RUS | RTTY (MSK) heard in March 2003 |
| 22100 | ? | Navy Anthorn | G | RTTY (MSK) heard in March 2003 |
| 23400 | DHO38 | Navy Ramsloh | D | RTTY (MSK) heard in March 2003 |
A complete list of VLF stations can be found Renato's VLF Open Lab site, see links at the end of this document. Or scan the net for Klaus Betke's LF pages.
Notes on certain VLF stations:
ALPHA is a Russion navigation system in the VLF band which you can receive
with a simple loop antenna (at least in Europe). Some ALPHA frequencies are
listed here . With a system of two orthogonal
loop antennas and a stereo soundcard, the transmission cycle can be analysed
with a preconfigured setup from SL's installation archive :
Load the application "AlphaTrg.usr" and see what happens on the
screen (most likely, you will need to adjust the 'contrast' and 'brightness'
controls for the waterfall because the input level from your VLF antenna
will be different). What the "Triggered ALPHA-Beacon Monitor" does is:
If everything works ok, the noise-reducing effect of the averaging may look like this (left screenshot without, right screenshot with 10-fold averaging ) :
The pretrigger was set to 1 second for this test, so the trigger pulse is visible on the blue amplitude bar. The VLF RDF antenna was deliberately placed in a noisy environment (indoors), so the ALPHA pulses were barely audible in the noise. The waterfall 'contrast' was set to a high value; the same settings were used for both screenshots.
How does it work ?
Because the ALPHA-beeps will always appear on the same screen positions, the average-buffers will slowly fill up with each new scan. The noise (if randomly distributed) will seem to get weaker with each new scan. For example, if there was a random 1-millivolt "beep" in one sweep, it will appear like a 0.1 mV signal after ten sweeps are averaged. After 1000 averages, 1 mV will be divided down to 1 uV. Ok, what else can you do with this ?
Of course, this principle can be used for all kinds of weak but periodic
tone signals - for example, for EME-testing purposes (Earth-Moon-Earth).
If several hundred VHF-"pings" were sent to the moon and the echo received
with a small yagi antenna, it is possible to see the echo on the 'averaged
synchronized spectrogram' even if a lot of dB's were missing to copy the
signal by ear ( successfully tested at the local amateur radio station, see
configuration EME_Ping_Test.usr ).
A similar configuration (which works without an off-air signal) is contained
in the SL installer: Load the configuration 'TrigSpectTest1.usr'.
When reprogrammed for the ALPHA beacon cycle, that configuration would also work
with a single (non-RDF) VLF antenna.
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If you are interested in VLF reception, check these sites...
Good luck !
Wolfgang ("Wolf", DL4YHF).