(old look of the component window, before the introduction of STEREO processing)
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: March 16, 2003.
Note: some links only work if this file is located in the help directory
of Spectrum Lab.
If so, you can return here to the main
index.
Check for the latest release of Spectrum Lab somewhere at
www.qsl.net/dl4yhf .
The PC is used to convert the VLF signal into an audible signal
A PC with at least 166(?)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 QRM source (like a TV set). 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).
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 should see this window (the component window looks different in later versions):
(old look of the component window, before the introduction of STEREO processing)
If one of the function blocks is colored red, the program has detected a malfunction- maybe the PC is too slow (ok with 166MHz-P2) or the soundcard is not supported.
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).
If you want to modify the VLF "RX-frequency", you must set the LO frequency to the desired RX freq minus 650 Hz (17200Hz-650Hz = 16550Hz in the example shown above). Click into the "Oscillator" block, the frequency can be edited then. (An easier alternative is to 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 that the 'connection' of the red frequency marker to the VFO is a programmed feature of the "VLF-Receiver" profile !)
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 the filter block), a small dialog opens where you can enter a factor (!). In a future release of the program it will be possible to "connect" a circuit parameters to a windows-style slider which can be moved with the mouse (check this, the program may be newer than this document...).
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 (guess how ... click 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 Windoze. 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.
Some of these stations are:
| Frequency (Hz) | Callsign | Station | Country | Mode, Remarks |
| 11905, 12500, 12649, 12695, 13281, 14881, 15625 (!) |
? | RSDN-20 ("Alpha") navigation system |
RUS | Pulses ("beep,beep") all heard in Dec. 2003 |
| 15100 | ? | ? | ? | |
| 15625 | -- | Your neighbour's 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:
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If you are interested in VLF reception, check these sites...
Good luck !
Wolf (DL4YHF).