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.

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Overview

- Basic Principle
- Required Hardware
- Operation of Spectrum Lab
- VFO Control
- Receiver Gain Control
- Filter Control
- List of some VLF radio stations

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 .

### Basic principle

The PC is used to convert the VLF signal into an audible signal

- Soundcard runs at a sample rate of 44100 or 48000 Hz (see Notes)
- Frequency is converted down to an audio frequency of 650Hz (or similar)
- 650 Hz "Audio IF" is passed through a narrow-band filter (100Hz BW)
- The amplified audio is sent to the soundcard's D/A converter.

### Required "hardware"

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).

### Operation of Spectrum Lab

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).

VFO Control

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 !)

Receiver Gain Control

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:

The new and easier way (since Spectrum Lab V1.7, tnx M0BMU):
  Click on the "Filter" block and switch to the tab "IIR-Design 3". Enter a filter center frequency and bandwidth of your own, then click "Apply". More information about the types of narrow-band audio filters can be found here. You can also control the audio filter's center frequency from a small slider on the frequency axis of the waterfall, quite like the VFO control.
The old way (before Spectrum Lab V1.7)
  Click on the "Filter" block and switch to the tab "IIR-Design 2". Load one of the "650-Hz-filter" designs which are part of the installation archive. The default "SAQ"-filter is a 4-stage IIR filter, each stage is a second order IIR-filter used as a bandpass. The gain, center frequency and bandwidth of each stage can be set individually. Modify a parameter in the filter designer table, then click "Calculate" to make the change effective. You may customize the filter for your own requirements, change the bandwidth, center frequencies or the number of stages used. But it's not easy to spread the poles to get exactly the frequency response you want, so you should use the "IIR-Designer 3" as mentioned above.

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### Frequency list of some VLF stations

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:

GBR
  Though GBR went QRT in 2003, you may find an old transmission from this station in morse code in your music archive ! For example, when Mike Oldfield recorded "Tubular Bells" in 1972 and 1973 at The Manor studio, some of the recording equipment was not shielded enough so the CW transmission from GBR was recorded on the master tape - possibly without the audio engineer's knowledge. You can see these these morse code transmissions in the spectrogram or make them audible again (play directly from CD or from a WAVE file, but do not try this with lossy MP3 files).

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Notes

Phantom signals when using 48kHz sample rate
  (December 1, 2001)
  Be careful when using 48000 samples per second. Most modern audio cards really support this sample rate which is nice to receive signals up to 24 kHz. Some cards don't and you'll get an error message when you try to use this sample rate. In this case, buy another audio card or stick to 44100 samples per second which is a widely used standard sampling rate. Other cards like the onboard "ESS AudioDrive" in the author's notebook play dirty tricks when you use 48000 samples per second, here one example:
  Phantom signals appeared on the waterfall, a signal on 20.7 kHz also appeared (a bit weaker) on 16.8 and 12.9 kHz !
  (go figure: 48kHz-44.1kHz = 3.9kHz ; 20.7kHz - 3.9kHz = 16.8kHz ; 20.7 kHz - 2*3.9 kHz = 12.9 kHz) It seems that some cards don't really sample the analog input at 48000 samples per second, instead they sample at 44100 samples per second and 'duplicate' a part of the samples.. what a lousy trick !
Even higher sampling rates
  (December 2002 and May 2004)
  There are now some expensive soundcards on the market which support true 96kHz sampling rate. If you have one of these cards, try to set the sampling rate to 96000 samples/second. With some luck, you may be able to receive VLF stations up to 40..44 kHz (to receive 48kHz is not possible because of the soundcard's anti-aliasing filter). Note: I once bought Creative Lab's "Extigy" card because it said "supports 96 kHz sampling" on the box, only to find out that the A/D converter in fact only runs at 48 kHz. I later tried the Audigy 2 ZS, and after some struggle with unwanted audio bypass from the "Line-In" to "Line-Out", got it working ok.

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User feedback is welcome. It is the most effective way to make the program better. Please send mail to dl4yhf (at) qsl (dot) net or dl4yhf (at) aol (dot) com - robots keep off ;-)
If you have optimized the settings to reveive other VLF stations, or got the 96kHz sampling rate running, let me know. I will post it somewhere on my website so others can also use them. Because the program "Spectrum Lab" is still under construction, some of the functions described above may be slightly different now, but the basic principle remains the same.

If you are interested in VLF reception, check these sites...

- Radio Waves below 22kHz by Renato Romero, IK1QFK and other members of the VLF Open Lab community
- SV1XV's World below 535 kHz by Costas, SV1XV
- Science.Netfold.com is a new amateur science site initiated by Phil Beastal, started in April 2002
- for ULF / ELF reception, consider using an external A/D converter (with "DC"-coupling) connected to the serial port.

Good luck !

Wolf (DL4YHF).