In testing my Chirpy 10m transceiver, I used the simple PC based VLF receiver by SM6LKM as a bolt-on SDR. This little VLF receiver works very well as a stand-alone VLF receiver that tunes the 0-22kHz frequency range. Simply by sliding the cursor to any frequency it produces an audible signal just as you would get by tuning a conventional receiver. Bandwidths are adjustable down to around 300Hz. For my video showing this in action listening to VLF signals (via my loop and small preamp) see http://www.youtube.com/watch?v=TDxRhQYg7lQ. With the SM6LKM VLF receiver you can clearly hear the Omega beacons around 11-14kHz, SAQ when transmitting on 17.2kHz, and various MSK military signals around 18-22kHz. It has also proved useful when carrying out earth-mode (through the ground) VLF communication tests.