Thank you for the help, I’ve made your suggested adjustments to the FFT setting settings.
Today I plan to add a second axis to the VLF RX antenna; hoping that steering the antenna (RDF mode?) in SpecLab will provide significantly improved SNR for distant signals (signals for which the required integration time is too long to allow manual steering of the antenna for best SNR).
73, Jim AA5BW
Yes, this is the signal. They do not send a continuous tone but "beeps", thus the sidebands. You need to chance the FFT settings. First, "complex with internal frequency shift", centered to 11904.76 Hz. Next chance the decimate factor, increase it from 1 to 2 and then 4 and then 8 and so on and see what happens. The FFT input size can be 8192 or so. Enable "1 pixel per FFT bin" in the first register card of the audio settings and automatic scroll rate. Move the displayed frequency so you can see the lines. You will see the S/N becomes better when going slower, as expected.
Others may give you additional advice...
Am 30.12.2013 17:20, schrieb hvanesce:
My stable/accurate time reference (GPSDO) should arrive in two weeks; in the meantime I am hoping to use 11904.76190 Hz (per your suggestion, thanks) to assess my soundcard clock frequency offset. I have been guessing (based on comparison with another uncalibrated but stable oscillator) that the frequency offset of my soundcard gives less than 1Hz error at 11904.76190 kHz.
I have attached a screenshot captured over the last 24 hours, showing a modulated signal centered at an indicated frequency (red marker) of 11905.1884 which would have a 0.426 Hz offset from 11904.76190 Hz.
The signal seems to be fading about 40dB on a 24-hour cycle, with a minimum at ~ 2000 UT (clock times in my screen capture are Arizona time, UT – 7 hours).
Could the modulated signal shown here be the 11904.76190 Hz signal?
73, Jim AA5BW