Jim
This looks like a lot of trouble for no real practical purpose, other than
maybe get a glimpse of a carrier an odd time on 8970 Kcs
The possibility of a real time QSO on this band over any distance looks
remote. In all it seems a lot of effort for little reward.
GL
de Mal/G3KEV
----- Original Message -----
From: "James Moritz" <[email protected]>
To: <[email protected]>
Sent: Sunday, October 17, 2010 2:27 PM
Subject: Re: VLF: Re: Detections of 5 microwatt transmission
> Dear Wolf, Markus, LF Group,
>
> I have been doing an experiment over the last couple of days to see if it
is
> practical to do this kind of reception with just the Spectrum Lab
"frequency
> compensation" facility, using an off-air VLF signal as a reference.
>
> As I noted during DK7FC's last VLF test, the internal sound card in my
shack
> laptop PC suffers from a short-term cyclic frequency drift, over a range
of
> a few ppm with a period of about 100s, which appears to be due to the
> cooling fan going on and off. This results in a spectral line in a
> millihertz-resolution spectrogram being spread out over several
millihertz.
> I tried altering the "power management" parameters of the laptop, but
> unfortunately none of these seem to result in the fan staying permanently
> on, which would probably cure the problem. The SpecLab frequency drift
> compensation facility had no problem locking itself to GBZ on 19.58kHz,
and
> gives practically perfect long-term frequency accuracy, but actually makes
> the short-term drift problem "spreading" worse. I guess this is due to the
> cyclic nature of the drift - the drift compensator shows corrections that
> vary in a more or less random way between measurements over a range of
about
> +/-2ppm, although the long term drift is quite small compared to that.
>
> However, I had a Maplin "budget" external USB soundcard, and tried that
> instead. This gives a great reduction in short-term drift, with no visible
> sidebands on the millihertz-resolution spectrogram above about -40dB on
the
> central peak. I then altered the FFT parameters to give 29.8 micro-hertz
> resolution (centre frequency 8.97kHz, fs 96kHz, decimate by 6144, FFT
length
> 524288 samples), and left it for a day or so to monitor an 8.97kHz signal
> from a Halcyon PFS-1 frequency reference, which is locked to the BBC R4
> signal on 198kHz. The resulting spectrogram is attached - the time markers
> are 5 hours, the amplitude divisions 10dB. after apparently settling for
> several hours, the signal is concentrated into a single peak at least some
> of the time, although it looks like there are some "abrupt" phase changes
> (at least compared to the 9 hour duration of the FFT samples ;-)). I am
not
> sure which part of the system these occur in - it could be due to
> propagation or other changes, either in the 198kHz signal , or in GBZ.
>
> As it is, the receiving system has frequency accuracy and resolution in
the
> several parts-per-billion range, which isn't bad considering the cheap and
> unstabilised nature of the components. I think this would at least stand a
> chance of seeing Markus' low ERP signal - will have to try next time!
>
> Cheers, Jim Moritz
> 73 de M0BMU
>
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