Dear Stefan, Paul, LF Group,
Yes... I used the SpecLab "hard limiter" function for reception of DK7FC in
the earlier tests in March. I used a 3kHz bandwidth filter (hardware in the
preamp output), and set the limiter threshold to "0dB above average level",
which seemed to give the best results in some rather un-scientific trial and
error tests. I estimated then that this combination gave around 10dB
improvement in the apparent SNR - this was certainly essential to
successfully receive the signal.
On last Sunday's test, I did not have the filter due to the change in TX
frequency. I used the Spec Lab noise blanker function with its default
settings, but this would have been less effective, due to the presence of
high levels of utility signals and low frequency mains noise, which would
have pushed up the blanker threshold. This was all done in a bit of a
hurry - looking at it since, it appears it would be possible to use the
downconversion / filter functions built into SpecLab to achieve the same
effect as with the hardware filter. Something to try next time - as well as
to remember to record the raw signal data!
Re: Paul's mention of using long periods to coherently integrate the signal,
several years ago I did some experiments on 136k with G4JNT, which involved
me transmitting a stable carrier (GPS disciplined reference) with 0.1
bit/second phase-shift keying in a 120 second sequence, which Andy then
coherently demodulated. The point was to show the feasibility of doing this
at low frequencies without clock recovery / carrier frequency tracking at
the receiver, taking advantage of the high carrier frequency stability that
can be achieved, and the stability of propagation, at low frequencies (it
seems that clock recovery is more difficult than recovering the data when
the SNR is very poor). Adequately low drift in phase was obtained with the
equipment we had - scaling to 9kHz, this would equate to a 30 minute
integration period.
Cheers, Jim Moritz
73 de M0BMU
----- Original Message -----
From: "Stefan Schäfer" <[email protected]>
To: <[email protected]>
Sent: Wednesday, August 04, 2010 6:35 PM
Subject: Re: VLF: DK7FC's 6th VLF kite experiment
Paul is asking who of the RX stations (except Wolfgang and me) has used a
noiseblanker or clipper?
Please a short info about the settings...
73, Stefan
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