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Message-ID: <A897DB4098B34AEDB34E7BD5C5144DF9@JimPC>
From: "James Moritz" <james.moritz@btopenworld.com>
To: <rsgb_lf_group@blacksheep.org>
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Date: Thu, 5 Aug 2010 01:50:21 +0100
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Subject: Re: VLF: DK7FC's 6th VLF kite experiment
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Dear Stefan, Paul, LF Group,

Yes... I used the SpecLab "hard limiter" function for reception of DK7=
FC in=20
the earlier tests in March. I used a 3kHz bandwidth filter (hardware=
 in the=20
preamp output), and set the limiter threshold to "0dB above average le=
vel",=20
which seemed to give the best results in some rather un-scientific tri=
al and=20
error tests. I estimated then that this combination gave around 10dB=
=20
improvement in the apparent SNR - this was certainly essential to=20
successfully receive the signal.

On last Sunday's test, I did not have the filter due to the change in=
 TX=20
frequency. I used the Spec Lab noise blanker function with its default=
=20
settings, but this would have been less effective, due to the presence=
 of=20
high levels of utility signals and low frequency mains noise, which wo=
uld=20
have pushed up the blanker threshold. This was all done in a bit of a=
=20
hurry - looking at it since, it appears it would be possible to use th=
e=20
downconversion / filter functions built into SpecLab to achieve the sa=
me=20
effect as with the hardware filter. Something to try next time - as we=
ll as=20
to remember to record the raw signal data!

Re: Paul's mention of using long periods to coherently integrate the=
 signal,=20
several years ago I did some experiments on 136k with G4JNT, which inv=
olved=20
me transmitting a stable carrier (GPS disciplined reference) with 0.1=
=20
bit/second phase-shift keying in a 120 second sequence, which Andy the=
n=20
coherently demodulated. The point was to show the feasibility of doing=
 this=20
at low frequencies without clock recovery / carrier frequency tracking=
 at=20
the receiver, taking advantage of the high carrier frequency stability=
 that=20
can be achieved, and the stability of propagation, at low frequencies=
 (it=20
seems that clock recovery is more difficult than recovering the data=
 when=20
the SNR is very poor). Adequately low drift in phase was obtained with=
 the=20
equipment we had - scaling to 9kHz, this would equate to a 30 minute=
=20
integration period.

Cheers, Jim Moritz
73 de M0BMU




----- Original Message -----=20
From: "Stefan Sch=E4fer" <schaefer@iup.uni-heidelberg.de>
To: <rsgb_lf_group@blacksheep.org>
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=20
> noiseblanker or clipper?
>
> Please a short info about the settings...
>
> 73, Stefan
>=20