Envelope-to: dave@picks.force9.co.uk
Delivery-date: Wed, 13 Jul 2005 21:32:14 +0100
From: "Dave Pick" <dave@picks.force9.co.uk>
To: rsgb_lf_group@blacksheep.org
Date: Wed, 13 Jul 2005 21:31:25 +0100
MIME-Version: 1.0
In-Reply-To: <42D5203E.2050506@freenet.de>
Thread-Index: AcWHxSw2C38LYL1/QQ2v7r1DW3rNFgAI8EBA
Message-Id: <E1DsntG-0005bV-KL@post.thorcom.com>
Subject: RE: LF: Rugby lines
Content-Type: text/plain; charset=US-ASCII; format=flowed
Content-Transfer-Encoding: 8bit
Sender: owner-rsgb_lf_group@blacksheep.org
Precedence: bulk
Reply-To: rsgb_lf_group@blacksheep.org

Wolf

I left the AGC on so that the wanted signal was "pushed down" by the AGC
action under the influence of the increased QRM.
I then loaded the two traces into Spectrum lab and measured the strength of
the carrier.
The blanker idea may work but when there is such a large proportion of
noise, if you manage to cut it all out there isn't much signal left
anyway... 

Nothing seen in the (clear) trans-Atlantic window last night.

73
Dave G3YXM

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Wolf DL4YHF
Sent: 13 July 2005 15:08
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Rugby lines

Hi Dave,


You wrote:

> That was done by measuring the "strength" of the tone when analysing 
> the wav files with Spectrum Lab. The receiver AGC made the peak noise 
> level the same I could read off the difference in tone level (in a 
> tiny bandwidth) .

Did you turn your receiver's AGC off (or to "manual") when recording the
files ? If not, the increase of the audio signal will not tell you the true
story.

Besides this, I played with some recent recordings which Jim sent me. 
Trying to subtract the spectrum of the Loran signal from the "wanted" 
signal, but this didn't work too well. And the automatic multi-notch
function (which also acts upon the FFT'ed audio stream) also kills CW-type
signals.. too bad. So the removal of the Loran noise must be done in the
time domain, maybe similar to a classic "QRM eliminator" on the rcvr's
antenna input. Here is how such a circuit could work:

1.) Build a broadband receiver for 100 kHz, which recovers the waveform of
the Loran pulses
2.) Regenerate the pulses (constant amplitude) by means of a
Schmitt-Trigger.
3.) Use these pulses to drive an analog switch to "blank" the 137 kHz
receiver input.

Or (a bit more complicated due to the need to demodulate "positive" and
"negative" pulses):
Add the harmonics of the regenerated Loran signals, with amplitude and phase
carefully adjusted, to the 137 kHz signal (no blanking but subtraction).
This may produce less unwanted sidebands, but requires an SSB-type receiver
for the Loran signal with controlled 100 kHz L.O. .

Well these are just a few thoughts... not verified the principle because
Loran is rather weak here (or other kinds of QRM too strong ! ).

Cheers,
  Wolf  DL4YHF .