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From: "james moritz" <j.r.moritz@herts.ac.uk>
To: <rsgb_lf_group@blacksheep.org>
Date: Tue, 11 Apr 2006 16:58:39 +0100
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Subject: RE: LF: Rugby LORAN measurement?
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Dear John, Stewart, LF Group,

I still have some .wav file recordings of the Rugby Loran TX starting up =
in
the mornings during their commissioning period last year, which gives a
direct "before and after" comparison. I think I have sent some of these
before to people on the reflector, but can do so again if anyone is
interested. Obviously they are quite big files. I recall that, using an =
RMS
voltmeter to measure the IF output of the receiver (with AGC off), the
increase in noise level with the Loran TX on was 15dB. This was using a =
loop
oriented to null out the Lessay Loran, which also approximately =
maximises
the signal from Rugby at my QTH. With an omni-directional vertical =
antenna,
the corresponding increase in noise when Rugby switched on was only =
about
9dB, due to the presence of considerable noise from Lessay. The actual
effect of this QRM on aural CW, PSK, etc., might be different than the =
power
ratios of course, and would also be affected by RX AGC or limiting, or =
noise
blankers and so on.

As far as the amount of error caused by low-level Loran interference at
frequencies close to the MSF carrier goes, I think the situation is a =
bit
like that of FM with an interfering signal - if we have two sine waves, =
V1
the wanted signal and V2 a smaller unwanted signal at nearly the same
frequency, then since the frequencies are different, the vector sum of =
the
wanted and interfering signals produces a resultant which is basically =
the
wanted signal with a small phase "wobble" at the difference frequency. =
The
peak instantaneous phase error will occur when the wanted and =
interfering
signals are in phase quadrature, and will be +/- arctan(V2/V1), which is
nearly +/-V2/V1 radians when V2 is small. This amounts to about +/-0.02
degrees for -70dB QRM level, about +/-1 nanosecond at 60kHz. The average
phase error would be zero, so averaging over a long period would reduce =
the
effect on this error on the frequency comparison. I expect other factors
would result in more than 1ns of jitter (the antenna moving 30cm in the
breeze, for instance!), so I guess a precision frequency standard based =
on
MSF would have to use long integrating periods in any case.

Cheers, Jim Moritz
73 de M0BMU

-----Original Message-----
From: owner-rsgb_lf_group@blacksheep.org
[mailto:owner-rsgb_lf_group@blacksheep.org] On Behalf Of Stewart Bryant
Sent: 10 April 2006 13:19
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Rugby LORAN measurement?


The issue that I have with this is trying to figure out is how to=20
convert the
voltage and amplitude of the interference into a time error. 70db is=20
1e-7 power and
only 3e-3 voltage. This seems big compared to 5e-11.

This clearly needs some more thought.