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: [email protected]
[mailto:[email protected]] On Behalf Of Stewart Bryant
Sent: 10 April 2006 13:19
To: [email protected]
Subject: Re: LF: Rugby LORAN measurement?
The issue that I have with this is trying to figure out is how to
convert the
voltage and amplitude of the interference into a time error. 70db is
1e-7 power and
only 3e-3 voltage. This seems big compared to 5e-11.
This clearly needs some more thought.
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