Which shows that the processing by eye is about as good as the processing by
a good CW ear about 10dB S/N in their respective bandwidths. I think I
would agree with the 30 Hz effective ear bandwidth when listening to a tone.
Think what could be achieved in a 0.03 Hz bandwidth !
Both these are energy detection methods, the electrical equivalent being a
rectifier on the output of the IF strip driving a chart recorder. Now if
instead of the signal being switched on and off, it was switched plus or
minus. The movement is twice as far and another 3dB is gained. That is
From: 'Geri' Kinzel, DK8KW <[email protected]>
To: LF-Reflector <[email protected]>; amrad <[email protected]>
Date: 15 April 2000 09:37
Subject: LF: Slow CW Sensitivity Measurments
I made some laboratory tests this morning to get some indication about the
ability to communicate with signals below noise level using Slow-CW.
I used a calibrated frequency synthesizer (Adret 2230), an 0-120 dB
attenuator in 1 dB steps (Schlumberger BMD500) and my Praecitronic MV61
Selective Level Meter. With a BNC t-connector I fed the normal band noise
including loran lines on 137.500 kHz (+/- 50 Hz) to one side of the
t-connector, and the output of the frequency synthesizer to the other side.
With the attenuator I made sure that a 0 dBm (50 Ohm) signal with the
synthesizer corresponds to a -80 dBu (75 Ohm) signal at the MV62
(plus/minus 1 dB).
The band was quite this morning, with a background noise around -110 dBu
(approx. S 4) and Loran lines clearly visible.
Using the 100 Hz bandwidth of the MV62 and the cascaded 250 Hz/500 Hz CW
filters of the IC-746 I checked the signal by ear as well as with the
Spectrogram software with the normal parameters I use for "3-5
second-dot-length" Slow-CW (5.5k sample rate, 16bit mono, 16384 points FFT
= 0.3 Hz resolution, 60 dB scale, 300 ms time scale, 10 x average) and
obtained the following results:
Injected Received Comment
- 20 dBm - 100 dBu good audible CW signal (approx. S6)
- 30 dBm - 110 dBu CW signal approx. equal to
background noise (S4), just can be copied
- 35 dBm - 115 dBu boundary for aural CW, signal just
detectable by ear
- 45 dBm - 125 dBu good "O" signal in Slow-CW, signal
same level as Loran-lines
- 50 dBm - 130 dBu still good readable Slow-CW signal
- 55 dBm - 135 dBu Slow-CW just detectable "T"
- 60 dBm - 140 dBu Slow-CW signal not any more
detectable with above listed parameters
Slow-CW has a 20 dB signal level advantage over normal (aural CW), which
means that the minimum detectable and/or readable Slow-CW signal that might
just allow communication lies 20 dB below the signal, that can just be
detected and/or decoded by a trained CW-operator's ear (yes, I consider
myself to be a trained CW operator ...). If I consider the "CW-operator's
ear/brain bandwidth" to be 30 Hz, this roughly corresponds to the
bandwidths used (0.3 vs 30 Hz).
I would be interested to get your comments or own measurements on this
subject. I do not yet have sufficient experience with Spectran to make full
advantage of this software, so I would like to hear about that software as
Geri, DK8KW (W1KW)