Topband: Weak Signal Reception (EME view)
btippett at alum.mit.edu
Thu Jul 17 23:37:46 EDT 2003
SM 5 BSZ - Receiving Weak CW Signals
(June 30 1997)
Among weak signal enthusiasts there are different opinions on what is the
best way to receive a marginal signal. The two main points of view are:
(1) Use a narrow filter to get better S/N.
(2) Use a fairly wide filter to avoid the ringing of a narrow filter.
The fact that both alternatives have many supporters is perhaps unexpected,
since the S/N, the signal to noise ratio is inversely proportional to the
bandwidth, and typical bandwidths for the two alternatives may be 1kHz and
25Hz - a difference in S/N of 16dB.
The explanation why the difference between the two methods is so small that
there is an argument on which method to choose lies in the fact that the
signal processing does not end at the ear phones !! The human brain is a
good signal processor and under optimum conditions it acts as a very narrow
filter. Nothing will be gained by putting a wider filter in front of it!!!
My own personal experience is as follows:
1. It is much easier to copy a weak signal if all external noise is
suppressed. I am using head-phones intended to protect the ears in noisy
environment while listening to something pleasant - radio or a CD... I can
not even hear when the telephone rings!!
2. Using both eras simultaneously through head phones is more safe than
using a loudspeaker in a quiet room. Multi path propagation of sound waves
may suppress certain frequencies. It may be a good idea to switch the two
wires to one side of the headphones now and then to produce a 180 degree
phase shift between the ears - this can relieve fatigue when trying to hear
a weak signal in noise.(Another way of relieving fatigue is to change the
pitch of the signal.)
3. If the bandwidth is too large, above 1kHz, the "signal processor"
between my ears gets overloaded and a significant loss of performance occurs.
4. Only at bandwidths below about 50Hz I can get any improvement over what
I get at 1kHz - but only at slow or moderate CW speeds, and with signals
that are very frequency stable. This indicates that my "brain bandwidth" is
about 50Hz. I can work for much longer times without loss of performance
when the bandwidth is low. I practically never use bandwidths above 100Hz.
Only for aurora, when a bandwidth of maybe 300Hz together with a high pitch
gives an improvement. (For aurora, using a matched filter one looks for
small amplitude changes of the noise without any change in the character of
the sound, so a high pitch is better)
5. Noise and hum in the AF part of the receiver (and all other parts as
well) has to be kept well below the noise floor originating in the pre
amplifier (or antenna). Some hams may argue that this is not necessary, but
I do not accept less than a 20dB increase of the voltage across my
headphones when switching on my pre amplifier up in the antenna. I do count
unnecessary losses by tenths of a dB.
6. When using bandwidths above about 30Hz it is important to avoid any
signal distortion for signals within the passband. The AF part has to be of
HIFI quality. 5% intermodulation between two carriers (same peak amplitude
as noise floor has in weak signal mode) means roughly that 5% of the signal
energy is smeared out into the noise, while intermodulation between noise
components raises the noise floor at the signal frequency by 5%. This kind
of degradation can not be removed by a good filter after the non-linear
part of the signal chain.
7. When using bandwidths below about 30Hz it may be a good idea to
introduce distortion signals within the passband. When the receiver has
already filtered the signal to the extent that the ear - brain can not hear
any difference between different frequencies within the passband, then the
only task for the ear - brain is to analyse amplitude changes. The ear is
not very good at this - it is logarithmic in nature and developed to watch
for signal character, not amplitude. Therefore expansion of the dynamic
range or symmetric clipping can make receiving much less tiring, and even
improve the detect threshold for a weak signal.
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