Topband: Weak Signal Reception (EME view)

Bill Tippett btippett at
Thu Jul 17 23:37:46 EDT 2003

SM 5 BSZ - Receiving Weak CW Signals
(June 30 1997)


Different opinions

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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