[TenTec] Topband: Fw: Noise Limiter Help - Thanks

[Ken Brown]

Hi John and group,

Since you brought up the topic of noise 
Blankers/Limiters/Cancelers/Reducers, I thought I would post this to the 
list. Originally it was a response to a long thread on another 
reflector. The subject was the effectiveness of various brands of Noise 
Cancelling devices, the kind that use phasing from a separate "noise 
antenna". The stated subject however was "Noise Limiter Help".

I think in order for us to have in intelligent discussion, we must agree 
on the definition of terms.

My understanding of the various terminology is as follows:

Noise Blanker = circuitry which switches off the receive signal path
during noise pulses. Noise blankers usually work in the receiver IF,
although some may work in the front end. The signal used to trigger the
blanking can come from the early stages of the IF of the receiver,
before narrow filtering, or in some systems it comes from a dedicated
noise receiver possibly even using a separate antenna.  The time
duration of  the blanking has to be long enough to blank the noise
pulses, and short enough that the gaps in the desired signal are not
objectionable.

Noise Limiter = circuitry which limits the overall signal level. This
can be implemented in the RF, IF or audio stages of the receiver.
Probably most often done in audio stages. It will limit any signal,
noise or not. Since noise pulses from lightning and man made electrical
noise are often the strongest pulses received, it will usually be
limiting only noise and not the desired signal. Noise limiters probably
don't improve readability of the desired signal as much as they protect
your hearing.

Noise Canceler = Circuitry which adds a 180 degree out of phase sample
of the noise to the received signal. The noise sample has to come from a
different antenna than the one used for the desired signal. The
amplitude and phase relation of the desired signal and the noise you
would like to cancel must be different on the two antennas, so that when
the phase and level controls are adjusted for maximum cancellation of
the noise, the desired signal is not also canceled. Noise cancelers do 
their work on the signal before it goes to the receiver front end.

Noise Reduction = Probably all of the above methods could be called
"noise reduction", however these days the term tends to refer to systems
which digitize the signal, either in the IF or AF stages, and then use
digital processing to reduce the unwanted noise, before the data stream 
is either decoded or turned back into an IF or AF analog signal.

Ken N6KB


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