[Top] [All Lists]

[TenTec] Weak Sig/Filter BW [was:Ten-tec noise and RX questions}

To: <tentec@contesting.com>
Subject: [TenTec] Weak Sig/Filter BW [was:Ten-tec noise and RX questions}
From: W8JI@contesting.com (Tom Rauch)
Date: Thu, 28 Oct 1999 09:57:58 -0400
> My opinion on the narrow bandwidth/noise/weak signal phenomenon is that
> when trying to copy a weak sig out of the noise, narrower is worse(er). 
> The reason being, the noise is limited to a frequency bandwidth that is
> *really* close to the frequency [say, 600 Hz] that you are trying to copy.
>  It's harder for your brain to distinguish that.  

Wider filters increase noise power outside the "passband" of your 
focus. Using a wider filter does not move the noise to a new 
frequency and improve S/N.

While S/N ratio in your "mental area of focus" remains exactly the 
same, the wider noise window simply increases noise power and 
eventually saturates your ears with so much noise you can't dig the 
signal out. Ears and even your brain, just like electronic equipment 
used to process signals, has limited dynamic range and IMD 

With non-white or non-smooth level noise, bandwidth restrictions 
increase duration of noise pulses or peaks. That's why noise 
blankers MUST be ahead of any selectivity in order to function 
correctly, and why DSP systems (even in the IF or in the AGC 
loop) do not work well on impulse noise when they follow selective 

Filter "ringing" is an unavoidable part of high selectivity, since any 
level change requires bandwidth to accomplish the change. When 
the bandwidth restricts the rate, we get what most of us call "filter 

Some filters have peaks in overall response or internal selective 
elements that cause excessive ringing for a given bandwidth and 
shape factor.
73, Tom W8JI

FAQ on WWW:               http://www.contesting.com/tentecfaq.htm
Submissions:              tentec@contesting.com
Administrative requests:  tentec-REQUEST@contesting.com
Problems:                 owner-tentec@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm

<Prev in Thread] Current Thread [Next in Thread>