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Re: LF: RE: Re: Testing now a new RX antenna on MF

from [Roelof Bakker] [Permanent Link][Original]
To: [email protected]
Subject: Re: LF: RE: Re: Testing now a new RX antenna on MF
From: "Roelof Bakker" <[email protected]>
Date: Fri, 08 Aug 2014 19:55:08 +0200
In-reply-to: <9556CA31D58145AFA57960399F67FE98@Clemens0811>
References: <095C9D4E586E45CAA3FE071EA4EA9818@HPG5401fr> <[email protected]> <64A3B1F0647248229C5C374887AA2FCB@F6CNI> <[email protected]> <1686C40AF97B4ABE99024972373C232A@HPG5401fr> <96662FC1BCA8402098A702D935EEC66F@F6CNI> <9556CA31D58145AFA57960399F67FE98@Clemens0811>
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Hello Clemens,
E.g., if your FFT is set to a bin width of say 5Hz and a carrier lies 20dB over the DANL (displayed average noise level) it will have a SNR of zero dB in a 
500Hz "IF"-bandwidth and therefore will be barely audible.
The brain of a skilled listener can simulate a bandpass filter with a width of 50 Hz. Hence your FFT bin width of say 5Hz and a carrier of 20dB over DANL will be clearly audible. Unfortunately the human brain needs white noise to work against, so a band full of interfering signals will exhaust the listener before long. On a quiet band it works great. This problem is solved by using a filter bandwidth of 10 - 20 Hz for aural copy of Morse Code. 

I believe that the "human brain filter bandwidth" is frequency dependent.
When I started chasing NDB's, a beat note of 500 Hz was used.
Over time this has changed to 300 Hz!
Such a low beat note can be a problem with an analogue receiver due to less than perfect filter skirts. With SDR's the problem does not exist. 

73,
Roelof, PAoRDT
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