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Date: Tue, 16 Aug 2011 13:33:43 +0200
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Subject: Re: LF: Ferrite RX antennas
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Jim, LF,

Am 15.08.2011 23:14, schrieb James Moritz:
> [...]  In a 300Hz CW bandwidth, this would be about 0.75uV of noise,
> so with a reasonably sensitive RX, no further gain would be needed
> (worthwhile checking if it IS reasonably sensitive though...).  [...]
>
> Cheers, Jim Moritz
> 73 de M0BMU

Today i did some new sensitivity measurements on my homemade RX (137 kHz 
-125 kHz => SpecLab).
I took an analog homemade VFO to generate a sine wave at 137 kHz, 20 mVp 
at 50 Ohm, measured with an oscilloscope. Then i added a 43 dB 
attenuator between RX (which has a 50 Ohm input) and VFO. In SpecLab 
(connected to L1) my background noise was -107 dB in 1 Hz and the 
achieved signal level was -30 dB.
So, an input signal of 100 uV rms (within the passband) causes 77 dB S/N.
To stay well above the soundcards noise, say 15 dB S/N is needed, so the 
input signal could be 62 dB lower, i.e. if the ferrite antenna generates 
a band noise signal amplitude of 0.08 uV, all is fine(?).

No idea if there is a reduction of the performance of the RX when the 
full LF spectrum is applied, instead of that single signal. If no 
non-linear effects (IM, etc...) occur, there should be no difference. 
And we are talking about a directional narrow band antenna so i do not 
expect to much trouble, beeing an optimist ;-)

That minimum input signal level is about 20 dB below your stated 0.75 uV 
but i am referring to a 1 Hz bandwidth (width of one FFT bin). You were 
talking about a 300 Hz CW bandwidth. But i am configuring SpecLab to 
convert the signal (talking about CW) to 700 Hz and apply a 300 Hz 
filter. This works very well, as presented in the HB9ASB.wav file.
So do i misunderstand some things about the bandwidth settings/calculations?

73, Stefan/DK7FC

PS: A signal of 20 dB S/N in 1 Hz is suitable for e real CW QSO.