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Date: Wed, 20 May 2015 22:18:37 +0200
From: DK7FC <selberdenken@posteo.de>
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Subject: Re: LF: RE: FZ-02-B mechanical filter input matching
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Status: O

Thanks Markus and Clemens for the feedback.
I will go on soon...

73, Stefan

Am 20.05.2015 20:58, schrieb Markus Vester:
> Hi Stefan,
>
> I'd suggest the following test:
>
> 1. with the current circuitry, measure the background noise floor on a 
> quiet day with little QRN (ie. not now ;-)
>
> 2. temporarily disable the BF981 stage, by disconnecting the supply, 
> or by pulling down the gate-2 bias. Measure the noise again, which now 
> comes only from the backend.
>
> If the noise level has gone down by more than 27 dB, you can spare the 
> ~ 17 dB gain stage and still be sure that the antenna noise dominates 
> 10x over that of the backend.
>
> If the difference is too small, you can try to reduce the ADC noise 
> figure by using a higher gain setting in the soundcard mixer, and 
> repeat the test.
>
> Best 73,
> Markus
>
> -----Ursprüngliche Mitteilung-----
> Von: Rik Strobbe <Rik.Strobbe@fys.kuleuven.be>
> An: rsgb_lf_group <rsgb_lf_group@blacksheep.org>
> Verschickt: Mi, 20 Mai 2015 7:38 am
> Betreff: LF: RE: FZ-02-B mechanical filter input matching
>
> Hi Stefan,
>
> regarding:
> Now my question: Is it better to remove a BF981 amp stage (about 16 dB 
> to 18 dB gain!) in front of the mixer (SBL-3) to get a lower RX noise 
> or do i better reduce the gain on the LT1028 stage on the AF side? The 
> AF pass band is 11...18 kHz (LO= 461 kHz)
>
> On MF one must have a very small RX antenna before the internal RX 
> noise becomes dominant to the external noise.
> Using my 475kHz antenna (lazy-L 12m high, 25m long) and a Kenwood 
> TS440 as RX I can attenuate the antenna signal by 40dB or more before 
> the antenna noise drops below the RX noise.
> So I think that in regard with SNR it will not really matter wether 
> you remove the preamp of reduce gain in the LF stage.
>
> 73, Rik  ON7YD - OR7T
>
> ...
>
>
>