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[Linrad] Re: system gain

In the old days, when every thing was analog, we would say that the 
system noise figure was set by the first amplifier's noise figure.  Assuming 
you have enough gain.  From the first amplifier noise is just added.  I can't 
visulize your system do you have any gain near the antenna?  The losses 
introduced by coax also adds to the system noise if the amplifier is after 
the line loss.

Maybe this will help.
John K1VBM

---- Original Message ----

From: Dave Sublette <k4to@xxxxxxxx>
To: linrad@xxxxxxxxxxxxxxxx
Subject: [Linrad] system gain
Date: Wed, April 08, 2009, 10:11:00

Good morning,

I have been watching the email thread on setting optimum gain and other 
settings for the LINRAD program. I have used the program, rather 
casually, for two years or so to look at weak signals on the 144 mhz IF 
outputs from my microwave transverters. I put a gain stage ahead of my 
SDR-14's direct input and that stage measures at 0.9 dB nf and 18 dB 
gain or so at 144 mhz.

When I remove this amplifier from the system, the noise floor on the 
main spectrum display on LINRAD drops 8 dB. I read in the emails that it 
should drop 15 dB, if I understand things correctly. I am looking at 
about a 160-170 Khz slice of the spectrum around 144.1 Mhz,

I do note that the visual display on Linrad is not as sensitive as my 
receiver on a splitter from the same IF.

My question is,  "Where should I look next to either find the problem or 
adjust the LINRAD parameters to bring the LINRAD system sensitivity up 
to standard?"

I continue to read the web page and previous emails from VK2DU on 
  Nothing I have tried seems to be working. I would list all my settings 
here, but perhaps that would be premature.

Any advice to be had out there?


Dave, K4TO

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