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Re: System Gain
- Subject: Re: System Gain
- From: Leif Asbrink <sm5bsz.com; leif@xxxxxxxxxxxxxxxx>
- Date: Fri, 21 Dec 2007 22:24:01 +0100
> I am getting absolutely no decrease in noise to the K2 or the SDR-14
> when the IF cable is removed. So, I don't think the SDR has a problem.
> Both the SDR and the k2 are very sensitive and detect very weak signals
Hmmm, my SDR-14 has NF=28dB. Not sensitive at all for a 144 MHz receiver.
> I just believe that with a little more post conversion gain in the IF at
> the top of the tower would improve the system even more.
Yes. But you might find "a little" to be a rater large number
(with an associated rather large system improvement)
> I remember when I did the original design of the setup, I was cautious
> about over driving the K2. My rule of thumb is that the noise should
> increase when you hook up the transverter, but the S-meter should not
> morve or at least should move only to S-1 or so in order to keep from
> over driving the IF radio in contest conditions.
This is the solution for the desperate victim of nearby QRO
startions. If the meter does not move, your noise floor might increase
3 dB when the transverter is connected. That implicates that about
50% of all the noise you hear does not originate in antenna or frontend.
The associated loss is 3dB. (compare with what we are willing to pay
for a twice as big antenna.....
> Now admittedly, I don't live in an area that has heavy microwave
> activity, but I still feel that following this guideline will result in
> a near optimum system response.
'optimum' is a difficult thing. What you aimed at seems to be the
optimum in a very different situation from the one you actually
Leif / SM5BSZ
> VK2KU wrote:
> > Dave,
> > I'm not sure exactly how the SDR-14 differs in architecture from my
> > SDR-IQ,
> > or how Leif has implemented the control of gain/attenuation through
> > Linrad,
> > or indeed how the 144MHz input works - presumably through a converter
> > down to 28MHz.
> > But ... the first thing I would be checking is that you are not
> > inadvertently using some
> > front-end attenuation in the SDR-IQ, as I found I was with the SDR-IQ.
> > When I unplug the 28MHz input to my SDR-IQ, the Linrad noise floor
> > drops by about 17dB.
> > My 2p-worth.
> > 73 Guy VK2KU
> > >
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