[Linrad] Re: system gain

[Edward Cole <acsalaska.net; kl7uw@xxxxxxxxxxxxxxxx>]

I have not made any recent measurements but a year ago I made 
comparisons between my SDR-iQ running Winrad and my FT-847 into 
JT-65.  My eyeball was the judge of waterfall SNR.

My RF system:  mgf-1801 LNA (0.15 dBNF, 16.8 dBG) + 2-dB line loss + 
1.0 dBNF line amp, 15-dBG, to 3-dB splitter.  One side feeds the 
8-dBNF FT-847 directly; the other goes to the 1.0 dBNF DEMI 144/28 
converter (est. 15 dBG), to a MAR-6 amplifier (18-dBG) to the SDR-IQ.

Results were that adding the MAR-6 IF Amp lowered the noise floor of 
the SDR-IQ 14-dB.  Without it the sensitivity was about the same 
(est. error of 3-dB).
I have not estimated what the system NF is for the SDR-IQ but the 
FT-847 is about 0.75 dB.

Oh I set SDR-IQ RBW to 3.4 Hz to be about equivalent with that of spectra-JT.

73, Ed - KL7UW

At 09:34 AM 4/8/2009, Leif Asbrink wrote:

>Hi Dave,
>
> > The antenna, preamp, power amp and transverter are mounted remotely at
> > the antenna. The system nf and gain to the IF is quite good, say .7 dB
> > nf and 25 dB gain.
>OK.
>
> > From there, the 144mhz IF travels through about 7 dB
> > of loss to a 3 dB splitter.
>Now we have 10 dB loss in total.
>
> > One side of the splitter goes to the IF
> > radio,
>Let me guess that the noise figure of the IF radio is 5 dB.
>That would give a noise figure at the other end of the
>cable of 15 dB. The composite system noise figure would
>be 1.04 dB
>
> > the other side to the .6 dB nf, 18 dB amp, and then into the
> > SDR-14 and on to the computer.
>At the antenna it looks like 10.6 dB NF and 8 dB gain. The composite
>noise figure becomes 0.82 dB with an associated gain of 33 dB.
>This is then connected to the SDR-14 direct input with a noise
>figure of 37 dB. The compsite noise figure for your Linrad
>part would be 5.7 dB. Totally unacceptable.....
>
>You should use the 0.1 to 30 MHz input which has a noise figure
>of about 17 dB. Then your system noise figure would be 0.91 dB.
>Probably just a little better than your IF radio.
>
>If I were you, I would move the 0.6 dB NF amplifier to the other
>side of the splitter and possibly insert an attenuator in front
>of the IF radio. The system noise figure for Linrad would then be
>0.84 dB for Linrad and the same for your IF radio if the attenuator
>is selected to provede a NF of 17 dB.
>
>73
>
>Leif / SM5BSZ
>
>
>
>
>
>
>
>
> >
> > The IF radio hears signals better than the LINRAD system displays them.
> > My whole point is to bring the LINRAD system up to or better than the
> > sensitivity of the IF radio. Of course this is a better problem to have
> > than the other way around--I think :-).
> >
> > I hope this helps you visualize my system. I don't believe the problem
> > is at the remote site since both the IF radio and the LINRAD system see
> > the same signal. The question is how to tweak the LINRAD parameters to
> > bring its performance up to that of the IF radio.
> >
> > Thanks for your thoughts.
> >
> > 73,
> >
> > Dave, K4TO
> >
> > radio1@xxxxxxxxxxxxx wrote:
> > > 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
> > > Cc:
> > > 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
> > > setup.
> > >   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?
> > >
> > > Thanks,
> > >
> > > Dave, K4TO
> > >
> > >
> > >
> > >
> > >
> > > >
> > >
> >
> > >
>
>

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