----- Original Message ----- From: "Gary Memory" gmemory@tks-net.com To: "'i8cvs'" domenico.i8cvs@tin.it; "'Amsat-BB'" amsat-bb@amsat.org Sent: Tuesday, September 18, 2007 8:26 PM Subject: RE: [amsat-bb] Re: New Preamp
Domenico:
Greetings once again. You sent me a note some time ago that helped to amplify my current belief concerning SNR.
I am willing to bet two things.
First, it is difficult (not impossible, just difficult) to obtain a better front end noise figure than what the original receiver manufacturer has created.
Hi Gary, N7BRJ/DA1BRJ
This is why we use a low noise preamplifiers antenna mounted. Make an example: The noise figure of a commercial receiver like a TS-736 is about NF=6.5 dB corresponding to an equivalent noise temperature T1= 1005 kelvin If you add a low noise preamplifier antenna mounted with a NF= 0.5 dB and if the overall noise figure of the system is degradated to NF= 0.7 dB corresponding to T2 = 51 kelvin then using the same antenna the improvement of S/N using the above preamplifier is: 10 log (1005/51) = 13 dB 10
Almost certainly the addition of an outboard amp will add more
noise. And this is not always a bad thing.
In the above example the more noise that you see on your S-meter is only apparent because the gain of the system probably is too great but you can reduce the noise reading of the S-meter without reducing the S/N ratio adding an attenuator and this is particularly easy to do if you use a preamplifier + downconverter + attenuator + IF receiver
A preamp can be a great addition despite the addition of more noise.
If you get more noise degrading the S/N it meens only that the NF of your preamplifier is greater then the NF of your receiver or its gain is too low.
But then you get into a curve of cost vs true value of gain. Is it worth it? Maybe so.
Adding a low noise preamplifier with the capability of improving the S/N ratio is always wortwhile.
Second, it is very unlikely that maximum signal strength will be at the same point as maximum signal to noise ratio (SNR).
The best noise figure NF of a preamplifier cannot correspond to the maximum gain as the S-parameters of a device for the lowest NF are showing.
Again, maybe this is not a bad thing in any one individual case. Almost certainly the best SNR will be at a point of gain somewhat less than maximum.
The important of the point is that even if the lowest noise factor F1 do not correspond to the maximum gain the gain G1 obtained from it be sufficient to get a satisfactory overall noise factor Ft of the system as the formula for more stages in series is showing:
F2-1 F3-1 Ft = F1 + --------+ --------- + .................. G1 G1xG2
I work with receivers and preamps that cost dozens of thousands of dollars. Cost and receive noise figure is almost a logarithmic curve. And it seems that only with the very best front ends are SNR and gain at the same point.
This is not strictly mandatory for as Radio Amateurs.
Once you get the best receiver, coax and antenna you can afford, that is all you can do....which seems like a dumb and obvious thing to say. Beyond that, true magic is hard to come by. And I am not knocking the preamp. Again, I own and use them all the time.
Once I get the best receiver and the best antenna I must reduce the thermal noise of the coax cable wich noise factor is F1 in the above formula so that to improve the S/N ratio I have to add a low noise preamplifier in F1 with the maximum possible gain in G1
73!
Gary, N7BRJ/DA1BRJ
Best 73" de
i8CVS Domenico