--- On Wed, 9/3/08, Russell Hltn <[EMAIL PROTECTED]> wrote:
> From: Russell Hltn <[EMAIL PROTECTED]>
> Subject: Re: [digitalradio] SignaLinkUSB freq response and S/N
> To: digitalradio@yahoogroups.com
> Date: Wednesday, September 3, 2008, 1:29 PM
> On Wed, Sep 3, 2008 at 6:11 AM, Peter G. Viscarola
> <[EMAIL PROTECTED]> wrote:
> >
> > Now, in terms of S/N ratio... I find it hard to
> believe that quieter
> > isn't better no matter how you look at it.
> >
>
> But at what point does it become insignificant? I've
> heard of people
> using low-noise preamps on antennas, but never low-noise
> audio
> amplifiers. (Is this because we "compress" the
> signal with the RF and
> AF gain controls?) Is the quest for low noise audio cards
> the
> equivalent of seeking gold-plated, oxygen-free, 10 Guage
> speaker wire
> for our rigs?
>
> This should be a straightforward math question for a rig
> designer.
> But I'm not a rig designer. Anyone know?
>
>
The noise induced by all the stages from the antenna to the speaker play a part
in the overall noise figure or noise factor. A long run of coax can have
noise to add even though it is a passive component and not an active device.
There is a long drawn out formula to determine the overall noise factor.
Hopefully the first stage or two is low enough and has enough gain to override
the following devices such as the mixer, IF and AF amps of the receiver. Low
noise stages in the audio will help, but you night not notice the help. Sort
of like if you have a truck load of bricks and add one more brick. You know it
is there, but it takes some special weighing equipment to notice it. The
driver of the truck would not know you added it by the way the truck drives.
Here is a link to the formula.
http://en.wikipedia.org/wiki/Noise_figure
In very low noise systems (say less than 2 or 3 dB )even the noise in the coax
is the limiting factor and not the loss of the coax. No mater how much gain
you have , you can not pull a signal out that is less (maybe much less) than
the noise without special equipment. A well designed system will balance the
gain and noise factors of each stage. For the low bands usually there is
usually enough atmospheric, man made and other noise that a very low noise
figure is not needed. Even the antenna will see enough of the earth to limit
the signal to noise ratio. You see number of around 270 deg K as the average
temperature. The antenna will pick that up due to the heat of the earth. By
having a good beam that is pointed above the earth such as a satellite dish ,
the antenna noise will go down and preamps in the 30 to 60 deg K will work.
I am sure the oxygen free, gold plated #10 guage speaker wires will help, but
it is like a fly landing on that brick truck.
Maybe more like the fly on the brick truck taking in a breath of air.
