David wrote:
On 28 Nov 2005 08:59:00 -0800, wrote:
David wrote:
Up until about 30 years ago, almost every radio station on the air was
feeding audio to their transmitters via unshielded wire phone lines.
No buzz.
Optoisolators as suicide devices.
+++++++++++++++++++++++++++++++++++++
Now I get it.
I am trying to recieve signals near or at the noise floor which is set
first by the local QRN. there are as amny different levels as
refference
books, so I will use John Doty's figures which are wildly optimistic in
my
experience. I will post links to my refferences so others may follow
them.
From:http://anarc.org/naswa/badx/antennas/grounding.html
"According to "Reference Data for Radio Engineers"
(Sams, 1975), the wintertime level of natural noise in
my area at 10 MHz should be about 32 dB above the
thermal reference level: this would produce a noise
floor of -104 dBm in this bandwidth with a perfectly
efficient antenna. "
A very handy chart that converts dBm to Volts can be found at:
http://www.minicircuits.com/dg03-110.pdf
Sherwood Engineeringhttp://www.sherweng.com/table.html gives
the R2000 noise floor as -130dB in the SSB Mode.
So I am trying to capture signals in the range of -130dB=0.058uV to
-119dB= 0.251uV.
Your refference to audio being delivered over balanced line is at 0dBm
(nominal) for a voltage of 0.775V. (*600Ohm) or 0.225 at 50 Ohms.
There is more then a slight voltage level diffference.
Can we agree that:
Adding 1uV of noise to a 1uV(-107dBm) signal will significantly degrade
the
SN ratio.
Adding 1uV of noise to a 0dBm 50 or 600 ohm line will result in
minimal,
as in I would like to see it measured, degradation of the overall SN
ratio.
And I bet you won't try to run the output of a moving coil phono
cartridge
over ~70' of balanced audio line.
In a uV world I will stick to single ended, unbalanced,
coaxial/shielded cable
interconnects. If you are lucky enough to deal with signals in the
0.1V range
then twisted or parallel cable will work just fine.
Terry