[opendtv] Re: Noise Figure
- From: Ron Economos <w6rz@xxxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Tue, 26 Feb 2008 04:55:34 -0800
It's not the temperature of the amplifying device that matters (except
for the ultra low-noise space systems that Dale is talking about), it's
the temperature of the background that the antenna is pointed at.
Here's the equations that can be used for either terrestrial or
space (satellite) communications:
1) Converts noise figure to noise temperature
Tr = 290((10^(Nf/10)) - 1)
Tr = preamp or receiver noise temperature in degrees Kelvin
Nf = preamp or receiver noise figure in dB
2) System noise temperature (including antenna, feed-line and
preamp/receiver noise)
Ts = Ta + 290(Lr - 1) + LrTr
Ts = system temperature in degrees Kelvin
Ta = antenna temperature in degrees Kelvin
Lr = feedline loss expressed as a ratio (3 dB = 2, 6 dB = 4, etc.)
Tr = preamp/receiver noise temperature
Note: Ta is consider to be 290K for terrestrial antennas pointed at
the horizon (although it can be much higher due to man-made noise
and cosmic noise at VHF). Ta can be much lower for antennas
in space communications when the antenna is pointed at cold sky.
3) Noise floor
dBm = -198.6 + 10 log B + 10 log Ts
B = bandwidth in Hertz
Ts = system noise temperature
If Ta (antenna temperature) is 290K (the antenna is pointed on the horizon),
then the equations above can be reduced to:
Receiver noise floor in dBm = -174 + 10 log B + line loss (dB) +
receiver noise figure (dB)
So a -83 dBm sensitivity is approx. 7.7 dB noise figure (with no line
loss, 15.5 dB C/N and
6 Mhz bandwidth)
Note that the typical run of 100 feet of good RG-6 has around 4 to 5 dB
of attenuation.
Even a 0 dB noise figure receiver ends up with a 4 to 5 dB system noise
figure
with that coax run. A low-noise preamp at the antenna is the only way to get
low system noise figures, making low-noise receivers not so useful.
Ron
Manfredi, Albert E wrote:
Those gremlins are hard at work.
In short, the NF is defined as SNRin - SNRout. And the SNR is assumed at
some temperature, typically 290 or 300 Kelvin.
If one specifies SNRin and SNRout, i.e. all the elements thereof, then
one doesn't need to also specify the NF. Matter of fact, if one does not
specify the temperature, then in principle the NF can be allowed to
vary, with no ill effect to the consumer. That's what I meant about
liquid nitrogen cooling of the receiver.
http://en.wikipedia.org/wiki/Noise_figure
That said, obviously 15.5 dB of C/N and -83 dBm of sensitivity required
from A/74 can be improved upon, as Samsung convincingly showed in the
Gemini receiver it shipped to the CRC for testing.
Bert
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