On Fri, 2006-07-14 at 06:47 -0500, James Duffer wrote:
> I was under the impression that noise was partially caused by temperature
> (molecular activity). I know that the sun causes noise, as I used to
> observe the sun strobe on the FAA long range radar I maintained for years
> near Memphis TN. Also a MDS of -114 dBm was considered an excellent figure
> and this was using a parametric preamplifier. And the "discernible" in MDS
> was by observing the pulse just as it came out of the noise. There was
> always noise, every day of the year. Believe me if there was no "grass" you
> had something broke!. I am not referring to man made noise.
>
> 73, Jim wd4air
>
-114 DBM is a pretty good MDS for radar bandwidth. For CW bandwidth, MDS
of -148 dBm is pretty good. Noise power goes with the square of
bandwidth. Noise is generated by random electron motion. Its a current
so the voltage developed depends on the resistor value and the resistor
temperature. It is possible to create a preamp that has no intentional
resistors so that the noise is significantly less than a resistor at
room temperature. e.g. a noise figure of a quarter dB or a noise
temperature of 20 or 30 degrees K. That is like the noise from a
resistor 20 or 30 Degrees C above absolute zero and its not quite
necessary for the preamp to be cooled way down though that makes it
easier.
When the radar receiver gets good enough to see sun noise, its hearing
lots of man made noise too.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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