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More on Temp, preamps and such



I appologize for the messed up formating on the previous post.  This
should be a bit easier on the eyes.



The medium length answer to this question harkens back a fundamental
tennant of physics:

Johnson noise.

This is the noise voltage produced by any real resistor.  The relation
that expresses this noise voltage has terms representing temperature (in
degrees Kelvin), resistance and bandwidth.  Not a specific frequency but
the range of frequencies over which you are measuring, f1 - f2.  This
difference in frequency should be positive if we are to avoid imaginary
numbers.  In our present case that is 4 MHz or the bandwidth of the 2m
band.  Room temperature is about 296 K.  Typicaly our loads and
transmission lines in our VHF, UHF and Microwave systems are 50 ohms.

If you plug those numbers into the equasion you will get about 1.88
micro volts of noise, at room temperature, simply due to the equivalent
resistance or real resistance of our transmission lines, terminations,
filters...etc.  This does not include other sources of noise due to
active components.

At my QTH the temperature extremes are from -28 C to 37 C (-20 F to 100
F).  If I take those extremes and add some at the top to account for the
sun beating down on a thermally unsheilded box or cable, the noise
voltage changes by 0.2 micro volts or so.  An FT-847 has a sensitivity
of 0.125 micro volts in the 2m and 70cm band.  This noise voltage,
inherant to our transmission systems and antennas, should get you
thinking about the theoretical limits of omnidirectional antennas vs.
gain antennas.

Since coax is a passive component there is not the same non linear
temperature dependence on normal opperation or best noise spec as you
would find in a device like a transistor or a diode.  This keeps things
simple so we don’t have to worry about non linear temperature behavior
unlike active components.

The point here is that yes there are some temperature effects but they
can be dominated by the inherent temperature restrictions we humans live
in.  Another factor that can hurt or hinder is the bandwidth.  The more
restrictive the bandwidth the smaller the noise voltage.  If you want
rock bottom noise levels you will also need to do what NASA does with
the DSN (Deep Space Network), which is also done by radio telescope
people, and that is to cool your MASER recivers down to within a few
degrees of absolute zero (-273 deg C).

Unfortunately if you do that to coax it will crack and break into a
million pieces when it is moved.  I’m not sure what would happen to the
dialectric properties of the plastics in any case.  Most of the
semiconductors produced these days, certainly all the ones we use in our
systems, become “iffyconductors” long before they reach such low
temperatures.  Indeed the operating temperature limits often are 10 or
20 degrees above and below the storage teperature limits.  Storage
temperature limits are often -55 C to +150 C.  This does not include
ICs.  ICs often have more restrictive but well documented temperature
limits.

At the other end of the temperature scale, most manufactures don’t
recomend that you opperate a device with the junction temperature above
150 deg C if you want it to last very long.  This is an issue for high
power devices but not so much of an issue for low power devices.
Remember that the junction temperature of a semiconductor is often
opperating at a higher temperature than the device case.  Keep that in
mind before you crank up your mast mounted power amp on the hottest day
of the year.  The heat sink may already be at 40 or 50 deg C, it may not
take much to push the junction close or beyond the 150 deg C limit.

I suppose if you look carefully you just might notice a difference
between the noise levels of your station in a climate with 140 deg C
swings between the darkest and coldest night of winter and the hottest
brightest day of summer.  The problem is that there are so many other
sources of RF noise that plauge us that it might be tough to see the
difference.

--
/***************************************
Bronson Crothers
LASST
5764 Saw. Res. Center
University of Maine
Orono  ME  04469-5764

Phone: 207 581 2252
Fax:.....207 581 2255

N1ZAQ

-... ...-

Email: bronson@eece.maine.edu
**************************************/


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