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VK2ZRG > TECH 01.10.08 10:44l 58 Lines 2139 Bytes #999 (0) @ WW
BID : 2707_VK2ZRG
Read: GUEST
Subj: Re Noise figure calcs...correction
Path: IZ3LSV<IK2XDE<F5GOV<F4BWT<F4DUR<CX2SA<VK7AX<VK2TGB<VK2IO<VK2WI
Sent: 081001/0751Z @:VK2WI.#SYD.NSW.AUS.OC #:2977 [Sydney] $:2707_VK2ZRG
From: VK2ZRG@VK2WI.#SYD.NSW.AUS.OC
To : TECH@WW
VK2ZRG/TPK 1.83d Msg #:2707 Date:01-10-08 Time:7:48Z
Hello to all technical minded amateurs,
In my recent bulletin on noise figure calculations I wrote :-
> System temp of the two amplifiers = K1 + K2/G1 = 75.1 + 3361/200
> (Pre-amp gain of 23 dB) = 75.1 + 16.8 = 91.9 Kelvins
>
> Converting 91.9 K to noise figure we get 1.317 dB
91.9 Kelvins is a 1.317 noise factor, not noise figure.
91.9 Kelvins is a noise figure of 1.196 dB.
If you would like to calculate the required gain in a pre-amp to achieve
an overall system noise figure or noise temperature, you can use these
formulas. (It's only slightly harder than using Ohms law!)
The system noise figure or noise temperature can't be less than the pre-amp!
First you must convert noise figures into noise factors if you are working
with noise figure. No conversion is needed for Kelvins.
Noise factor = Anti Log(Noise figure/10) Using base 10 or common logs.
Using noise factors:-
F1 is noise factor of the first stage (pre-amp).
F2 is noise factor of the second or following stage.
F3 is the desired system noise factor.
Gain required = (F2-1)/(F3-F1)
In dB, 10 * Log((F2-1)/(F3-F1)) Using base 10 or common logs.
Using Kelvins:-
K1 is noise temperature of the first stage (pre-amp).
K2 is noise temperature of the second or following stage.
K3 is the desired system temperature.
Gain required = K2/(K3-K1)
In dB, 10 * Log(K2/(K3-K1)) Using base 10 or common logs.
Simple isn't it! No need to make wild guesses!
One thing to keep in mind when calculating the gain is what the added
gain will do to the intermod performance of your receiving system.
The pre-amp should be as close as possible to your antenna because all loss
in front of the pre-amp adds directly to the system Nf/temperature. If you
are using two pre-amps of lowish gain, then both should be situated close
to the antenna to get the best system Nf/temperature.
73's de Ralph VK2ZRG @ VK2WI
P.S. to G8MNY You may not use this bulletin in your TECH bulls without
my permission.
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