[Moon] Noise Factor measuremnt answers

[SM5LE]

Hi again
And again the MOONBOUNCE / MOON mailing list whas very useful ! Tnx to all for "Noise Factor measuremnt" coments.

73 Sven

Here are the anvers  :

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HP 346C is an high ENR source. So 0.3 or 0.4 don't  take it so dramatic.

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... for noise figures as low as you are measuring you need an ENR 
from your noise head of around 6 dB so the 346B should be used 
with an attenuator in line and ideally the combination of noise 
head an attenuator should be calibrated. 
Noise temperature is effected by temperature when measured because 
T cold is now not 290K which is the instrument default so there will be an error, T cold must be adjusted. 
Also beacuse noise power is related to kTB (Boltzman's constant * temperature * bandwidth) 
then temperature will increase the noisefigure.

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On the 8970A it is "Special Function 6.0" that you need to change. The 
default according to my manual is 296.5 K (23.35 deg C).

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The difference in noise temperature for the preamp between 17 and 30 degrees is 13 degrees higher 
noise temp on the preamp...
... But the 8970 works it out for you, alter your Tcold by 13 degrees and it takes it into account.

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There will be a measurable difference between noise figures made at different 
ambient temperatures.

It's probably better with low noise amplifiers to think in terms of noise 
temperature. Noise figure defined formally is a logarithmic expression of 
noise temperature related to 290degrees Kelvin. 

So:   NF = 10*log10*(1+noise temp/290)

We can measure noise by comparing the output power of an amplifier driven by a 
noise source or sources capable of generating two different, defined, noise  
temperatures. Conventionally this is a noise head like the HP346 I use here, 
which will have outputs of ambient temperature when the head is off and, say 
1000K when the source is turned on. From this we can derive the K factor, and 
then noise figure. If the ambient temperature varies from 290K the K factor 
will change. (There's also the potential thermal dependence of the 'on' noise 
temperature to consider, but not in a short email!)

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That is about normal for the LNA -   

 
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