Re: Measuring Noise Figure

Andy G4JNT

I did it once with boiling water and room temperature on what was supposed to be a low noise amp (144MHz) of the late 1980s era.  I saw  a genuine "fraction of a dB" but back then didn't have the means to measure mean noise power reliably or accurately, especially in a wide bandwidth.

Assume a 1dB noise figure LNA with sufficient gain to overcome any following stages.
1dB is equivalent to a noise temp T of  75K   (  since NF = 10.LOG(T / 290 + 1)

So the delta noise power seen at the output of the system for input at 100C = 373K and  Room Temp 290K 
= :
10.LOG[ (373 + 75) / (290 + 75) ]  = 0.89dB

Which ought to be measurable with modern wide bandwidth SDRs.  It wasn't reliable using a diode on the output of an SSB Rx in 1988 !

Andy  G4JNT

On 19 July 2016 at 11:54, Richard Baker perwick@... [ukmicrowaves] <ukmicrowaves@...> wrote:

Any noise measurement you make with an open transmission line is going to be dependant on its length.

Rather than me try to explain the theory I suggest you try the following experiment.

Connect a good 50 ohm dummy load to your 432MHz or above receiver under test and note the noise level (reading 1). Now disconnect the dummy load and take a series of readings (readings 2) with different lengths of open transmission line attached. Easily done by adding adaptors or short lengths of coax. You should find reading 2 is dependant on the length of the open transmission line. Finally add a dummy load to the end of your adjustable length transmission line to confirm you get the same value as reading 1.
If you are looking for a minimum equipment noise estimate I suggest you put your dummy load in the deep freeze for a few hours and while still very cold connect it to your receiver and note the noise level. Now let it warm to room temperature and note the reading again. You are looking for a very small change in noise level and indeed you might need to heat your dummy load to about 100C to see an easy to read difference. This is the hot and cold dummy load method and it should be reasonably accurate as long as the impedance of your dummy load does not change much with temperature.
I would be interested in what readings people get.

On 18/07/2016, 18:19, "richard@... [ukmicrowaves]" <ukmicrowaves@...> wrote:


"Thoroughly disagree"  Thanks for your reply Chris. I am not clear with which approach you thoroughly disagree with however.

a) the gain method of measuring noise figure
b) measuring without an input load
c) measuring with an input load; or possibly
d) Derek's approach.

73 Richard G3CWI

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