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Noise figure Temperature (Read 18541 times)
aaron_do
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Noise figure Temperature
Oct 16th, 2007, 1:58am
 
Hi all,


by definition, NF must be quoted at 290K however, most receivers are designed to operate over a broad temperature range. So what is the convention for defining NF at different temperatures? I'm asking because i was writing a paper and one of the reviewers pointed out that it is incorrect to define NF away from 290K...he didn't offer any alternative however...


thanks,
Aaron
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Frank Wiedmann
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Re: Noise figure Temperature
Reply #1 - Oct 16th, 2007, 2:20am
 
You might be able to use the difference between effective noise temperature and ambient temperature (see http://en.wikipedia.org/wiki/Noise_figure).
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aaron_do
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Re: Noise figure Temperature
Reply #2 - Oct 16th, 2007, 8:09am
 
Thanks for the link

that makes me question though, in spectre, if we simulate NF versus temperature, is the source temperature taken as 290K or the operating temperature when calculating NF?

thanks,
Aaron
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Frank Wiedmann
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Re: Noise figure Temperature
Reply #3 - Oct 16th, 2007, 8:27am
 
If you read the definition in the standard (mentioned in the link), you will find that the temperature of the input termination is always assumed to be standard noise temperature (usually 290 K) when noise figure is calculated.
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pancho_hideboo
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Re: Noise figure Temperature
Reply #4 - Oct 16th, 2007, 8:37am
 
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aaron_do
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Re: Noise figure Temperature
Reply #5 - Oct 16th, 2007, 5:50pm
 
Ok thanks,

does that seem like the wrong way to measure NF though? From my understanding the choice of 290K is arbitrary and is only used so people have a point of reference. i.e. the actual source noise depends on temperature.

Aaron
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pancho_hideboo
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Re: Noise figure Temperature
Reply #6 - Oct 16th, 2007, 10:57pm
 
Hi.

ENR table is defined as noise ratio of Tcold(290K) and Thot.
But actual temeperature(Tambient) is not 290[K].
So user have to compensate of temperature difference from 290[K].
Default value of Tambient is 296.5[K] not 290[K] in actual instruments such as
Agilent N8974A or N8975A Noise Figure Analyzer.
These NF analyzer have a temperature sensor built in a noise source, so Tcold can be auto corrected.

In legacy NF meter such as Agilent 8970B, there is no auto correction for Tcold.
Unless user enter temperature of Tambient, NF meter assume Tambient is 290[K].

Anyway actual NF meter with correct setting calculates NF as T0=290K regardless of Tambient.
If this is corresponded to simulation(e.g. Cadence Spectre), port temperature is 290[K],
analysis temperature is Tambient.

http://cp.literature.agilent.com/litweb/pdf/5952-3706E.pdf
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« Last Edit: Oct 17th, 2007, 8:06am by pancho_hideboo »  
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Frank Wiedmann
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Re: Noise figure Temperature
Reply #7 - Oct 16th, 2007, 11:42pm
 
aaron_do wrote on Oct 16th, 2007, 5:50pm:
Ok thanks,

does that seem like the wrong way to measure NF though? From my understanding the choice of 290K is arbitrary and is only used so people have a point of reference. i.e. the actual source noise depends on temperature.


Yes, of course the actual source noise depends on the temperature. However, the official definition of noise figure is just what it is and it's not very useful to argue about it.

This is why I suggested that you use the effective noise temperature together with the ambient temperature to characterize your results. For example, you could define a quantity 1+Te/Tphys (just don't call it noise figure, see http://en.wikipedia.org/wiki/Noise_figure for the definition of the quantities in the formula), which for an attentuator with attenuation L is always equal to L, independent of the ambient temperature Tphys.
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Ken Kundert
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Re: Noise figure Temperature
Reply #8 - Oct 18th, 2007, 9:48am
 
It is also important to realize that the temperature that is needed for noise figure is the temperature of the source not the temperature of the circuit. The temperature of the source is the temperature of the channel, and 290 is probably a reasonable approximation to that.

-Ken
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Frank Wiedmann
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Re: Noise figure Temperature
Reply #9 - Oct 19th, 2007, 1:10am
 
Thanks for emphasizing this, Ken. I had (perhaps incorrectly) assumed that the noise of the source is equal to kTphysB.
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pancho_hideboo
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Re: Noise figure Temperature
Reply #10 - Oct 19th, 2007, 6:43am
 
Hi.

"1+Te/Tphys, the ambient temperature Tphys" is wrong.

Again see my append. Sad
The followings are summary.

From NF definition, NF is defined as NF=1+Te/T0,
where T0 is reference temperature(=port temperature)  
and Te is equivalent noise temperature of DUT
(Te is dependent on analysis temperature(=Tambient)).  
Standard T0 is 290K.

In sp analysis of ADS, ADS treats port temperature as 290[K] even if you set any value as port temperature.
While in Spectre, user can set any tempereture value in port but it's default(blank) value is 290K.
So in Spectre, port temperature should be 290K or blank.
In both ADS and Spectre, user can set any value as analysis temperature(Tambient).
http://www.designers-guide.org/Forum/YaBB.pl?num=1190112138

Actual NF meter with correct setting also calculates NF as T0=290K regardless of Tambient.
http://cp.literature.agilent.com/litweb/pdf/5952-3706E.pdf

[Conclusion]
The following three are all matched, that is, NF=1+Te/T0, T0=290[K] regardless of analysis temperature(Tambient).
(1) Actual NF meter with correct setting
(2) Agilent ADS NF
(3) Cadence Spectre NF with 290K or blank as port temperature

In simulation, analysis temperature means temperature of DUT.
In actual measurement, analysis temperature means ambient temperature which is almost same as temperature of DUT.


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aaron_do
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Re: Noise figure Temperature
Reply #11 - Oct 20th, 2007, 7:52am
 
Thanks for all the replies.


I read that 290K is used for satellites which are aimed directly at the earth such that the beams width is completely covered by the earth. When the beam covers space, then space temeperature is used which is as low as 3K. So it seems that if your channel temperature is not 290K, then the NF is not exactly the best way to measure noise performance...


cheers,
Aaron
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Re: Noise figure Temperature
Reply #12 - Oct 20th, 2007, 8:18am
 
Hi,
I think that all noise due to the channel it's just putted in the antenna temperature, To=290 it's an standard used since Friis paper I think, it's like talking about why a meter is a meter. NF is a convention so all the designs can be compared in equality.
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aaron_do
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Re: Noise figure Temperature
Reply #13 - Oct 20th, 2007, 9:15am
 
Hi,

I understand the logic of having a basis for comparison, but from a designer's point of view we still need to know what are the design requirements in terms of thermal noise...


Aaron
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manresa,spain
Re: Noise figure Temperature
Reply #14 - Oct 20th, 2007, 10:54am
 
Hi,
The system requirements are dictated by the link budget given a modulation,necessary Eb/No for a given BER is obtained,the you can calculate the SNR at the input of the demodulator, from here you can obtain NF of the receiver. You must fullfill the requirement over the expected range of temperature, as pancho said:
Quote:
From NF definition, NF is defined as NF=1+Te/T0,
where T0 is reference temperature(=port temperature)  
and Te is equivalent noise temperature of DUT
(Te is dependent on analysis temperature(=Tambient)).  
,the DUT noise obviously will vary with temperature,but To it's standarized to 290.
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