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    dbm/hz

    Let me make a simple question.

    Is it possible to have output noise power of a system at room temperature bellow kT? It can be stated that filter with just noise at input can attenuate it in stop band bellow the level of thermal noise. But, in practice, the output impedance of the filter posses also some resistive component that makes output noise power density again equal kT. Puzzling, ain't it?

    What do you think?

    Alternative question can be: Is it possiblle to build a transmitter that radiates less than -174 dBm/Hz at some particular frequency (with 0dBi antenna)?

    Thank you very much
    D.

    •   Alt29th November 2004, 11:47

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    174 dbm hz

    Absolutely not possible.



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    174dbm

    It's impossible observe noise less than KTBp
    p:= Planck correction. p≈1 for F<50 GHz
    T:= the observed physical temperature

    If your antenna look at a body placed to a phisycal temperature T the output available power is KTB.

    Of course, if, in your laboratory, you'll point the horn to the sky, at the zenith, you'll get a noise below -174dBm/Hz, because the sky is mutch colder than the room temperature.


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    •   Alt29th November 2004, 18:12

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    174 dbm

    Thank you both for your answers. Since I am still not sure that I got all right I will reformulate:
    Supose I have high Q filter of bandwidth Bf which one port is terminated with 50Ω and second port is connected to measurement equipment with bandwidth Bs, so Bs>Bf. Will observed noise power be equal kTBf or kTBs?

    Regards,
    D.



    •   Alt29th November 2004, 19:00

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    ktbf noise

    Quote Originally Posted by Debeli
    Thank you both for your answers. Since I am still not sure that I got all right I will reformulate:
    Supose I have high Q filter of bandwidth Bf which one port is terminated with 50Ω and second port is connected to measurement equipment with bandwidth Bs, so Bs>Bf. Will observed noise power be equal kTBf or kTBs?

    Regards,
    D.
    this is a different question. :)

    you shoul use a bandwidth of the meas setup lower than Bf. Please Read Application note agilent 57-3 : you'll find the answer to this topic.



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    noise dbm/hz

    Thank you again, but once more I am not sure if you got actualy my question.
    I do not want to measure exact noise in a system, I was just wandering if a filter can suppress the out-of-band noise bellow kT. Therefore I choose hypotetical Bs>Bf for the experiment. If the noise can not be suppressed bellow kT than result is kTBs, otherwise it should be kTBf.

    Regards
    D.



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    antenna noise power temperature conversion dbm/hz

    Quote Originally Posted by Debeli
    Thank you again, but once more I am not sure if you got actualy my question.
    I do not want to measure exact noise in a system, I was just wandering if a filter can suppress the out-of-band noise bellow kT. Therefore I choose hypotetical Bs>Bf for the experiment. If the noise can not be suppressed bellow kT than result is kTBs, otherwise it should be kTBf.

    Regards
    D.
    Take a passive 2 port network. Close the input port with a termination at T0 temperature. The noise power density from the termination is kT0.

    The noise power density at the output port of the 2 port is kT0.

    Thje noise power is kT0B where B is the observing band.

    In other words: the noise from the input termination is reduced by the attenuation of the 2 port, BUT at the same time the 2 port has (inside) noise sources that generate noise so that, at thermal equilibrium, you'll have again kT0 at the output port.


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    •   Alt30th November 2004, 10:58

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    -174 noise power

    Yes, it was my assumption in my 1st post, and this consideration about thermal equilibrium can be found also in Pozar book. I wanted to get some more oppinions abut it, while there is missconception in telecommunications connected with minimum possible irradiated energy. Similar discussion has been conducted on RFGlobalNet several years ago in several forums. Interestly, there were more people disputing thermal equilibrium statement. You can find one discussion at following link address, if interested, there are also some more at other forums:

    http://rf.rfglobalnet.com/forums/act...sages/9464.asp

    I thank everyone that contributed to this topic since understanding noise is one of fundamental topics in communications and every discussion makes our knowledge deeper.

    Regards,
    D.



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    174 noise

    Quote Originally Posted by Debeli
    Thank you again, but once more I am not sure if you got actualy my question.
    I do not want to measure exact noise in a system, I was just wandering if a filter can suppress the out-of-band noise bellow kT. Therefore I choose hypotetical Bs>Bf for the experiment. If the noise can not be suppressed bellow kT than result is kTBs, otherwise it should be kTBf.

    Regards
    D.
    Of course everyone may have it's own (and different) opinion regarding your question.
    i may write my opinion, but tell me the possibility to write that is the usual, canonical and well accepted in my field of operation (radioastronomy).

    I'll call G the available gain. I'll call T the Operative Temperaure (Agilent AN57-1).
    Noise power = K ∫ T(f) G(f) df
    What about T(f)?
    1) inside the flat zone of Bf, supponing the reflection coefficient very close to 0, T(f) is supposed to be constant and is the sum of Noise Temperature of the receiver + The noise generated by the filter "Bf". The second is term is numerically same to the actual ambient temperature.
    2) Very far outside Bf but inside Bs, The noise power coming from the filter is very low (supponing |Γ|≈1- see note) but the noise temperature of the receiver change because Te is dependent on Γsource.
    3) on the the rising and falling zone close to the flat zone, the situation is more complicated.

    The result is the following:
    1) Noise power at the output of a network may change vs. T and Gain, it's impossible to discriminate the cause.

    2) In order to sicriminate the cause of the noise power, a 2 port network should be examinated.

    3) In any case the thermal equilibrium must exist. It is impossible speak or think that a receiver may see a target with a noise temperature less than it's phisycal temperature.

    Note: the available noise power still KTB, but if the |Γsouce| is close to 1, the power delivered to a matched load is very low.



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