Thermal noise

What will be the thermal noise floor if we connect these two components HMC752 and ADF5904?

 If we have a setup as: 

Noise Source (LO + Noise) ->  HMC752 -->ADF5904 -> Spectrum Analyser

As per my understanding: Thermal noise floor = kTBF (in dBm)

kT = -174 dBm/Hz

B = 1 MHz = 60 dBHz

F = ? dB

What floor I will get on Spectrum Analyser?

What shall be the SA settings?

  • +1
    •  Analog Employees 
    on Mar 27, 2019 11:51 AM over 2 years ago


    - Thermal Noise =kTB=-174dBm/1Hz therefore if you want to calculate for different bandwidth for example 1kHZ

    Thermal Noise = -174dBm + 10*LOG1kHz= -144dBm/ikHz

    - Noise Floor in SA is combination Thermal Noise and Noise Figure of SA( Note:usually there is additional 1.5dB or 2dB as part of SA but you can neglect it for rough estimation. 

    Noise Floor can be adjusted by changing Resolution Bandwidth.

    Example: NF of SA=20dB; Noise Floor=-144dBm/1kHz

    Noise Floor = -144dBm+20dB=-144dBm/1kHz.

    I am sendng you plot below so you can see how by changing RBW, Nosie Floor is changed.

    - If you measure Low Levels, you need lower Noise Floor, otherwise you should not worry about that.

    Settings always depends on what you are trying to measure.


    Ivan S

  • Our SA noise floor is -117dBm/10kHz.

    In the testup HMC752 input is terminated. Its output is connnected to ADF5904. We are interested to measure noise floor of the HMC752+ADF5904 chain at IF=500kHz.

    For converting the differential output of ADF5904 to a single ended (to SA), a AD8130 with 1dB voltage gain is used.

    What is expected noise floor of HMC752+ADF5904 combination ?

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