# 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?

Parents
• Hi,

- 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.

Regards

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 ?