How do I calculate the power and temperature of this microwave signal acquired by the radiometer I built?

[hiphip]

Hi all!

I'm working on building a radiometer. The ultimate goal is to measure the temperature of the desired object. To do that I have to first know the power radiated by that object.

My solution is to build a radiomter. The structure of the radiometer is shown in this graph:

So I have a scalar feed horn receives the microwave signal radiated by the object, the frequency of the signal is converted from 12.45GHz to 1.2GHz by the LNB. The signal is feed to a spectrum analyzer. The 10MHz output of the spectrum analyzer is feed to a RF Millivoltmeter. The analog output is feed to NI USB-6002. Finally the signal goes into my computer.

I have two questions:

1. How do I know the power of the signal that goes into my computer. I can find the voltage of the signal by NI DAQmx. Then I think I have to know a resistance since P=(V^2)/R? The only resistance I can find in the system is the load resistance of 50 ohm from the probe of RF Millivoltmeter. So is the power of this 12.45GHz I acquire = V(value from NI DAQmx)squared/50?

2. I found in several papers that Power = k(boltzmann's constant) * T(temperature in K) * B(bandwidth). Therefore I also have to know the bandwidth of the signal to calcualte the temperature of the object. Which bandwidth am I supposed to look at in my case? Because the original signal is processed through several steps. I'm confused about which bandwidth I should be looking at.

Thank you for your patience！ Any help is greatly appreciated!

[BingbingWang · Accepted solution]

Hi，if you want to measure the power that goes into your computer through USB-6002, I do not recommend you use V^2/R because this formula is used for pure resistance system where all power is transformed to heat. You may also have to measure the input current.

As for P=kTB, I think it is usually used in calculating the power of thermal noise.