# Why the Receiver Sensitivity theoretical value doesn't match its experimental value when using USRP B200mini in GNU Radio?

I'm using a USRP B200mini SDR card for my GNU Radio energy detector project. I wanted to measure its Receiver Sensitivity ($$RS$$) experimentally and compare the experimental value to the theoretical value. Assume the minimum needed Signal to Noise Ratio ($$SNR$$) threshold for detection is $$0\ dB$$ and assume the bandwidth ($$BW$$) to monitor is $$20\ MHz$$, then using the $$RS$$ equation: $$RS=N_{Floor}+N_{Figure}+10\times \log(BW)+SNR$$,

where, $$N_{Floor}$$ is the noise floor, and $$N_{Figure}$$ is the noise figure.

We have: $$N_{Floor}=10\times\log(kTB)=10\times\log(1.38×10^{−23}×290×1)=−204\ dB=−174\ dBm$$,

where, $$k$$ is Boltzmann's constant, $$T$$ is room absolute temp in kelvin, and $$B$$ is $$1Hz$$ bandwidth.

$$N_{Figure}<=8\ dB$$ according to the Ettus website.

Hence, $$RS=−174+8+10\times\log(20,000,000)+0=−93\ dBm$$.

The problem now is that I don't see a match between the theoretical value which is $$−93\ dBm$$ and the experimental value which is $$−63\ dBm$$. Note that I found the experimental value based on the definition of receiver sensitivity, which is a measure of the minimum signal strength that a receiver can detect. Based on this definition, my energy detector can start detecting at a signal power around noise power level which is around $$−63\ dBm$$ (see the attached flow graph). There is around a $$30\ dB$$ difference between the theoretical value and the experimental one! Note that I didn't connect any antenna to the SDR card and kept the SDR gain at $$12\ dB$$.

Notes:

I put the SDR card gain at $$12\ dB$$ in order to calibrate the average power calculations from the flow chart to match the average power value provided by a Keysight power sensor U2000A. I used a sine wave source to do the calibration.

Note that the Keysight power sensor U2000A also gives a noise power level of around $$-63\ dBm$$ without an antenna connected.

The "Receiver Gain (dB)" label is given by GNU Radio itself and I don't know how to change it, but it should be "Power Spectral Density (dBm)" for this flow chart. But anyway, the problem remains the same, there is a big gap between the calculated $$RS$$ and the measured one.

What am I missing here? I'm not sure how to explain this difference, any help is appreciated. Thank you very much.

• Hello and welcome to ham.stackexchange.com! It's nice to see someone using MathJax right from the start ;) Feb 1, 2022 at 14:50
• Thank you for having me, yes it's a nice tool.