Low noise amplifier to measure resistor noise

Noise is all around us, and while acoustic noise is easy to identify with our ears, electronic noise is difficult to identify even with the right tools. The spectrum analyzer is the most convenient tool for noise measurements, but it also adds its own noise to whatever signal you’re looking at. [Limpkin] He has been working on measuring very small noise signals with a spectrum analyzer, and he shared his results in Comprehensive blog post.

His self-set goal was to measure the noise generated by a 50 ohm impedance, the most common impedance at the input and output of RF systems. Formula The power of Johnson Nyquist noise It tells us that the expected noise voltage in the single Hz bandwidth is only 0.9 nanovolts — small by any standard, and an order of magnitude smaller than the floor noise of a typical spectrum analyzer. [Limpkin] So he designed an amplifier and signal buffer to boost the noise signal by a factor of 100, using very low noise amplifiers running from a pair of 9V batteries.

However, there was a problem with this circuit: any stray DC voltage present at its inputs would also be amplified to levels that could damage the analyzer’s sensitive input port. to prevent this, [Limpkin] He decided to add a scissor circuit to his amplifier. This consists of a pair of comparators that constantly monitor the amplifier’s output voltage and disconnect it through a silicon switch if it exceeds 200mV. [Limpkin] He packaged his circuit in a beautifully crafted case and ran various tests to ensure the clipper operated reliably even in the presence of fast transient inputs.

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With the scissors in place, it was safe to take the planned noise measurements. The final result? about 0.89 nV, just as predicted by theory. Measuring nanovolt level signals usually requires very precise equipment and a lot of noise reduction tricks. Sometimes, noise is just what you need to make a radio transmitter. thanks for the tip, [alfonso32]!

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