Noise is all around us, and while acoustic noise is easy to spot with our ears, electronic noise is hard to spot, even with the right tools. The spectrum analyzer is the most widely used tool for measuring noise, but it also adds noise to whatever signal you’re looking at. [Limpkin] He worked on measuring very small noise signals with a spectrum analyzer, and shared his results at Full blog post.
His goal was to measure the noise produced by a 50 ohm impedance, the most common impedance on the inputs and outputs of RF systems. Formula The power of Johnson Nyquist noise It tells us that the expected noise voltage at one hertz bandwidth is only 0.9 nanovolts — small by any standard, and an order of magnitude smaller than the floor noise of a typical analyzer spectrum. [Limpkin] So he designed an amplifier and signal buffer to increase the noise signal by a factor of 100, using a very low profile amplifier running on 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 avoid this, [Limpkin] She decided to add a scissor circuit to her amplifier. This is a pair of comparators that constantly monitor the amplifier’s output voltage and disconnect it via a silicon switch if it exceeds 200mV. [Limpkin] He packaged his circuit in a beautifully shaped case and ran various tests to ensure that the clipper operated reliably, even in the presence of fast transient inputs.
With the mower in place, he was sure to take the planned noise measurements. The final result ? about 0.89 nV, as predicted by theory. Measuring nanovolt level signals usually requires very precise equipment and a lot of noise reduction tricks. Sometimes, however, noise is just what you need to make a radio transmitter work. Thanks for the advice, [alfonso32]!
