You may not know this, but using a spectrum analyzer to directly measure that 50 ohms thermal noise isn’t really doable as even top of the line Rohde & Schwarz FSWs state a Displayed Averaged Noise Level (in short, an instrument’s noise floor) between 71nV and 224nV RMS (-130dBm & -120dBm) at a 1Hz resolution bandwidth around kHz frequencies.
Moreover, every spectrum analyzer out there that is rated from a few Hz up also comes with a no DC input requirement! While the easiest solution to get around this issue usually is to use inline DC-blocking pass-through adapters, they however typically come with a low-pass filter corner frequency of a few hundred Hz… so what’s the solution?
A simple amplification circuit based on a low-input noise operational amplifier, with a clipper circuit at its output.
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