I understand that db is based on a ratio of voltages, but in order for the signal to have a certain voltage at a certain frequency, the measuring device must be admitting signal in a certain bandwidth around that frequency. For example, even a very strong signal if measured in a bandwith of 1/1000 of a Hz (not really practical) would have a virtually nil voltage, because the signal power within that bandwidth would be nil. Analog spectrum analyzers have selectable bandwidths, so you know what you are dealing with. I am just trying to figure out what the bandwidth is for Picoscope. My guess is that it is the total sweep bandwidth divided by the number of FFT samples.