Relationship between oscilloscope and spectrum analyzer amplitude

This note explains the relationship between oscilloscope and spectrum analyzer amplitude.

Application

Both oscilloscopes and spectrum analyzers measure electrical signals.
The oscilloscope measurement results are displayed on the screen as horizontal axis: time, vertical axis: amplitude.
On the other hand, the measurement result of the spectrum analyzer is displayed on the screen as horizontal axis: frequency, vertical axis: amplitude.
This note describes the relationship between the amplitude of a sine wave signal measured with an oscilloscope and a spectrum analyzer.

The graph above shows the waveform (blue line) when a sine wave signal with a maximum value of 1 mV (Peak-to-peak = 2 mV) is measured with an oscilloscope.
When this signal is measured with a spectrum analyzer, the amplitude is displayed as an rms value and the sine wave (rms value) = (maximum value) ÷ √ 2, so it is about 0.707mV.
The notation in logarithmic units, decibels (dB), is often used in spectrum analyzers, so in that case, 0.707mV becomes 20 * log (0.707mV) = about -3dBmV = about -63dBV = about 57dBμV .
(Conversion from dBmV to dBV is minus 60 dB, and conversion from dBmV to dBμV is plus 60 dB.)
Also, considering the impedance at 50Ω, 0.707mV is about 1 × 10-5 mW, and 10 * log (1 × 10-5 mW) = -50dBm.
These relationships are summarized below.