MICRONIX

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.

グラフ

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.

Oscilloscope Spectrum analyzer
MAX Peak-to-peak Voltage Power (impedance 50Ω)
1mV 2mV 0.707mV -3dBmV -63dBV 57dBμV 1×10-5 mW -50dBm

[System configuration]

Handheld signal analyzer MSA500 series
Handheld spectrum analyzer MSA400 series

*MICRONIX Corporation reserves the right to make changes in design, specification and other information without prior notice.

[Products introduction]

2020/04

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