This post features a series of calculators to find the Root-mean-square (RMS) voltage of different waveforms. VRMS can be used to compute the power in Watt or dBm.
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How to Calculate RMS Voltage
To use the calculators, use the drop down menu to specify one of the following:
- Peak Voltage – Vp
- Peak-to-peak Voltage – Vpp
- Average Voltage – Vavg
The units of VRMS are the same as those of the input voltage.
Time domain samples
Calculate the RMS value using any number of time domain samples. These samples can be used to specify an arbitrary waveform.
DC
Direct current is a signal level that has a fixed value and does not vary with time. The RMS voltage in this case is the same as the peak voltage. Since there is no variation, the peak to peak and average values are also the same.
Formula
VRMS = |Vp|
A DC/fixed value can also be entered into the time-domain sample RMS calculator to confirm this.
DC is shown in the picture above (red line) relative to other signal types.
Calculator
Sine Wave
This is one of the most common waveforms used in RF engineering labs. Also known as a continuous wave (CW) signal. The default output from an RF signal generator is a sine wave. It is used to test various RF components such as amplifiers, filters and splitters.
Formula
The time varying sinusoidal waveform is
y = Vp*sin(2πft)
The RMS voltage is
VRMS = Vp/(√2) = Vpp/(2√2) = π*Vavg/(2√2)
If there’s a DC offset VDC , the RMS voltage is
VRMS-DC = √(VDC2 + VRMS2)
Calculator
Modified Sine Wave
This is the sum of two square waves, one of which is delayed 0.25 of a period relative to the other.
Formula
The time varying signal is given by the following equations:
y = 0, frac(ft) < 0.25
y = Vp, 0.25 < frac(ft) < 0.50
y = 0, 0.50 < frac(ft) < 0.75
y = -Vp, 0.75 < frac(ft) < 1
The RMS voltage is
VRMS = Vp/(√2) = Vpp/(2√2) = π*Vavg/(2√2)
If there’s a DC offset VDC , the RMS voltage is
VRMS-DC = √(VDC2 + VRMS2)
Calculator
Half-wave rectified Sine Wave
Formula
RMS voltage is
VRMS = Vp/2 = Vpp/4 = π*Vavg/2
If there’s a DC offset VDC , the RMS voltage is
VRMS-DC = √(VDC2 + VRMS2)
Calculator
Full-wave rectified Sine Wave
Formula
The time varying waveform is represented as
y = |Vp*sin(2πft)|
RMS voltage is
VRMS = Vp/(√2) = Vpp/(2√2) = π*Vavg/(2√2)
If there’s a DC offset VDC , the RMS voltage is
VRMS-DC = √(VDC2 + VRMS2)
Calculator
Square Wave
Formula
The time varying signal is given by the following equations:
y = Vp, frac(ft) < 0.50
y = -Vp, frac(ft) > 0.50
RMS voltage is
VRMS = Vp = Vpp/2 = Vavg
If there’s a DC offset VDC , the RMS voltage is
VRMS-DC = √(VDC2 + VRMS2)
Calculator
Triangle Wave
Formula
y= |2*Vp*frac(ft) – Vp|
RMS voltage is
VRMS = Vp/(√3) = Vpp/(2√3) = π*Vavg/(2√3)
If there’s a DC offset VDC , the RMS voltage is
VRMS-DC = √(VDC2 + VRMS2)
Calculator
Sawtooth Wave
Formula
y= 2*Vp*frac(ft) – Vp
RMS voltage is
VRMS = Vp/(√3) = Vpp/(2√3) = π*Vavg/(2√3)
If there’s a DC offset VDC , the RMS voltage is
VRMS-DC = √(VDC2 + VRMS2)
Calculator
Pulse Wave
Formula
The time varying signal is given by the following equations:
y = Vp, frac(ft) < D
y = 0, frac(ft) > D
Where D is the duty cycle expressed as a percentage (%).
RMS voltage is
VRMS = √D*Vp = √D*Vpp/2
If there’s a DC offset VDC , the RMS voltage is
VRMS-DC = √(VDC2 + VRMS2)
Calculator
Phase-to-phase Voltage
Formula
y = Vp*sin(t) – Vp*sin(t-2π/3)
VRMS = Vp*√1.5 = (Vpp/2)*√(1.5)
Calculator
Frequently Asked Questions
Is the RMS voltage a positive or negative number?
The Root mean square value of voltage is always a positive number. Note the definition given by the formula:
VRMS = √(1/n)(V12 +V22 + … + Vn2)
where V1, V2, … Vn are the corresponding values of voltage. The square of each value is positive and therefore VRMS will be positive.
References
[1] Root-mean-square on Wikipedia
[2] Duty Cycle on Wikipedia
[3] Continuous Wave on Wikipedia
[4] Modified Sine Wave on Wikipedia