RL high pass calculation

Calculator and formulas for calculating the parameters of an RL high pass

RL high pass online calculator

This function calculates the properties of a high-pass filter consisting of a resistor and a coil. The output voltage, attenuation and phase shift are calculated for the given frequency.

Calculate RL high pass

Input voltage
Decimal places
Reactance XL
Ouput voltage U2
Attenuation dB
Phase shift φ

\(\displaystyle L\) = Inductance [H]

\(\displaystyle R\) = Resistance [Ω]

\(\displaystyle U_1\) = Input voltage [V]

\(\displaystyle U_2\) = Output voltage [V]

\(\displaystyle X_L\) = Reactance [Ω]

\(\displaystyle f_g\) = Cutoff frequency [Hz]

\(\displaystyle φ\) = Phase angle [°]

Formulas for the RL high pass

Calculate the voltage ratio

The output voltage U2 of an RL high pass is calculated using the following formula.

\(\displaystyle U_2=U_1 ·\frac{2 · π · f ·L} {\sqrt{R^2 + (2 · π · f · L)^2}}\)

or simply if XL is known

\(\displaystyle U_2=U_1 ·\frac{X_L}{\sqrt{R^2 + X_L^2}}\)
\(\displaystyle X_L=2 π · f ·L\)

Attenuation in decibels

At the resonance frequency, the damping is 3 dB. It can be calculated for the various frequencies using the formulas below. If the input and output voltage are known, the attenuation can easily be calculated using the following formula.

\(\displaystyle V_u=20 · lg \left(\frac{U_2}{U_1} \right) \)

If the voltages are not known, the following formula is used.

\(\displaystyle V_u=20·lg\left(\frac{2 · π · f ·L} {\sqrt{R^2 + (2 · π · f · L)^2}}\right)\)

or simply shown

\(\displaystyle V_u=20·lg\left(\frac{ω · L} {\sqrt{R^2 + (ω · L)^2}}\right)\)

Phase shift

In an RL high pass, the output voltage leads the input voltage by 0 ° - 90 °, depending on the frequency. At the resonance frequency, the phase shift is 45 °. At frequencies that are higher than the cut-off frequency, it tends to 0. At lower frequencies, the phase shifts in the direction of 90 °. The phase shift can be calculated using the following formula.

\(\displaystyle φ=acos \left(\frac{U_2}{U_1} \right)\)
\(\displaystyle φ= arctan \left(\frac{R}{ω ·L}\right)\)

Cutoff frequency

At the cutoff frequency fg or ωg the value of the amplitude-frequency response (ie the magnitude of the transfer function) is 0.707, which corresponds to -3dB.

\(\displaystyle 0.707= \frac{1}{\sqrt{2}}\)

Cutoff frequency formulas

\(\displaystyle ω_g= \frac{R}{L} ⇒ f_g=\frac{R}{2·π·L}\)
\(\displaystyle R=2·π·f_g·L\)
\(\displaystyle L=\frac{R}{2·π·f_g}\)

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