2/13/2024 0 Comments Ltspice pi filter design![]() ![]() Let’s say we would like to build a simple high pass filter. In principle, you could build a simple filter using nothing more than a resistor and a capacitor and, as you might have guessed, LTspice once again comes at rescuing us from our wandering around. But the crucial question from most of us was the following: how would you build such filters, and perhaps more importantly, how would you tune them? asc file and play with it to get deeper understanding of all the parameters involved.During my last engineering class I learnt about the frequency response of a system and how this thing can be applied to solve simple problems. Note, Zo is the same in both cases and how the Q gives the indication of “peaking” in impedance. This can clearly be seen in the following plot. The value of characteristic impedance directly determine the quality factor, relative sharpness of the peak in impedance profile.įor parallel circuit, only thing change is the definition of Q = R / Zo. ![]() meas statements in figure 4.Ĭharacteristic impedance is a crossover point where capacitor’s impedance is equal to inductor’s impedance. Using this value to find resonant value on frequency axis (see the. First I calculated the min impedance value. meas statements to calculate resonant frequency. It’s easy to measure resonant frequency in LTspice using cursor or. Resonant Frequency: Fres = 1 / (2*pi*sqrt(L*C).There are four main parameters of a RLC circuit: Since R is constant and will remain, so this will determine the lowest point of impedance. At some point in frequency sweep, the complex impedance of inductor will cancel with the negative complex impedance of capacitor. At low and high frequency capacitor and inductor has high impedances, so total impedance should be high at low and high frequencies respectivily. In the next two examples, let’s take a look at the series RLC and the parallel RLC circuits.įigure 4: Simulation Circuits for Series and parallel RLCįor series RLC circuit, let’s intuitively guess the overall impedance. If impedance decrease (-20dB/decade), phase is close to -90˚. ![]() If impedance increases (20dB/decade) phase is close to 90˚. Typically impedance profile do not show the phase because by knowing the amplitude behavior we know how the phase behaves. Now what about the phase? Glad you asked as it can also be easily plotted-launching the Right Vertical Axis dialog box by clicking on the right axis.įigure 3: Phase is shown in dashed lines for 100mΩ resistor, 1♟ capacitor, 1µH inductor V(Imp_X) where X=R or C or R either a voltage in V or impedance in Ω.If there is no DC path, simulation will fail. R2, R5 and R6 are optional, it some version of SPICE, a DC point need to be calculated first.ac command is used to plot 10kHz to 10MHz with 100 point/decade Note complex number j encodes phase information.įigure 2: Impedance response of 100mΩ resistor, 1♟ capacitor, 1µH inductor For capacitor and inductor we expect impedance to linearly decrease or increase respectively with frequency in log-log plot per following equations: ![]() We expect resistor impedance, normally called resistance, to constant across frequency. Let’s illustrate how this work in LTspice using basic passive components.įigure 1: Simulation circuit for a resistor, a capacitor and an inductor We can measure voltage easily in LTspice that essentially equal to impedance. This is really the basis to do simulation in LTspice.Ĭhoose I=1A constant current source, then V = Z. The math involve complex algebra, but doable for simple circuits. In the frequency domain it’s really the magnitude and phase of sine wave that are used for voltage and current. This definition of impedance applies to both in the time domain or the frequency domain. What is impedance? Impedance is simply defined as voltage divided by current (Z = V/I). ![]()
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