Transmission Line

There are tons of things you can do with this. Just vary the resistors on either end to adjust the impedance matching (it’s a 50Ω line so a 50Ω resistor is the only perfect match) and play around the the voltage input and time scale and you can learn a lot about impedance lines and how signals move through them. So if you’ve ever wondered how a voltage gets from one end of a wire to the other and how the information of how much current to flow based on the resistors at either end travel across the wire, this is how. The switch on the left switches between automatic and manual mode. In automatic mode it just connects a 1MHz sine wave source to the impedance line, although I do recommend trying out other frequencies and voltage sources (square wave, triangle wave, etc.). In manual mode you can toggle the voltage yourself between 1V and 0V to see how the transmission line responds. You might notice that at certain frequencies the transmission line resonates depending on how well it’s impedance matched at either end with poor matching leading to better resonance at the resonant frequencies. You might also notice that at a specific frequency (around 434kHz or so) and it’s odd multiples it resonates when the right resistor is too large and on the even multiples of the frequency it encounters an anti-resonance where it resonates so poorly that there’s actually less voltage and current than when it isn’t resonating. If the resistor is made to be too small instead of two large then the resonances become anti-resonances and vice versa so 434kHz and it’s odd multiples become anti-resonances and it’s even multiples become resonances. In order to find the resonant frequency I highly recommend using the bode plot as just multiplying 434kHz by integer values won’t work since this is only an approximation of an impedance line (a real one is continuous and can be modeled as an infinite number of infinitely small inductors and capacitors, this approximation here just uses a finite number of finite inductors and capacitors). Also in order to resonate and build large voltages the left resistor will need to be very small and the right resistor very far from 50Ω. If the left one is at 50Ω then it’ll still resonate as long as right resistor isn’t at 50Ω but it won’t be able to gain any large voltage or current. If the left resistor is larger than 50Ω then the signal will simply be too attenuated by the resistor.

published 3 years ago