Impedance Matching – How to Match Impedance in Distributed Transmission Line Model

analogcadenceimpedance-matchingsimulation

I'm trying to do impedance matching for an inverter transmitting signal through a transmission line in Cadence. The transmission line is modelled as a number of RLC elements(\$R_{TL}=0.1\Omega\$,\$L_{TL}=250nH, C_{TL}=100pF, and N = 10\$) connected in series (as shown below).

enter image description here

The model approximately represents 1 meter of RG-58 coaxial cable (with resistance of \$ 50\Omega\$). I suppose the idea is to keep the output impedance, the impedance of the transmission line, and the impedance of the load equal. So I put a \$ 50\Omega\$ termination resistor at the end of the transmission line, and put a \$ 40\Omega\$ resistor between the inverter and the transmission line since the output impedance of the inverter is approximately \$ 10\Omega\$(measured by simulation).

However, the waveform at output node does not improve a lot after impedance matching. The top one is the Output before impedance matching, and the bottom one is after impedance matching. I'm wondering if the approach is correct and why the waveform still has glitches. I put part of the schematic at the end(there are 10 singleRLC modules in the middle), if that helps.
Output before impedance matching

Output after impedance matching

enter image description here

enter image description here

Best Answer

If I understand your screenshot, you're simulating a cable using discrete 10cm segments. For that model to be accurate, your rise time must be much longer (order of magnitude) than the time for your signal to propagate 10 cm, which is about 0.5 ns. How fast do your transistors switch?

My guess is a few nanoseconds at most, since for a real piece of RG-58 if you put a 50 ohm terminating resistor and measure the output of a square wave generator you'll get a relatively clean edge with minimal reflection.