I need to be able to simulate communications with a sensor device over a large length of wire (0-10km). This is for quite low-speed comms (10khz max, usually 1-2khz though). This would be FSK... but at some point I may have to handle a low baud RS232-like signal as well.
Mostly, I'm looking for voltage drop and signal distortions. Delay doesn't matter much.
How would you go about it?
I've been able to determine the cable is indeed a (fairly nonstandard) type of coax. I now know resistance and capacitance per unit length, cross section geometry, and that the insulation resistance is high enough not to matter. It wasn't initially clear if the return line was a separate run or not.
This would be a test setup for multiple target devices. Most are FSK of various frequency choices under 10khz, some are ASK (you could almost use a standard UART after bandpass/filtering). All are riding on a high DC offset (comms over power).
In the past, I've seen people build a simple rotary switch that swaps in resistors, capacitors, and maybe inductors to simulate a given line length. Could that be good enough?
I'm currently trying to build a few simulations in LTspice.
Okay, if I go with just adding resistors, caps, and inductors... what does the model look like? The RLGC network below is assuming the grounds are at the same potential I believe (a safe assumption on PCBs w/ground planes). The return in this case is through the outer shell, and it's resistance is probably 3 times higher than the inner conductor. Does that change things significantly? Do I just add another resistor on the bottom rail, and split the capacitance on both sides of it?