# Source (inner) resistance

1. Oct 6, 2005

### temujin

Dear Group,

I need to simulate a HF circuit (matched to 50 ohm ) connected (via a 50 ohm coax cable) to a signal source with 50 ohm output resistance.
Simulating this with e.g. Spice, should I use a 50 ohm resistor in series with the source?

Does the fact that the source is matched to 50 ohm output resistance have anything to do with any equivalent resistance connected in series with the source?

hope I managed to express myself properly here

regards
doneirik

2. Oct 6, 2005

### Ouabache

I am surprised you do not include an antenna at the end of your 50 ohm coax, otherwise you really don't have a load attached to your HF circuit. Yes you can use a 50 ohm resistor to model the 50 ohm coax and matched antenna. In practise that is called a "dummy load".

Last edited: Oct 6, 2005
3. Oct 7, 2005

### temujin

Actually I do include the antenna...(I am afraid to write antenna because I´m working on magnetically coupled RFID, not really antennas...)...I was actually wondering about how to depict the source...

So if I use a 50 ohm resistor to model the load and the coax cable, is this resistor also modeling the 50ohm output impedance of the source? I think this 50ohm output impedance has nothing to do with the source´inner resistance, but I´m not sure...

regards
doneirik

4. Oct 7, 2005

### Staff: Mentor

The simple SPICE model would have an ideal RF voltage source with a series 50 Ohm resistor to model the RF signal source, and that drives a 50 Ohm resistor to ground to model the transmission line and its termination (the antenna if it is well matched to the 50 Ohm coax).

The next level of modelling would have the same RF source (ideal AC voltage source and 50 Ohm series output impedance), and you would use a transmission line model in SPICE to model the coax, and you would use a complex impedance at the end to model your antenna. That way you would get a more accurate view of the SWR and power radiated.

You can also add more detail to your SPICE model by doing a more accurate representation of the transmitting amplifier as well, instead of just the ideal RF voltage source and 50 Ohm output impedance.