I am trying to instrument 100BASE-T Ethernet. Sounds like it's not an RF question but I think it is.
I want to tap into an ethernet pair without loading it any more than necessary for purposes of testing the signal. I don't want to demodulate it. I just want to do a basic envelope detection on it - an AM radio. It's a differential signal, so I need to turn it into a single-ended signal. I need a BALUN.
My first thought was to use a normal Ethernet isolation transformer without any termination resistors, but there's still the magnetizing inductance which is fairly high at 350uH. So then I thought about putting resistors between the transformer inputs and the network pair. Solves the loading problem and I can deal with the attenuation but results in a lot of distortion. So I feel like I need to shunt the resistors with small capacitors, like an oscilloscope probe. Then, I thought, well maybe I don't even really need the resistors.
At this point I'm in over my head mathematically and have no idea how to figure it out. I think I can safely assume I'm dealing with a 31.25 MHz signal since that's the whole point of the MLT-3 modulation scheme. So somehow I want to resonate out the transformer inductance and make it look resistive at 31.25 MHz but at the same time minimize the loading.
Maybe what I really need is to start from first principals and design a balun instead of trying to shoehorn in an Ethernet transformer.
Now you see why I think this is an RF problem.
I want to tap into an ethernet pair without loading it any more than necessary for purposes of testing the signal. I don't want to demodulate it. I just want to do a basic envelope detection on it - an AM radio. It's a differential signal, so I need to turn it into a single-ended signal. I need a BALUN.
My first thought was to use a normal Ethernet isolation transformer without any termination resistors, but there's still the magnetizing inductance which is fairly high at 350uH. So then I thought about putting resistors between the transformer inputs and the network pair. Solves the loading problem and I can deal with the attenuation but results in a lot of distortion. So I feel like I need to shunt the resistors with small capacitors, like an oscilloscope probe. Then, I thought, well maybe I don't even really need the resistors.
At this point I'm in over my head mathematically and have no idea how to figure it out. I think I can safely assume I'm dealing with a 31.25 MHz signal since that's the whole point of the MLT-3 modulation scheme. So somehow I want to resonate out the transformer inductance and make it look resistive at 31.25 MHz but at the same time minimize the loading.
Maybe what I really need is to start from first principals and design a balun instead of trying to shoehorn in an Ethernet transformer.
Now you see why I think this is an RF problem.