Need help for RF DC co-transmit network design

[ggyy0715]

Hello folks,

How can I prevent DC(1nA) and RF(2 GHz~4 GHz) from interfering each other when they're sequentially going through a joint T-shape SMA adapter? My intuitive design is to add a DC block on RF side, and an isolator on DC side. Is this OK?


I'm a totally outsider of this field, it'd be more than appreciated if you could provide any advice or better design.
Thank you! :grin:

 

[jiripolivka]

Hello folks,

How can I prevent DC(1nA) and RF(2 GHz~4 GHz) from interfering each other when they're sequentially going through a joint T-shape SMA adapter? My intuitive design is to add a DC block on RF side, and an isolator on DC side. Is this OK?


I'm a totally outsider of this field, it'd be more than appreciated if you could provide any advice or better design.
Thank you! :grin:

The device you need is called "DC BIAS Tee". The RF is coupled via a capacitor (500 pF for 2...4 GHz), and DC current passes via a choke coil. The choke is made so it presents a high RF impedance to the RF line. For your RF of 2... 4 GHz, I usually wound a 10-turn coil with 0.2 m dia. wire (0.5 A max DC current) on ~3 mm diameter, the pull the turns so between them are ~0.2 mm gaps.
Such simple RF/DC separator works fine in 50 or 75 Ohm coaxial lines.
You can buy commercial models, e.g. at "www.minicircuits.com" for a reasonable cost.

 

[ggyy0715]

Thank you very much!! It helps a lot!
One more stupid question, are signals in "Bias Tee" allowed to transmit reversely? Which means, sequentially input RF and DC into DC+RF port and they will go out respectively from their own ports?

The device you need is called "DC BIAS Tee". The RF is coupled via a capacitor (500 pF for 2...4 GHz), and DC current passes via a choke coil. The choke is made so it presents a high RF impedance to the RF line. For your RF of 2... 4 GHz, I usually wound a 10-turn coil with 0.2 m dia. wire (0.5 A max DC current) on ~3 mm diameter, the pull the turns so between them are ~0.2 mm gaps.
Such simple RF/DC separator works fine in 50 or 75 Ohm coaxial lines.
You can buy commercial models, e.g. at "www.minicircuits.com" for a reasonable cost.

 

[jiripolivka]

Thank you very much!! It helps a lot!
One more stupid question, are signals in "Bias Tee" allowed to transmit reversely? Which means, sequentially input RF and DC into DC+RF port and they will go out respectively from their own ports?

Sure the DC can go in reverse direction as well as RF. There is no directional isolation.
The values of C , 500 pF and the chokeke I estimated for your RF range of 2 to 4 GHz, and line impedance 50 to 75 Ohms. Wire diameter limits the maximum DC current to 0.5A approx.

 

[ggyy0715]

Which level of isolation do you suggest? Is 40 db OK?
Thank you!

Sure the DC can go in reverse direction as well as RF. There is no directional isolation.
The values of C , 500 pF and the chokeke I estimated for your RF range of 2 to 4 GHz, and line impedance 50 to 75 Ohms. Wire diameter limits the maximum DC current to 0.5A approx.