Need help for RF DC co-transmit network design

ggyy0715 said:

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:

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jiripolivka said:

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.

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ggyy0715 said:

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?

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jiripolivka said:

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.

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