2 way power divider with DC block

[kumar16]

schema dc block

I have to design a 2 way wilkinson 500-2500Mhz but one of the output ports must have a DC block and the other just a DC pass. I have the wilksion working perfectly to specs for the freq range. But when i add a DC cap to the DC blocked output port, it blocks DC at the DC pass port too. Any idea of why this is happening and what would be a good cap value to choose? I am using 2700pF right now, tired 0.1uF but that lets DC pass on both ports.

 

[karote]

Your question does not make any sense. Could you explain it more explicitly or draw a schema? DC blocking is not related to the value of the capacitor. Fully functional capacitor will always block DC current from flowing.

 

[kumar16]

Ok, i have a 2 way wilkinson working at a particular frequency range. One of the output ports needs the DC signals to be blocked and the other need the DC signals to be passed. The frequency of interest is 500-2400mhz and i dont care about the performace at DC (which it cant and was not designed to), but all i need at DC is that one of the ports pass DC signals and one blocks it. The way this is done from what i have seen is by placing a cap b/w the output of the wilkinson and the output connector. See this **broken link removed**
The problem i am seeing is when i hook the unit up to a network analyzer and see the port with the DC block (with the cap) there is very high VSWR and IL (infinite, like a open) which is ok since it is supposed to block the DC signals. But i am seeing the same thing in the output port without the DC blocking cap (the DC pass port). Why is this?

 

[Azulykit]

I am assuming that you have a microstrip of stripline output. I'll see if I can describe the suggestion in words............


Consider a long trace. Now, simply cut the line and cut a long notch in one side, centered and slightly wider than 1/3 of the line width. From the cut on the other side extend a finger into the notch, silghtly less than 1/3 width wide. Make the notch about λ/4 (midband) as a starting point. The trace should look like the cross section of a male/female connector pair where the contacts do not quite touch. A little time fooling with a simulator and you can find the perfect dimensions.

It is not clear why your cap does not work. Could you have a box of faulty caps?

 

[kumar16]

thanks for your reply but i am confused. I have this fabricated on rogers4003 microstrip board which is screwed into an enclosure with BNC connectors. I tried different caps but it dosent seem to be the problem. I will try to see if i can solder the board to a different enclosure, i have had better results if the board are soldered into and not screwed into the enclosures, especially with the IL parameter (its better when its soldered). Please feel free to shed more light into the matter.

 

[Azulykit]

I would be really suspicious of BNC connectors at the frequencies you cite. They would not be my first choice.

 

[kumar16]

i hate BNC's too at these frequencies, but it is a requirement. I am going to try building one up without any DC block just to see if the power divider makes the specs just by itself and then add the cap.

 

[yendori]

You commented that you see a high impedance at the port with a DC Block, as well as a the port without a DC block.

At the operating frequency the dc block "cap" should be transparent.

Are you measuring below the capability of the analyzer? Typical low end frequency is 300KHz or 30KHz.

Rod