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What exactly is DC blocking capacitor?

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Antenna (^.^)

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Hi everybody,

I have to use an RF IC switch in my circuit and it needs DC blocking capacitors for it work.
In the datasheet it recommends 100pF for above 500MHz operation.

I'm just wondering, can I just use any capacitor with a 100pF value (but one for high freq?) or does it have to be a special "DC Blocking Capacitor"?
 

schmitt trigger

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Some coax cable schemes, in addition to the RF signals, carry DC power for remote amplifiers or downconverters.
You don't want that DC voltage entering your TV, only the RF signals.
Therefore you put a capacitor in series. ALL CAPACITORS WILL block a DC voltage. Your concern should be that the capacitor's voltage rating must be higher than the DC voltage you are attempting to block.
 

fouwad

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Any capacitor of 100pF and higher voltage ratting 500V would be suitable. It should be non polar capacitor
 

jiripolivka

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For frequencies above 500 MHz please use SMD ceramic capacitors . Check the maximum DC voltage that may occur in your circuit, and select the capacitor that can stand it. For RF transmission, the capacitor value should be "negligible" against 50 Ohm line impedance, so its reactance at a particular frequency should be less than 5 Ohms.
Otherwise check IC Specifications for details. Many IC makers offer a Test Circuit details., with also D C block capacitor type recommended for the best results.
 

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So can I use the same capacitor value (e.g 100pF) for a range of frequencies, (e.g. 2.4GHz - 0.9GHz) and expect it to DC block the same for all the frequencies as long as the reactance at all those frequencies are less than -5j?
 

jiripolivka

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So can I use the same capacitor value (e.g 100pF) for a range of frequencies, (e.g. 2.4GHz - 0.9GHz) and expect it to DC block the same for all the frequencies as long as the reactance at all those frequencies are less than -5j?

Yes, correct. Above 3 GHz , try not to use caps larger than 100 pF as some types resonate. The smallest SMD versions are safe (if the DC voltage rating is OK with your DC component on the line).
 

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