hi guys,

can someone explain what is the difference between RF choke and bypass capacitor ? i know that we use capacitor to pass AC to ground , and i know that we use RF choke (coil) to block AC signals , so what is the difference and how can i know when to use each one .


thanks !

 

I have two questions for you

1. What is the impedance of an RF choke as a function of frequency?
2. What is the impedance of a capacitor as a function of frequency?

The answers to these two questions will lead you to the answer.

 

i know that the impedance of the choke increase with the frequency , and the impedance of the capacitor decrease with the frequency in order to pass ac to ground .

but how the choke block the AC ? what are the considerations to use a capacitor and GND , and not to use RF choke ?

 

chokes have resistance, in high current aplications, they might drop too much voltage. the best is to use both if you really want to get rid of stray ac or rf signals.

 

i know that the impedance of the choke increase with the frequency , and the impedance of the capacitor decrease with the frequency in order to pass ac to ground .

but how the choke block the AC ? what are the considerations to use a capacitor and GND , and not to use RF choke ?

So choke in series with a power rail will pass DC and low frequencies while blocking high frequencies -- right?

 

it depends on the inductance of the choke. the higher the inductance, the lower the frequencies it will block. unfortunatly, that means more wire which means more resistance. or a (usually) ferrite core, which you have to choose the ferrite mix to determine the frequency also.
if using inductors and caps together, watch out for resonance, strange things can happen if you hit a resonant frequency, the impedance could go way up or way down.

 

thanks a lot guys !
you were very helpfull !

 

Wire wound RF chokes (always in conjunction with a capacitor to ground) are usually only used when required in high frequency RF circuits where maximum suppression of an unwanted signal is needed (such as not getting to a high gain node and causing oscillations).

Another use of RF chokes is as a collector load for an amplifier stage instead of a resistor. This creates higher gain since the choke impedance can be made much higher at the signal frequency then the resistor could be while still passing the required bias current.

To avoid the problem of resonance noted by alfacliff you can use ferrite bead inductors, which are lossy enough that they usually don't show significant resonant issues. But they are also used in conjunction with a capacitor to ground for maximum attenuation of the unwanted signals.