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# Formula for RF choke (inductance) calculation

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#### Murlix

##### Member level 2
How do we calculate the inductance(rf choke) required for a biasing network.
my design is 12GHz LNA with Zin=15.143+j9.6410 and ZL=17.75+j4.99. Vdd is 8.3V with Rd=680ohm.I am using 50 ohm terminations for input and output.
Can anyone pls show the calculation and explain the formula?

Actually i used microstrip but i want to know the formula for Rf choke for approximation calculation.

thanks

#### nanoYasser

##### Junior Member level 1
Re: RF choke calculation

I'm not expert...

But as far as i know, RF Chock coil can be simply a coil wonded around ferrite core too many times (say 300 turns on length of say 2cm)..

It is like blocking capacitor, you don't need to calculate it exactly, unless you wanna block certain freq. specifically..

We'll learn here.. Correct me if i am wrong ..

Best Wishes to all of us,
Yassorty,

#### Murlix

##### Member level 2
Re: RF choke calculation

thanks for the reply but i really need a method for calculating the inductance because I need to explain my design to my supervisor. the frequency that need to be blocked are ku band frequencies(11.7-12.2GHz)

Can anyone juz pls explain the calculation in detail.(I know the SRF must be lower than the operating frequency but i need a formula for the RF choke inductance approximation)

#### rogerynt

Re: RF choke calculation

Can you use microstrip line for bias choke ?

If yes, a quarter lambda line with appropriate decoupling cap. at the far end may solve your problem.

khungbopro

### khungbopro

Points: 2

#### boy

RF choke calculation

Radial stub and high impedance transmission line is a good alternative.

khungbopro

### khungbopro

Points: 2

#### rfsurfer

##### Junior Member level 3
Re: RF choke calculation

Several points:
First the SRF should be higher than your operating frequency.
Second, for choking purpose, inductor is usually followed by decouple cap, i.e. an AC ground. In a good design, DC bias is very often be integrated into matching circuit. Only in ideal case, choke impedance is infinite.
A simple estimation of the impedence is good enough (Z=2*PI*Freq*L). In real case, you need CAD tool and inductor model for accurate simulation cause things are more complicated than idea calculation. For example, Inductor Q is dependent on frequency.

#### drabos

##### Full Member level 6
RF choke calculation

RF choke blocks the RF, so you need minimum value of the inductance which has 10 times larger impedance than the other impedances, for example in 50 Ohms system your RF choke has minimum 500 Ohms resistance at your operating frequency range.
I think you have to use some microstip based RF choke in this frequency, maybe meander.

#### 7rots51

Re: RF choke calculation

how to design microstrip choke?

#### drabos

##### Full Member level 6
RF choke calculation

a very thin line is good, just optimize the line width to the desired inductance value in a simulator, I always use this method.
meander is also possible

or see some example: