dc blocking capacitor

[spide]

dc block capacitor

can anyone let me know how to choose the best series of dc blocking capacitor which are available in RS.the value of my capacitor is 120pF.i am working in 2.4GHz.

 

[flatulent]

Look at the data sheets for the self resonant frequency. Select one that has the highest value.

 

[hanm]

120pF is too large,it's my point of view.
In fact,you should trade off bewen couple loss and capacitive load.
Chose a resonable value that your circuit work properly.

 

[IanP]

RS Components has a range of surface mount 0603 (and bigger) COG series capacitors in the range of 0.5pF to ≈100pF which are suitable for RF coupling and RF decoupling applications.
The part numbers start from: 264-4466 (0.5pF) .... 264-4494 (10pF)

 

[supra924]

Hi ,
you can also take the point of considering low ESR value capacitor. But it will added up to more cost

 

[boy]

You must get the model of the capacitor and simulate it . Or you can test it with a network analyzer. A genaral rule is that the operating frequency have to less than the self resonance frequency (SRF). The SRF depends on the size and manufacturer of the capacitor. The smaller size, the larger SRF.

 

[tjuby]

Reviving an old topic to pose some new questions. (Nice eh? )

I'm using an ATC600S cap (47pF, 0603 size) as a DC-block at the input and output of a switch (operating between 3-4 GHz). I've been trying to read up on ESR, reactance, resonant frequencies etc etc.. but I'm having trouble connecting the dots.

See, I need the capacitor to have very good (low) Insertion Loss. And it IS good. But I need to justify this by some theory.

Below is a monologue by me, where I pose some questions that I may or may not answer correctly. Id very much like it if someone could give me any input regarding this

ESR:
So, the ESR is a resistance in series with the capacitance, caused by the resistive materials (dielectric, terminals etc). Am I getting this right? And, the larger ESR, the more power the capacitor will dissipate, yeah? Which leads to more heat generated, which will increase the noise figure in my system.

I can't imagine anything other than that this ESR effect would be baked in the final result that is the Sparameters of the device, am I right? And since S21 is the Insertion Loss, I would be correct if I calculate the Noise Figure by using this data?

Monologue finished.

Resonant Frequencies.
What is the difference between Series Resonance and Parallel resonance? The frequency of full resonance is easily spotted by looking at the S-parameters, but how do I know if its one or the other?

There is resonance at approximately 4.5 GHz in the ATC600S470, this should be good right, since my system operates between 3-4.. Or is it dangerously close? I wouldnt know..

Many thanks to whoever manages to get though this post and answers

Best Regards,
Alex

 

[spide]

hi tjuby,
In order to understand the phenomena of the resonant mode frequency, you actually study the equivalent circuit of a general capacitor or the equivalent given by the manufacturer. it is clear that the first resonance occur are due to the series resonance between capacitor and the inductance(lead and plate).
Once the capacitor exceed this series resonance, the inductance due lead+plate will be the dominant factor, until next resonance occur due to the parasitic parallel interlead capacitance and the inductance(lead+plate). this is known as parallel resonance because the parasitic interlead cap resonance with the lead+plate inductance. since this is parallel mode, the capacitor becomes a very large resistor.

 

[volker_muehlhaus]

Resonant Frequencies.
What is the difference between Series Resonance and Parallel resonance? The frequency of full resonance is easily spotted by looking at the S-parameters, but how do I know if its one or the other?

At the series resonance (SRF) your capacitor will be low impedance. At this frequency, the capacitance and the parasitic series inductance cancel. If you use the capacitor in series, as an RF thru, this works fine. Your capacitor does not have the exact capacitance, but it passes your RF signal.

However, you want to stay away from the parallel resonance frequency (PRF) which means high series impedance -> RF blocked.

The question is if 47pF is the right value for 3-4GHz operation. If I look at the SRF/PRF on page 4 of the data sheet linked below, your frequency would be above SRF and PRF for that value and parasitics will dominate. I would use a smaller value and stay away from PRF.

**broken link removed**

Best regards
Volker