[SOLVED] capacitor frequency range

[ktx2222]

Hi guys,
I want to design a analog circuit which works at 10-100 MHz. I need input/output caps to block DC current. What the caps' value should I use? Can I use 1nF cap? If not, what value do you suggest? Please give me some advice. Thank in advanced.

 

[chuckey]

1 NF should do, work out its reactance at 10 MHZ to check that this is low compared to your load impedance.
Frank

 

[crutschow]

It depends upon the equivalent resistance of the circuit. The -3dB rolloff point for the circuit would be f = 1/(2πRC) where R is the equivalent resistance and C is the capacitance. You want f to be well below 10MHz.

For example, for a 50 ohm circuit the capacitance should be greater than 320pF so 1nF should be more than adequate.

 

[Audioguru]

The value of a coupling cap is determined by a simple calculation using the capacitor's load resistance and the lowest frequency you want.
10MHz into a load resistance of 100 ohms needs a capacitor value that is about 160pF. 10MHz will be reduced a little to -3dB (0.707 times).

 

[Borber]

At 10MHz coupling capacitor to 50ohm load will bring some signal loss or attenuation. Cap 1n will attenuate signal for 5% and 3.3n only 0.5%. Cap calculated from -3dB at corner frequency 10MHz will attenuate signal for ~30%. What do you want?

 

[D.A.(Tony)Stewart]

My rule of thumb depends on choosing Zc to be much lower an load, R

Zc = 1/(2*pi*fc*C)

For fc=10MHz, R=50

C=1/(2pi*R*fc)=1/(100*pi*1e7)=313pF

to reduce loss on C by-20dB make C=>10x or 3300 pF


same as Borber and others...

 

[ktx2222]

Thank you guys,
I want the DC block caps does not affect to AC signal (minimum power loss). I want to use 100 nF but I heard someone said that 100 nF capacitor cannot work at 100 MHz (because of limited frequency of devices).

 

[Borber]

You should use few caps to cover whole frequency bandwidth. For example 3.3n+330p+33p in parallel.

 

[BigBoss]

Thank you guys,
I want the DC block caps does not affect to AC signal (minimum power loss). I want to use 100 nF but I heard someone said that 100 nF capacitor cannot work at 100 MHz (because of limited frequency of devices).

No..
A ceramic 100nF capacitor can work up to 500MHz or more..

 

[D.A.(Tony)Stewart]

No..
A ceramic 100nF capacitor can work up to 500MHz or more..

Small SMD size is essential to get SRF above 100MHz or very low ESR.

If they do not specify ESR or SRF, they do not qualify. Leaded parts would not qualify.

Here are some references of ESR and self resonant freq (SRF) for 3.3nF to 470nF from one of the best suppliers of ceramic, TDK. Chinese caps won't compare. Shunting smaller caps is a good suggestion, unless you are 100% certain of future buys.

 

[BigBoss]

Small SMD size is essential to get SRF above 100MHz or very low ESR.

If they do not specify ESR or SRF, they do not qualify. Leaded parts would not qualify.

Here are some references of ESR and self resonant freq (SRF) for 3.3nF to 470nF from one of the best suppliers of ceramic, TDK. Chinese caps won't compare. Shunting smaller caps is a good suggestion, unless you are 100% certain of future buys.

Murata 100nF has been qualified up to 6GHz by s-parameters.

https://ds.murata.co.jp/software/simsurfing/en-us/index.html

Every cap has absolutely a self resonance frequency but it does not mean that it cannot be used beyond this point.It's valid inductors only but caps can definitely be used beyond their resonance frequency for coupling purposes.They cannot be used for critical applications such as tank circuits, filtering etc.

 

[D.A.(Tony)Stewart]

Yes of course even plastic film is better than X5R but if we use design rules for the cheapest grade ceramic then any substitute will work.

This is why I would recommend 3.3nF in 603 or 805, then supplement that with 100pF for harmonics of 100MHz.

Why spec exotic single sourced parts.?

 

[Borber]

This is overkill. We did not ask about DC voltage capacitors has to block neither what kind of capacitors will be used or technique circuit will be build.

 

[ktx2222]

Thank for all comments. I got experience from your discussions.