Why all RF designs bypass capacitors are much larger than blocking capacitor?

[Terminator3]

I read few appnotes from Agilent, Hittite, Minicircuits, and all of them have such thing. Why not use the same capacitor value?

For example,
10GHz LNA: Cbypass=1000pF Cblock=1pF
Up to 12GHz prescaler: Cbypass=1000pF Cblock=100pF
MMIC amp around 10GHz: Cbypass=0.1uF=100000pF Cblock=0.001uF=1000pF
Some paper Cbypass=680pF Cblock=2pF

Why not choose something around 1pF for Cbypass at those freqs?

 

[G4BCH]

Very often it is not only the signal frequency that needs to be bypassed. Many circuits have a number of bypass capacitors to decouple either a wide bandwidth or particular frequency bands. Care needs to be taken with this approach to aviod unwanted resonances.

 

[biff44]

RF integrated circuits often have very large gain at low frequency, such as 1 MHz. so you use a much larger capacitor to keep low frequency oscillations from happening. But the RF series elements only have to pass microwave frequencies, so they can be smaller value (in fact, the smaller values work BETTER at microwave frequencies--fewer resonances)

 

[Borber]

Bypass or decoupling capacitors are usually placed across low impedance power supply sources (~0.1 ohm). To acheive enough attenuation of noise greater the capacitor is. Blocking or coupling capacitor must have enough low reactance regarding to source and load impedances (50 ohm) not to introduce attenuation losses.

 

[afz23]

Role of capacitors is different in different places:
Bypass capacitors presents low impedance path to range of frequencies
coming from DC power supply,hence uF, kpF and pF covers very low to high
frequencies.These are in shunt.
Whereas the blocking capacitors are meant to block DC from going to preceding or succeeding stages,they must offer no insertion loss to RF frequencies,and pF range has high frequency SRF ,hence suitable for MW range.

 

[Terminator3]

Thank you all for the answers! It gived me idea on topics to search. I read some more about..

I come to some (crazy) conclusions, please connect me if i am wrong:

1)DC bypass capacitor:
Usually connects thin dc-supply lines with ground. At operating frequency of 5...10GHz such lines can be around 100...200 Ohms (1.6mm substrate). As i understand, it would me most effective if bypass capacitor impedance matches between line impedance and gound plane connection point impedance.
What would be ground connection point impedance at 5..10GHz? I think it would be very low, but i am not sure, did not found information about that. So, assume ground impedance is very low, supply line impedance is around 100..200 Ohm. Best matching will occur if we use quarterwave transformer with sqrt(Zline*Zgnd) impedance. As DC bypass capacitor values are pretty high in example above, it is used above SRF, where it start to act as inductor (which equal to 90deg microstrip line as i understand). So capacitor above SRF would work as quarterwave transformer.

From 10GHz examples in my first message most Cbypass capacitors have value around 1000pf=0.001uF, it may be good down to 10MHz and after SRF around 1000MHz will act as inductor (=quarterwave transformer?) .

2)DC block capacitor:
Usually stays on 50 Ohm microstrip lines. Assume we working in X-band with 10GHz sinusoidal signal. If we choose capacitor which SRF is exactly at 10GHz, then capacitor impedance will be extremely low, which makes impedance mismatch to 50 Ohm line. So we need too choose capacitor with higher SRF, so capacitors impedance would be around 50 Ohm at 10GHz.
Here i can't understand why using capacitor above SRF is not recommended in RF. Reactance becomes inductive above SRF, but impedance magnitude is raises very slowly. Also all SRF graphs are given in logarithmic frequency scale, so 0...1MHz we have drop from 100 Ohm to 0.1 Ohm impedance, and from 1MHz to 10GHz(!) we have raise from 0.1 Ohm to 100 Ohm. So why not using capacitor above SRF in DC-block application?

 

[vfone]

The answer is simple. There are a lot of applications where capacitors ARE used as DC-block above their SRF.

 

[volker@muehlhaus]

2)DC block capacitor:
Usually stays on 50 Ohm microstrip lines. Assume we working in X-band with 10GHz sinusoidal signal. If we choose capacitor which SRF is exactly at 10GHz, then capacitor impedance will be extremely low, which makes impedance mismatch to 50 Ohm line. So we need too choose capacitor with higher SRF, so capacitors impedance would be around 50 Ohm at 10GHz.

No, multiple wrong ideas here.

First mistake: The series cap must be as low impedance as possible, because it is in series with the 50 Ohm load. So we certainly do NOT want it to be 50 Ohm impedance.

Second mistake:
The capacitor impedance would/could never be 50 Ohm (real) anyway. It could be reactive 0 -j50 Ohm which is not matched to 50 + j0 Ohm line impedance.