For 900MHz band operation, I have seen some ac-coupling capacitors in the order of 100pF and others in the order of 10nF. If I use generic NPO/X7R caps, which is better?
A typical 100pF NPO cap has self resonant freq around 1GHz while 10nF is well below 100MHz. Since above the self resonant frequency, the impedance goes up with frequency, looking like an inductor, I thought it was better to use a 100pF cap. But then again, with the 10nF cap, since the impedance is so much lower before it turns inductive, even at 900MHz, its inductive reactance is still small so maybe it doesn't really matter???
Can anyone out there tell me if I am right or wrong?
So the inductive reactance is not (in relative terms compared to capacitive reactance) small at all, it's thousands of times bigger.
If the circuit impedance is 50 ohms to a few hundred ohms, the impedance of the 10nF capacitor will have a measurable effect (gain down a fraction of a dB to a few dB). Whether a fraction of a dB or a few dB matter or not depends on what the circuit is there for.
It still decouples... which is probably the only reason it's there!
Why does the 10 nf cap have a higher impedance than the 100 pf? If the capacitors are the same types (say, both 0805's of similar construction) the esl will be pretty much independent of capacitance, and the bigger cap may well have lower esl (more layers.)
I routinely do multi-GHz dc blocks along microstrip transmission lines, and TDR them to see how well they work. The only visible difference between a 1 nF 0603 cap and a 2.2 uF 0603 is the improved low-frequency coupling due to the obvious capacitance difference.
About the only time a lower-value cap will have a lower impedance is when it hits series resonance.
Our observation is that, in general, bigger caps behave like bigger caps.
IC manufacturers all think the world revolves around their part, so they want three different caps on every power pin. The idea of staggering the srf points of bypass caps is, in my opinion, silly. A power plane or island adjacent to a ground plane, scattered with a few
0.33 uF or whatever surfmount caps, is good enough for most cases.
I found exactly this too. When I got a spectrum analyser with tracking generator, the first thing I did was look at the impedance of various chip caps.
Manufacturers datasheets often show power supply bypassing with paralleled chip capacitors of say 100pF || 10nF || 1uF. What do you think about this? Is there a reason not to just use say 3 x 1uF?
I read in sci.electronics.design that John Devereux wrote (in ) about 'Capacitor self-resonant freq question', on Thu, 10 Mar
2005:
The Z/f characteristics depend critically on where, and how large, the parasitic inductances are. Consider, for example, the difference between the Z/f characteristics of three equal caps and three with very different values, if they are measured via 30 mm long tracks.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk
NPO. X7R is IIRC +/-15% over its temperature range, NPO is close to zero. X7R also has a voltage coefficient of capacitance - as you increase the voltage across the cap, its capacitance drops. This isnt much, around 10% or so, but again NPO is almost perfect. X7R is also a bit piezoelectric due to the materials used, whereas NPO aint.
So if you want a particular capacitance over a range of applied voltage, temperature and thumping, NPO is the way to go.
If you only ever use NPO caps, that limits you to about 1nF or so. X7R gets you up to about 1uF, but RF caps are almost always much smaller than that.
if they are in the same package (eg 0603) then the inductance is roughly constant, ie SRF varies with the square root of capacitance. To answer your question, model each cap as R + L + C. use the same fairly low R (1-10mOhm, can read off a datasheet maybe, or look at tdk website for math models) and the same L. plot the magnitude of impedance vs frequency for both caps, and look.
lets use 100pF and 10nF in the same package, with 0.01Ohms esr
100pF, 1GHz - L = 250pH (wow, thats low) Z = 1.59 Ohms ESR = 0.01 Ohms Qu = 159
10nF 100MHz - L = 250pH Z = 0.025 Ohms ESR = 0.01 Ohms Q = 2.5
so the 10nF is going to have a not-very-sharp dip from 0.025 ohms down to 0.01 Ohms (resistive) at 100MHz. At 1GHz the impedance will be about
1.6 Ohms inductive, at 10GHz its 16 Ohms inductive and at 10MHz its about 1.6 Ohms capacitive.
The 100pF cap is about 160 Ohms capacitive at 10MHz, 16 Ohms capacitive at 100MHz and 16 Ohms inductive at 10GHz. At 1GHz it has a sharp resonant peak down to 0.01 Ohms resistive.
So around the resonance of the 100pF capacitor the impedance is a *LOT* lower than the 1.6 Ohms inductive of the 10nF cap. Obviously the actual ESR is critical, as it directly controls the unloaded Q of the device.
always model your parts as RLC circuits and you wont be wrong.
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