Self resonant frequency of a capacitor


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Self-resonant frequency of a capacitor

Nonsense about so-called “self-resonant frequency” of a capacitor.

Many pages cited below have been removed.


If you cut off a capacitor’s legs at the knees, you will double its self-resonant frequency – Ivor Catt.

  *1 Self-resonant frequency
The upper frequency limit at which the capacitor functions. At frequencies exceeding the self-resonant frequency, the capacitor loses the function essential to a capacitor and begins to function as a coil that has properties different to those of capacitors.”


It is particularly clear with their capacitor that it is a transmission line, with no series inductance. Perhaps their mechanism for trimming introduces inductance - Ivor Catt. 31oct03

Pure nonsense. Ivor Catt 31oct03

“The frequency at which both impedances are equal is known as the self-resonant frequency. This frequency is set by the materials used and the construction of the capacitor.”

Why are we not told about the mechanism? Because nobody has ever proposed one, see , published 25 years ago. Why have none of these guys ever risen to the bait, and proposed a mechanism?   - Ivor Catt  31oct03

“Power Supply By-Passing

Power supply bypassing is essential at microwave frequencies to ensure stable high frequency operation. It is important to choose a capacitor such that the impedance of the capacitor is the lowest at the frequency one is trying to decouple. For example, a 1000pF capacitor is not a good choice for high frequency decoupling since the lowest impedance for a 1000pF capacitor would likely occur below several hundred MHz. At 5GHz, the self-resonant frequency would make it look more like a lost inductor! So for high frequency decoupling, a capacitor of typically less than 10pF should be placed close to the IC. Also, for low frequency decoupling, a 1000pF and 0.01uF capacitor combination is a good choice but they don't have to be located immediately at the IC pin.”

The usual misconception, using an untested LCR formula. They do not realise the reason why it is thought that a high value capacitor seems worse, not because of its construction, but because of its high C and this effect on the formula (frequency squared) = 1/LC . As pointed out below by me, nobody has noticed that the hi C in the formula gives the false impression of a lower self resonant frequency. – Ivor Catt 31oct03

In the surreal world created with inappropriate mathematical stunts by physically ignorant operators, a capacitor is looked on with disdain, not because it has more L, but because it has more C.      Ivor Catt  18may02





Recap. Take the formula for the resonant frequency for an inductor-capacitor tank circuit.


The frequency (in radians per sec.) squared equals (1/ inductance x capacitance)


Thus, either increase in inductance or increase in capacitance reduces the resonant frequency. This has led physically ignorant mathematical mugwumps to think, not that the best capacitor has the least capacitance, which even they might realise is ridiculous, but that the best capacitor has the least inductance, making it able to perform to a much higher frequency up to its higher resonant frequency. They have failed to realise that they would realise their dream, of a high self resonant frequency, by reducing the capacitance just as well as by reducing the inductance. They think that it is an accident that lo value capacitors have the highest self resonant frequency. They think it is because of the difference in inductance, which it is not.


However, all this is nonsense when decoupling digital logic. What matters with digital logic is the transient performance of a decoupling capacitor, when some switching logic wants to grab as much charge as possible to launch down a transmission line  towards the next logic gate. The true model, which should have replaced the series L C R model for a capacitor, was already published in 1978,  , and has been ignored for 24 years by radio men who continue to teach and publish the old model which is inappropriate and damaging in digital electronics. Note that today, most capacitors are used in DC voltage decoupling.


The only way out of this impasse is for students to create problems during the lecture when lecturers continue to pump out the old, wrong drivel. Otherwise these lecturers and text book writers will continue to copy and repeat each other from a bygone age when electronics was about radio, and such a misconception about the physical nature of a capacitor was not so damaging.   Students have much to gain by disrupting their lectures. It is probably more difficult to learn and be examined in material which is false. Ivor Catt.   18may02.




In 1965, living in the USA, I telephoned the design engineers in Sprague, who manufactured capacitors. They told me that they tested for the high frequency performance of a capacitor by testing at 5kHz and 50kHz, and deduced its performance at 1MHz and above using the series L C R model. Thus, the published self-resonant frequency of a capacitor is the result of lo frequency testing extrapolated using the L C R model.


By making this error, engineers in the capacitor manufacturers might have doubled their companies’ sales, ensuring that a second “high frequency” capacitor would be added to every 1uF decoupling capacitor in every digital system.          Ivor Catt   18may02.




Academic apes




I make the commitment that anyone wishing to counter any assertion made on this site will be guaranteed a hyperlink to a website of their choosing at the point where the disputed assertion is made.

Ivor Catt. 18june02


Scandals in electromagnetic theory



(Possibly we need a standard word for this. I suggest "Riposte", or the symbol [R] .) Ivor Catt, 30june02.