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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. http://www.murata.com/ninfo/nr0261e.html
“ *1
Self-resonant frequency
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 http://www.ecircuitcenter.com/Circuits/cmodel1/cmodel1.htm Pure nonsense. Ivor Catt 31oct03 http://www.qsl.net/kf4trd/varactor.htm “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 http://www.ivorcatt.com/z001.htm , published 25 years ago. Why have none of these guys ever risen to the bait, and proposed a mechanism? - Ivor Catt 31oct03 http://www.maxim-ic.com/appnotes.cfm/appnote_number/1756/ln/en “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. http://www.ivorcatt.com/em_test04.htm 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, http://www.electromagnetism.demon.co.uk/z001.htm , 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. http://www.ivorcatt.com/em_test04.htm
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. @@@@@@@@@@@@@@@@@@@@@@@@@@@ xx |
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Riposte Ivor Catt. 18june02 Scandals in electromagnetic theory http://www.ivorcatt.com/28scan.htm x |
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(Possibly we need a
standard word for this. I suggest "Riposte", or the symbol [R] .) Ivor Catt, 30june02. ivor@ivorcatt.com |