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Riposte to http://www.ivorcatt.com/2603.htm http://www.logbook.freeserve.co.uk/riposte%20capacitor.html “I had
thought that Ivor had deleted this section from his website as it is clearly
erroneous.” Extraordinary that Green does not give a hyperlink to the
material he is criticising. What section? |
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Riposte Ivor Catt. 18june02 Scandals in electromagnetic theory http://www.ivorcatt.com/28scan.htm (Possibly we need a standard word for
this. I suggest "Riposte", or the symbol [R] .) Ivor Catt,
30june02. ivor@ivorcatt.com @@@@@@@@@@@@@@@@@ ----- Original Message ----- From: "Leslie Green" <logbook@lineone.net> To: <ivor@ivorcatt.com> Sent: Saturday, August 09, 2003 3:29 PM Subject: Riposte > for page @@@@@@@@@@@@@@@@@ http://www.logbook.freeserve.co.uk/riposte%20capacitor.html RIPOSTE: Ivor Catt's view of Capacitorsby Leslie Green CEng MIEEI had thought that Ivor had deleted this section from his website, since it is clearly so faulty. Given that electromagnetism is evidently a subject of great interest to engineers, this site attracts a respectable volume of readers. It is therefore worthwhile correcting one of the most blatant and demonstrable errors on the website. Ivor claims that capacitors do not have self-inductance if measured without their leads. He makes this claim on purely theoretical grounds. The problem with this assertion is that it relates to no known real-world components! The application note from American Technical Ceramics (ATC), rubbished by Ivor, is actually excellent, and is well worth reading. A capacitor has capacitance. It also has parasitic inductance and resistance. These are measurable and quantifiable. Nowadays electronics designers working on high density or high frequency designs use surface mount components almost exclusively. There are no lead wires to worry about, and the inductance is then due to the path length within the body of the part. The inductances are quite small, of the order of 2nH, but are nevertheless important in RF designs. Good text books make the distinction between low frequency capacitance and high frequency capacitance. Why? Capacitance changes with frequency, even for solid dielectric capacitors! Now I am not just talking about rubbish dielectrics such as X7R and Z5U. These Class II and Class III dielectrics are no use at all for any sort of "analog" applications because the capacitance changes with temperature, time and frequency by extraordinarily ridiculous amounts (see manufacturer's data sheets). Even for good dielectrics like NP0 ceramic, polyester and air, the capacitance INCREASES with frequency. Now you know that the impedance of a capacitor decreases with frequency You learnt this at school. Well using complex arithmetic, by which I mean maths in the form of a+jb, where j is the square root of minus one, you can work out what the measured value of a capacitor would be if you consider a real world device to have a small parasitic inductance in series with the capacitive element. The maths shows that the measured capacitance actually increases with frequency until the capacitor hits its first self-resonant frequency. If you are unwilling or unable to do the maths, then you can look up the measured impedance curves given by all volume manufacturers of capacitors. The impedance curves drop at 20dB/decade initially, but as they approach the self-resonant point, the curve slope increases slightly, showing a rising capacitance! Whether the parasitic inductance of a surface mount capacitor is identically equal to a body-length sized wire is a debatable point. It is also true that making the capacitor body shorter, as for example used in microwave capacitors, is an important way of increasing the self-resonant frequency of a capacitor. But in the real world the capacitor does have a finite amount of self-inductance which does adversely affect its performance in real-world applications. Ivor has got himself a bit confused about transmission lines and real components. Ivor says that capacitors are really transmissions lines and should be treated as such. This is a bit backwards. According to electromagnetic theory, everything is based on Maxwell's theory, transmission lines, waves, fields and so forth. All very complicated. Rather than confront the huge mass of differential equations necessary to solve even simple problems, practical engineers have come up with "lumped element models". Rather than consider a coil of wire as a transmission line, it is easier to consider it as "an inductor". This approximation is only valid up to a certain limiting frequency where the phase shift of the current in the wire becomes too great. When the phase shift is relatively small the system is described as "quasi-static" and the simple lumped element approximation is used. We know it is not exact, but it is good enough for engineering purposes. Thus Ivor has "invented" non-quasi-static systems, something known about for over a century! It has to be said in Ivor's defence, however, that such descriptions are not usually seen in modern electronics books, but were common in good text books between say 1930 and 1955. - LG 10aug03 @@@@@@@@@@@@@@@@@ Discussion by Ivor. Compare (1) LG, above; “….Ivor claims that capacitors do not have
self-inductance if measured without their leads. He makes this claim on purely
theoretical grounds. The problem with this assertion is that it relates to no known real-world
components!” - LG with (2) Ivor Catt, in the original 2603 article; “ …. Ivor Catt 22apr02 In
1963 I bought the EH-125 pulse generator. This delivered a –10v step with a
100picosecond fall time into a 50 ohm load (e.g. 50 ohm coax.). The pulse generator could
also deliver a –ve 10v spike with a width of 150psec. I decided to try to
create a positive 10v spike. I cut into the 50 ohm coax, and joined the incoming
inner to the outgoing outer via a red 1uF tantalum capacitor. I also joined
the incoming outer to the outgoing inner via another 1uF tantalum capacitor.
Further downstream I found that I had a positive 150psec spike with no
discernable degradation (in rise time or pulse width) compared with the
initial –ve spike. That is, I had a +ve 10v spike with a width of 150psec. …. Note 1. Anyone who wants to play
with frequencies can be told that the fundamental of the 150psec spike will
be around 3GHz. Put that in your “self-resonant” pipe and smoke it! IC …. ” 22apr02 Compare (1) LG, above; “Good text books make the distinction
between low frequency capacitance and high frequency capacitance. …. Why?
(see manufacturer's data sheets). - LG with (2) Ivor Catt, in the original 2603 article; “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.” – IC Compare (1) LG, above; “According to electromagnetic theory,
everything is based on Maxwell's theory, transmission lines, waves, fields
and so forth. All very complicated. Rather than confront the huge mass of
differential equations necessary to solve even simple problems, practical
engineers have come up with "lumped element models". - LG with (2) Ivor Catt, The Hidden Message in Maxwell’s Equations “ …. So the only information about electromagnetism contained in the apparently sophisticated equations (9) and (10) is about the two constants in electromagnetism: the fixed velocity c, and that E, H at every point are in fixed proportion Z0. The remaining content of Maxwell’s Equations is hogwash. We have to conclude, with respect, that what Maxwell and his sycophants do not say about a tapering, disappearing plank of wood isn’t worth saying. …. I am sure that Maxwell was sincere, and did not knowingly shroud the very heart and soul of science, Electromagnetism, in confusion and nonsense for over a century.” – IC Ivor Catt 22apr02/13oct03 |
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Copy of the original
article http://www.ivorcatt.com/2603.htm
which generated the riposte and my reply (above). Self-resonant frequency of a capacitor Nonsense about so-called “self-resonant frequency” of a
capacitor. This
captures web page http://www.ivorcatt.com/2603.htm
as of aug03 and Leslie Green’s Riposte in aug03 If you cut off a
capacitor’s legs at the knees, you will double its self-resonant frequency –
Ivor Catt Martin Eccles takes
the biscuit My 1994 book Electromagnetism 1 is at http://www.ivorcatt.com/em.htm Nigel
Cook on Ivor Catt’s ideas, (London) Electronics World (was Wireless World),
aug02, pp46-49 More
nonsense is at http://www.ivorcatt.com/2605.htm Yet more nonsenseScandals in Electromagnetic Theory http://www.ivorcatt.com/28scan.htm [This is http://www.ivorcatt.com/2603.htm
as it stood on 10aug03, when the L Green Riposte turned up.] http://www.atceramics.com/pdf/technotes/effective_capacitance_vs_fr.pdf talks about self-resonant frequency. The “parasitic inductance” does not exist inside the
capacitor. However, the article is very primitive, because the graphs go in
the wrong direction, with C increasing with frequency, anyway. The alleged L
reduces the C, not increasing it as the author seems to think. http://www.capacitors.com/multicap/phase-esr/phase-esr.html “the capacitor can be used
up to the natural self-resonant frequency or” http://www.educatorscorner.com/experiments/pdfs/exp79.pdf Unfortunate students are
made to measure the “self-resonant frequency” of a capacitor. Tell them to
cut off the poor capacitor’s legs! http://www.capacitors.com/consider/consider.htm This guy is stumbling in the right direction, but he gets his number wrong. Inductance caused by the legs should be proportional to the length of the legs. “Lead length alters a capacitor's range of operating frequency. Here a 2 uf capacitor's self-resonance decreases from 490 kHz to 290 kHz when its leads are lengthened from 3/8 inch to 3 inches. In other words, the capacitor's usable operating range is reduced by almost half.” More nonsense; http://www.qsl.net/kf4trd/varactor.htm your poor
capacitor behaves just like an inductor! The frequency at which both impedances http://www.emf-emi.com/dosanddonts.shtm Ensure that the SRF (self-resonant frequency) of capacitors is above the highest frequency to be bypassed. …. 10.) Select and mount decoupling capacitors having self-resonance (SRF) above logic band-width (1/) http://www.cypress.com/pub/appnotes/decouple.pdf This writer is in a bad way. Pure fantasy. – IC What a pity this poor
fellow has been pulled from the www by his paymasters. The hyperlink now
jumps straight to another page, bypassing the nonsense I saw on 30jan02.
Perhaps I have influence! Ivor Catt. 5may02 http://www.benchmarkmedia.com/appnotes-a/caig/caig06.asp However, all capacitors have
their own self-resonant frequency @@@@@@@@@@@@@@@@@@@@@@@@@@@ The amount of nonsense
drifting around the world, of which the above are examples, is vast. See my 1978 article at http://www.electromagnetism.demon.co.uk/z001.htm
; “Series inductance
does not exist. Pace the many
documented values for series inductance in a capacitor, this confirms
experience that when the so-called series inductance of a capacitor is
measured it turns out to be no more than the series inductance of the wires
connected to the capacitor. No mechanism has ever been proposed for an
internal series inductance in a capacitor.” The key point in my article is that “No mechanism has ever been proposed for an internal series inductance in a capacitor.” The IEE and IEEE have
helped to cause the confusion to escalate by suppressing my 1978 article http://www.electromagnetism.demon.co.uk/z001.htm
, which puts an end to a capacitor’s series inductance. Also, competent
experimentation will show that a capacitor has no internal series
inductance. http://www.ivorcatt.com/em_test04.htm – Ivor Catt, 30jan02 @@@@@@@@@@@@@@@@@@@@@@@@@@@ Ivor
Catt 22apr02 In
1963 I bought the EH-125 pulse generator. This delivered a –10v step with a
100picosecond fall time into a 50 ohm load (e.g. 50 ohm coax.). The
pulse generator could also deliver a –ve 10v spike with a width of 150psec. I
decided to try to create a positive 10v spike. I cut into the 50 ohm coax,
and joined the incoming inner to the outgoing outer via a red 1uF tantalum
capacitor. I also joined the incoming outer to the outgoing inner via another
1uF tantalum capacitor. Further downstream I found that I had a positive
150psec spike with no discernable degradation (in rise time or pulse width)
compared with the initial –ve spike. That is, I had a +ve 10v spike with a
width of 150psec. It
is interesting to calculate the physical width of a 150 psec wide spike
travelling down normal coax, which has a dielectric with a dielectric
constant of 2. Whereas light travels one foot in vacuo in one nsec, it would
travel 8 inches in material with a dielectric constant of 2. Thus, a 150psec
spike in the coax has a width of about one inch. So I sent a TEM spike with a
width of 1 inch through these 1uF capacitors. [Note 1] Obviously, I kept
their legs short. It is sad that during the ensuing 40 years the New York
IEEE and the London IEE prevented me from informing electronic engineers that
they did not have to add “high frequency” decoupling capacitors to their
logic boards, that the 1uF would do perfectly well on its own. This
obstruction has cost the industry many millions of pounds. However, a bolshie
IEEE and a bolshie IEE cost us a lot more than that in other ways. Ivor
Catt 22apr02 Note 1. Anyone who wants to play
with frequencies can be told that the fundamental of the 150psec spike will
be around 3GHz. Put that in your “self-resonant” pipe and smoke it! IC Note 2. As the spike passes the
capacitors placed to each side, the situation is as in http://www.ivorcatt.com/2_1.htm
Figure 14. The characteristic impedance of each capacitor is very small, less
than 1% of 50 ohms. Thus, the mismatch is less than 2%, causing a minimal
reflection of less than 1%. At the same time, if the
legs of the capacitors are kept down to a total of one quarter of an inch in
length, and the two parallel legs represent a quarter inch transmission line
of characteristic impedance 150 ohms, then the mismatch will cause a
reflection of 50%, see http://www.ivorcatt.com/1_4.htm
Figure 11 and the reflection formula. This will be reduced by the fact that
the 150psec spike covers a distance of one inch and a half, so that the
reflections on entering the 150 ohms region tends to be masked by the
opposite mismatch on re-entering the 50 ohm impedance of the next section of
coax. This reduces the reflection to one sixth, i.e. 8%. @@@@@@@@@@@@@@@@@@@@@@@@@@@ 18may02 More drivel. Fig. 2 at http://www.ultracad.com/seminar_caps.htm Google Hit no. 7 for
“self resonant frequency” + capacitor This article high on the
Google hit list has row of capacitors, and each one decouples (digital
electronic equipment for) its particular frequency range. This farce is obvious if
one realises that a 2uF capacitor is made by glueing together two 1uF capacitors.
Thus, a supposedly “high frequency capacitor” is merely the front little bit
of a 1uF capacitor. Of course, you can ruin the performance of either by
leaving it with long legs, making a series one-turn inductor to stifle its
performance. However, the idea that a 10pF capacitor has shorter legs than a
1uF capacitor is based on nothing at all. What is so tragic is that
the formula these clods use for self resonant frequency, 1/ sqrt LC , means
that if C is big, then the resulting calculated “self resonant frequency” is
low. This is a sensible idea if a resonant circuit is being designed out of a
discrete C and a discrete L, where L can be varied. But if, as in our case,
we (have legs of fixed length and) can only vary the value of C, then the
calculation deludes. If we start with a pair of legs of fixed length, that
is, with a fixed external L, then the bigger the C, the lower the resonant
frequency according to the formula w = 1/ sqrt LC. These buffoons are buying capacitors for the very reason that they
have less capacitance, not that they have less L. They buy these “high
frequency capacitors” for the very reason that, lacking much C but helping
the w = 1/ sqrt LC formula, they are inferior at doing the decoupling job
that they have been bought to do. All their nonsense is counter-productive.
This was pointed out in my book more than 20 years ago. Since digital
electronics took over from radio as the majority of electronic engineering 40
years ago, it is high time the radio men gave us at least some access to the
IEEE, the London IEE and to Cambridge and MIT. Even a single digital
electronics course by someone who understood the subject at either MIT or
Cambridge University would help a lot. I would love to give it. However, I am
sure the radio men will continue to shut me and my colleagues out, as they
have done for a number of decades, hoping that their antique radio theory
will continue to appear to address the needs of digital electronic
equipment. http://www.ivorcatt.com/em_test04.htm Ivor Catt 18may02 Since a capacitor is a
two-conductor transmission line with very low characteristic impedance, the
transient impedance that it presents to a step is resistive, not reactive.
This is the way it behaves when decoupling digital circuits; as a local
energy store for the 5v supply with a very low resistive source impedance,
not a reactive source impedance. Calculation of the impedance is made by
using the normal formula for the characteristic impedance of a transmission
line made up of two parallel plates with width a and separation b. See p73 of
my book “Electromagnetics 1”, pub. Westfields 1975. The (resistive) impedance
is very low because the dielectric constant is very high indeed, and the
separation b is tiny. Ivor Catt 18may02 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 |