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Jane Lince, 28 Penryn Road, Walsall, WS5 3EU 20 December 1997 LETTER TO THE EDITOR The Editor, Electronics World, Quadrant House, The Quadrant, Sutton, Surrey SM2 5AS Dear Sir, Misunderstanding Capacitors Well done, Cyril, in
his January 1998 letter, for squaring the circle. Everyone is correct, and we
all live happily ever after in a deluge of prestigious confusion. "Indeed
I first proposed in internal company reports that capacitors be considered
transmission lines as long ago as 1968 while employed as design engineer for
electrolytic capacitors at the old Erie company." - Cyril, EWW, January 1998, p45 However, when
Cyril quotes from the 1975 catalogue, the phrase "transmission
line" is missing. It only contains the unremarkable idea that leads have
inductance, and says that the capacitor also has "inherent
inductance", directly contradicting Catt 1978; "Series inductance
does not exist". This response in
e-m theory is discussed by Oliver Heaviside, who developed the concept of the
transmission line, but failed to apply it to the capacitor, as Cyril did
later in secret in 1968. Heaviside wrote; "If
you have got anything new, in substance or in method, and want to propagate
it rapidly, you need not expect anything but hindrance from the old
practitioner - even though he sat at the feet of Faraday..... he is very
disinclined to disturb his ancient prejudices. But only give him plenty of
rope, and when the new views have become fashionably current, he may find it
worth his while to adopt them, though, perhaps, in a somewhat sneaking
manner, not unmixed with bluster, and make believe he knew all about it when
he was a little boy!" Oliver Heaviside, "Electromagnetic
Theory Vol. 1", p337, 1893. Please, Cyril,
publish the Erie 1968 internal memo, where, as you say, the capacitor as
transmission line is mentioned. Also perhaps comment on its implication for
the concept of Displacement Current, which plays no role in transmission line
theory, and compare with Wireless World December 1978. You deserve the credit
for an historic first, wrongly attributed to Catt and Co. But memory is
insufficient, as O.H. implied. Yours sincerely, Jane Lince. (A lynx is a big cat.
Neither type of cat was allowed to publish in Eccles’s EW. The new editor is
making up for lost time; see EW aug02, jan03, perhaps feb mar03. Ivor Catt
dec02) Ivor Catt 121 Westfields, St. Albans AL3 4JR 20dec 1997 LETTER TO THE EDITOR The Editor, Electronics World, Quadrant House, The Quadrant, Sutton, Surrey SM2
5AS Dear Sir, Misunderstanding Capacitors Cyril (EW dec97 p1001
and jan98 p45) seems to think that when in a hole, he should dig. More or
less the whole world is now digital, and nonsense deriving from rf days must
not continue to cost digital systems dear. A capacitor does not
have a series inductance, and the best decoupling capacitor for the 5v supply
is 1uF tantalum, which has no series inductance to limit the
capacitor's ability to decouple. "At any given frequency, the [reactive]
term .... can be simplified into its series equivalent" (Cyril in dec97)
is rubbish, however often it has been taught and written. (The capacitor has
no ESR either. These punka-wallahs have trotted out this rubbish far too long,
from their lecture notes and text books. Losses do not follow from such a
lumped term, any more than they do in your TV aerial feeder cable, which is a
long capacitor with narrow plates and poorly chosen dielectric.) I put a 150psec wide,
10v spike generated by the E-H 125 pulse generator through a 1mF tantalum capacitor buried in the
inner of a 50 ohm coax. There was no degradation. This means we must ban the
'series L' mythology. You can see such spikes in photographs in my dec1967
paper in IEEE TransComp EC-16 No. 6. There is no series L in a capacitor,
and, like any other transmission
line, a capacitor's initial response is resistive, not reactive. (This
resistance is a very low, lossless characteristic impedance, not ESR.) ".... any two
conducting surfaces are both a capacitor and a transmission line - albeit
possibly of very short electrical length." (Cyril jan98) is misleading.
Since u is so high, the propagation velocity across a capacitor is very slow,
so the "electrical length" is long. These facts about a
capacitor are unknown to Cyril and the rest working in companies
manufacturing and testing capacitors. At Erie in 1968, Cyril only tested
capacitors on an a.c Wheatsone Bridge at 5kHz and 50 kHz, and merely deduced
their performance at higher speeds on the basis of an assumed L - R - C
model. ".... better measurement equipment .... now commonplace ....
" (Cyril jan98) presumably means a breathtaking rise from 50kHz to 500
kHz on the same Wheatstone Bridge. We must escape from these antiquarians
with their silly maths in today's era of 1 nsec logic. Cyril thinks we can
blame a capacitor for its leads. Imagine leaving long leads on a high speed
transistor, and claiming it's much slower! However, in that case, less would
be sold. These capacitor salesmen get us to buy a further 'rf' capacitor to
boost the big one's alleged shortcomings, which result solely from its long
leads. There's no money to be made out of telling the customer to crop the
leads. Yours sincerely, Ivor Catt |
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