Nonsense about self resonant frequency


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



More nonsense about the so-called “self-resonant frequency” of a capacitor.


Scandals in Electromagnetic Theory

Generally, this is nonsense. In particular,

This approach depends critically on the self resonant frequencies of the particular bypass caps chosen, and also on the ESR (Equivalent Series Resistance.) It turns out, many of the assumptions designers routinely make regarding these issues are not true, as we demonstrate in this seminar is nonsense.

Did the author ever do experiments with capacitors, and is he capable of wiring a circuit together competently in order to avoid strays? 

This brings a score of 0/10 for Douglas Brooks
President, UltraCAD Design, Inc

- Ivor Catt    22apr02

The point of minimum impedance (ESR) marks the frequency at which L and C form a series-resonant circuit, where the inductive reactance equals the capacitive reactance. Above this resonant frequency, the capacitor functions as an inductor. For many applications, the capacitor's series resonant frequency will be a circuit's useful upper frequency limit, especially where the phase angle of the capacitor is expected to maintain a 90-degree (tan ð = 0) or near 90-degree voltage/current relationship. This is a common assumption in filter network design.

Nonsense. The fact that this nonsense has been trotted out for nearly a century does not stop it from being nonsense.  Ivor Catt  22apr02.

This has some nonsense in it about two capacitors in parallel being worse for voltage decoupling than a single capacitor. He is a mess, even though he says that a 1uF capacitor will do fine. He does not understand the nature of voltage decoupling, thinking that each frequency component is individually decoupled. The truth is, the C is an energy source. Frequency is irrelevant.  Ivor Catt 22apr02




Ivor Catt  22apr02

In 1963 I bought the EH-125 pulse generator. This delivered a –10v step with a 100psec 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 150v 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