[volt-nuts] PCBs with ceramic substrates

[Bruce Griffiths]

Hook is merely a manifestation of the variation of dielectric constant with frequency.

It affects the frequency response and transient of resistive attenuators by requiring more complex compensation than merely adjusting a trimmer cap to equalise the low frequency and high frequency attenuation. Since the dielectric "constant" (relative permittivity) of all dielectrics is frequency dependent all dielectrics will exhibit hook to some extent. Some PCB substrates like some versions of FR4 and G10 exhibit a significant variation in the dielectric constant from the dc value to a somewhat lower value for frequencies even in he audio range let alone frequencies of several MHz. Achieving a flat frequency response where the dielectric associated with circuit board capacitances exhibits significant hook is a complex task. Circuit board hook even affects the impedance of printed transmission lines (eg stripline, microstrip, CPW etc).

Bruce

> 
>     On 17 April 2017 at 13:54 cheater00 cheater00 <cheater00 at gmail.com> wrote:
> 
>     Are conformal coatings the right way to handle this?
> 
>     I understand there are kinds of FR4 and G10 that don't have hook. What does
>     one do about hook - how are those substrates improved? How does hook
>     manifest in circuits?
> 
>     On Mon, 17 Apr 2017 03:48 David, <davidwhess at gmail.com> wrote:
> 
>         > > 
> >         FR4 has problems with consistency. Samples can have problems with
> >         hook, dielectric absorption, leakage, and sensitivity to humidity.
> > 
> >         On Sun, 16 Apr 2017 12:08:07 +0100, you wrote:
> > 
> >             > > > 
> > >             Hi Chuck
> > > 
> > >             But the context is "PCBs with ceramic substrates". Are any of *those*
> > >             tough? They may well be, perhaps you know of some? It does not help us
> > >             with the subject much if there are ceramics with these amazing
> > >             properties if they are not available as PCBs.
> > > 
> > >             There is also the question of exactly what properties of FR4 are
> > >             limiting for "metrology" use.
> > > 
> > >             John
> > > 
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> >     >