[hpsdr] ALEX - Non Linear Capacitors

Thu Mar 8 12:56:55 PST 2007

That makes sense, and there's no need for any non-linearity to have a
problem :-).

On 3/8/07, Pieter Ibelings <sietetrescincoprimo at hotmail.com> wrote:
> Larry,
>
> Even if the capacitance vs. voltage curve is perfectly linear, it will still
> cause crossmodulation between signals on the same filter. As one big signal
> modulates the capacitance, the small signals will get modulated by the
> varying reactance.
>
> Pieter
>
>
> >From: "Larry Gadallah" <lgadallah at gmail.com>
> >To: grahamh at verizon.net
> >CC: hpsdr at hpsdr.org, "Pieter Ibelings" <sietetrescincoprimo at hotmail.com>
> >Subject: Re: ALEX - Non Linear Capacitors
> >Date: Thu, 8 Mar 2007 10:23:16 -0800
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> ><45F041CA.4080107 at verizon.net>
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> >
> >Graham, Pieter:
> >
> >I understand the voltage coefficient of capacitance, at least
> >conceptually, but what surprises me is the idea that the relationship
> >between applied voltage and effective capacitance is significantly
> >non-linear. In fact, if you look at curves for varactor diodes that
> >exploit a similar phenomenon, they are very linear. Perhaps the
> >problem with passive front end passive LC filters comes more from the
> >SMT inductors than the capacitors? IMHO, the best bandpass filters
> >would contain axial lead NPO caps as mentioned below and conventional
> >toroidal inductors. Naturally, these are both (relatively) large and
> >expensive :-).
> >
> >Cheers,
> >
> >On 3/8/07, Graham Haddock <grahamh at verizon.net> wrote:
> >>There were several war stories told in the ALEX
> >>discussions about field problems with "non-linear"
> >>capacitors and the resulting degradation of IP3
> >>in a preselector.  Someone said that they did not
> >>understand how a capacitor could be non-linear.
> >>
> >>So I did a little research.
> >>
> >>Well, if your mama told you that capacitor values
> >>stayed constant with voltage, she didn't tell you
> >>the whole truth.
> >>
> >>Did you ever wonder what all the different kinds
> >>of dielectrics in the chip capacitors in the
> >>Mouser Catalog were all about?
> >>
> >>Did you ever hear of "Voltage coefficient of Capacitance" ?
> >>
> >>Read the following single page ".pdf" files.
> >>
> >>http://www.niccomp.com/Products/General/NPOvolt-capcoeff1200.pdf
> >>
> >>http://www.niccomp.com/Products/General/X7Rvolt-capcoeff1200.pdf
> >>
> >>http://www.niccomp.com/Products/General/Y5Vvolt-capcoeff1200.pdf
> >>
> >>
> >>Well, NPO (C0G) capacitors are what you thought a capacitor
> >>was all about.  This is EIA Catagory I - Ultra Stable
> >>
> >>The XR7 and Y5V capacitors are EIA Catagory II - Stable
> >>
> >>Apparently "Stable" is a relative term.  ;-)
> >>They could be used as varactors !
> >>
> >>If you put anything other than an NPO (C0G) capacitor
> >>into a tuning or critical filter circuit bad things
> >>can happen.
> >>
> >>XR7 capacitors work great for bypass caps.  If you use
> >>them for RC bandpass filtering at baseband, some goofy things
> >>could happen.  They are sold in values down to 47 pF.
> >>Beware.
> >>
> >>--- Graham / KE9H
> >>
> >>==
> >>
> >>Pieter Ibelings wrote:
> >> >
> >> > Larry,
> >> >
> >> > Most NP0 and C0G capacitors have extremelly low distortions. I have
> >> > performed tests on these type caps with as many as 132 carriers and
> >> > 0dBm each and the performance is very very good. As you start going up
> >> > in value, the materials required to increase the capacitance per unit
> >> > area become more and more unstable and non linear. Some of these
> >> > materials exhibit a capacitance over voltage characteristic. So as the
> >> > voltage on the cap varies, so does the capacitance. This tends to
> >> > created mixing products since a change in capacitance (modulated by
> >> > one carrier's voltage) causes the whole filter/resonator to be
> >> > modulated in phase and amplitude.
> >> > An even bigger problem are the ferrite beads that are used in the high
> >> > value chip inductors. These can have very non linear characteristics
> >> > specially if the core is conductive and the enamel coated wire touches
> >> > the core.
> >> > The powder iron cores from Micrometals are also extremelly good as
> >> > well as the wirewound surface mount inductors that have no ferrite.
> >> >
> >> > Pieter, N4IP
> >> >
> >> >
> >> >
> >> >> From: "Larry Gadallah" <lgadallah at gmail.com>
> >> >>
> >> >>
> >> >> Hi all:
> >> >>
> >> >> One more comment regarding bandpass/lowpass/highpass filters: Using
> >> >> very small SMT components for these may cause problems with IM,
> >> >> dynamic range, and non-linear operation. I had an early model of the
> >> >> AOR AR7030 receiver, and there were problems in the 1.6 - 2.5 Mhz
> >> >> range because of SMT parts in the front-end bandpass filters and the
> >> >> high signal levels found at the lower end of this range (i.e. MW
> >> >> broadcast stations). AOR issued an ECO
> >> >> (http://www.aoruk.com/7030bulletin.htm#ip3) whereby they replaced the
> >> >> SMT components with normal types and this solved the problem. I have a
> >> >> hard time visualizing how a SMT capacitor becomes non-linear, but
> >> >> apparently they do.
> >> >>
> >> >> Cheers,
> >> >>
> >> >>
> >>
> >>
> >
> >
> >--
> >Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT
> >com
> >PGP Sig: 616D 4E52 CF1F 3FEC FFFB  F11B 7DB9 C79A EA7E B25B
>
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-- 
Larry Gadallah, VE6VQ/W7                          lgadallah AT gmail DOT com
PGP Sig: 616D 4E52 CF1F 3FEC FFFB  F11B 7DB9 C79A EA7E B25B

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