[hpsdr] ALEX - Non Linear Capacitors

Thu Mar 8 10:38:21 PST 2007

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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>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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