[hpsdr] ALEX - Non Linear Capacitors

[Graham Haddock]

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,
>>
>>


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