Ceramic capacitor value changes drastically with DC bias

Yeah, I got acquainted with that particular problem in the 1980's. High-K ceramics have all sorts of non-ideal behaviour. Capacitance varies with temperature and bias voltage, they are piezo-electric, hysteretic and tend to crack. If they couldn't be made so tiny, no one would want to use them.

There was one thing in your link that *did* surprise me: One commenter claims to have used a batch of caps that would oscillate all by themselves at 900 MHz when biased. I won't believe that until I see it.

Jeroen Belleman

It's been talked about in passing here, where the idea of using a Z5U (or Y) as part of a low frequency VCO.. (Hey Joerg how 'bout a cermaic cap for you 10kHz VCO?)

George H.

You could make a parametric amplifier with them. Power gain would be huge.

Jeroen Belleman schrieb:

Hello,

a parametric oscillator? If I would see it, I would try to remove all semiconductors from that board to see if it still oscillates.

Bye

Scratch, scratch... (google google)... OK I'm not sure how a parametric amplifer works. I 'did' a parametric oscillator in the past, but even that is a bit vague now. (I recall a plot of drive amplitude versus frequency that looked like a semi-circle, with a threshold.)

Got a link (or pic) that might explain it? (It's a mixer type thing, no?)

George H.

.highlandtechnology.com  jlarkin at highlandtechnology dot com

Run high-frequency AC through a cap, and then rectify and drive a load. Vary the DC bias on the cap to change the power transferred into the load.

Parametric amps and oscillators used to be popular for RF stuff, using PN varicap diodes, but regular semis got faster and the paramps faded away.

a-0-33--F-capacitor

It's been discussed. It's a known issue with high dielectric constant ceramics, and part of the reason that you want to avoid them unless you really need the capacitance in the space you can provide.

It's been talked about in passing here, where the idea of using a Z5U

That's sort of a mag amp/saturable reactor approach, which would be easy to do.

A real paramp (one with pump, idler, and signal resonances) might be kind of hard to build, since Y5V caps below about 4.7 nF have basically disappeared, even in the really small sizes. I still have a few hundred

I've never actually built one, but I've always wanted an excuse. I wonder how much gain you could get out of an 0201 capacitor?

Actually, one pretty cool application would be a floating isolation amp. A paramp doesn't necessarily need DC bias, so you can power it just from the RF pump.

Cheers

Phil Hobbs

There were potted-brick parametric opamps that had pretty good specs, Philbrick maybe. The floating front end differential stage was a pair of varicaps pumped at a MHz or so, all transformer coupled. I recall that TI (?) even did an IC opamp with a varicap front-end.

Nonlinear ceramic caps have been used to make shock lines, to speed up the leading edges of pulses. They work in the kilovolt range. I have a paper on that, somewhere.

I have used the effect for moving a filter around. It doesn't get any cheaper than that.

Right now I am looking for capacitors with the worst microphonics because I want to use them ... as contact sound sensors. But has to be low enough in capacitance so I can reasonably squeeze 100Hz bandwidth out of them.

Also, don't automatically assume that X7R is always golden and never has that problem. The very high density ones often do and it can be very tough to pry some data about that out of manufacturers. BTDT, a lot. Murata is pretty good when it comes to furnishing such data so they are currently my preferred source.

Delta-c over C should be independent of C, so if you charge a cap through a big resistor, and look at the voltage, bandwidth should be OK for a big cap.

I did a little testing about ceramic caps generating noise from tapping the board and didn't see anything. We lost phase lock when a front-panel SMB connector was pulled out... turned out to be the XO. We had some tiny springs fabricated to shock isolate the dip14 oscillator.

How about a cheap piezo chirper disk?

I once designed a circuit that crashed a product if the cover was removed, using a phototransistor to detect light. I never used it.

If you take a ceramic capacitor and apply the rated DC voltage across it and heat to above the curie point, when it cools it will exhibit the piezo effect.

With MLCC one never knows. All kinds of weird effects.

When I got the Fluke 8845A, or rather after UPS (literally!) threw it over the backyard fence, I took an older ceramic cap and hooked it up to it. Turned it to AC, all 6-/12 digits of it. Every time I clapped my hands near that cap I could see the excursion.

This meter is nice, no fan. But the relays clicking around when in ohms-mode are a bit annoying.

They are not very good at very low frequencies, sub-Hertz and such.

Or design one where it quits working for a minute every time someone says "s..t!" :-)

I wonder what the feedback sounds like with that.

Like this: Dang!

For customers in the south: Daiang! :-)

High capacity surface-mount ceramics also make good microphones when they've got a lot of bias on them. They're great blocking caps if you have a low-level signal riding on a lot of DC that you want to corrupt with ambient vibration.

I have an 8845A, replacing the horrible Chinese rebrand Keithley I sent back. Great meter, the Fluke.

The relay click is a good audible continuity indicator!

Ah, that's what it's for :-)

It is a great meter but one thing I'll never understand: It's not the slowest on the continuity beep but if I drag the probe across a set of traces too fast it misses. The cheapo Chinese meter doesn't! So for continuity tests I'll keep using one of the Chinese meters. I wish the large companies would talk to their clients some more.

I'll still have to try out LAN and PC operation. It shipped with a little USB-RS232 gizmo. For the LAN I'll wait for warmer weather because it's a crawl space job to get Ethernet there. It came with some sort of Fluke software but (as usual with many US companise ...) that seems to only be some teaser kit.