Does capacitors cause noise?

[fonograph]

I know about resistor noise,I know about transistor noise,but I am not sure how noisy capacitors are.If we ignore the equivalent series resistance of capacitor,is there some sort of noise that is intrinsic to capacitors?

This is purely theoretical example,I have low noise OCXO with super low noise levels,I pass the output through capacitor,how much will that capacitor degrade the noise performance? Do generaly capacitors add such negligible levels of noise that its safe to say that its ok to not even bother thinking about it?

Will two capacitors cause equal amont of noise no matter what type they are as long as their ESR is same? If my goal is to make as low noise circuit as possible,are some types better than others?

[tszaboo]

Yes. Ceramic Class 2 for example has microphonic effect.
Most of the time though, capacitors reduce noise, since they are there to reduce the bandwidth (in some way/shape/direction etc). And less bandwidth means less noise.

[Gyro]

There are possible noises capable of being generated by capacitors. In practice you are unlikely to be bothered by them in something like a low voltage ocxo circuit:

1. Leakage resistance - all dielectrics have some degree of leakage resistance. Any resistance can generate a noise voltage. At low voltage and with a good dielectric (eg film) this isn't going to trouble you.

2. Microphony - Many capacitors exibit some level of sensitivity to vibration, either piezoelectric (some ceramics) or mechanical shifting of the film (some film caps). Only an issue in very specific circumstances. If your OCXO is super low noise then you might need to look at capacitor selection and mechanical damping. Some capacitors produce audible noise in pulse operation too.

3. Corona discharge - at high voltage, voids in capacitor construction can encourage local corona discharge. Again, not an issue at low voltages.


ESR isn't really an issue unless you're dealing with power circuits, where you won't have the same interest in super low noise anyway.

[GreggD]

I don't think fonograph is talking about mechanical induced noise but thermal / Johnson noise. I do not have that answer but I do want to know how to get a good loud "beep" sound from say a 0805 sized ceramic cap. A normal piezo beeper element is just a ceramic cap that had voltage applied to it when it was fired (hot!!) during manufacturing. These are normally no smaller than 8mm dia. I want something much smaller that is Tape&reel, pick&place.  X5R, Z5U, .01uF, 10uF   There must be a mounting direction (rotation) that is best so a cap that is thicker/thinner than it is wide so you can tell the mounting rotation is a must. Is a slot in the pcb going to help the sound ?  Suggestions anyone ?

[Ian.M]

The pizeoelectric effect will cause a MLCC capacitor to expand/contract dominantly in a direction normal to its layers (which IIRC are horizontal, as crosswise expansion would cause the solder fillet and terminations to fatigue with cyclic voltage change), which would be vertically.  I don't think there's any way of harnessing that to get the board to bend.     If you were to sandwich two such caps, close together, driven in antiphase between two PCBs you could get them to flex tilting inwards/outwards

[fonograph]

There are possible 1. Leakage resistance - all dielectrics have some degree of leakage resistance. Any resistance can generate a noise voltage. At low voltage and with a good dielectric (eg film) this isn't going to trouble you.

what is leakage resistance?   I know what is capacitor leakage and capacitor ESR,but what is leakage resistance I have no idea.

[Kleinstein]

Leakage is modeled with a resistor in parallel to the capacitor. Usually this is not a problem at higher frequencies. It can be a problem if you look at 0.1-10 Hz noise.

Any loss the a capacitor can give rise to a kind of Johnson like noise. However most capacitors are low loss, especially in the higher frequency range. There is more loss in electrolytic caps (not just ESR) and class 2 ceramics. As the loss factor is usually less than 1%, this is normally not a big deal.

Though not strictly noise, capacitors can cause an upset if they have an internal resonance in the frequency range of interest. This can cause fluctuations in the impedance of the "capacitor". Noise like behavior would come in due to thermal variations of board stress.

[fonograph]

So for ultimate low noise performance I want not only low ESR,but also low leakage? Interesting...

[BMK]

I don't think fonograph is talking about mechanical induced noise but thermal / Johnson noise. I do not have that answer but I do want to know how to get a good loud "beep" sound from say a 0805 sized ceramic cap. A normal piezo beeper element is just a ceramic cap that had voltage applied to it when it was fired (hot!!) during manufacturing. These are normally no smaller than 8mm dia. I want something much smaller that is Tape&reel, pick&place.  X5R, Z5U, .01uF, 10uF   There must be a mounting direction (rotation) that is best so a cap that is thicker/thinner than it is wide so you can tell the mounting rotation is a must. Is a slot in the pcb going to help the sound ?  Suggestions anyone ?

Yes possibly PCBMOTOR.com. I thought about misusing their smd piezo elements as buzzers. Never got round to trying though.