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New for September 2008! And under construction. Ceramic capacitors are used in almost all microwave assemblies as bypass capacitors (often stabilizing power amplifiers), as DC blocks, as speedup caps in pulsed applications, sometimes as tuning elements in filters. Depending on the dielectric material and geometry they can perform well into the microwave spectrum. However, many failures can be attributed to ceramic capacitors because not everyone takes time to understand them.
In microwave applications we use surface mount capacitors, not old-school caps with axial leads. Remember, if you want lumped element behavior, you want small parts and low-inductance connections.
Avoiding capacitor troubles
In our microwave mortuary there are at least two examples of failed ceramic capacitors. How do you avoid this fate?
First, we recommend that you discuss your application with a technical guy from a reputable capacitor supplier. While you're at it, point them to this page and tell them to sponsor it and rewrite it! These guys know far more than the rest of us, but we'll offer a few pointers.
Temperature coefficient of capacitance can be a problem. Beware, many dielectrics (especially EIA Class 3) can vary tremendously over temperature. Once upon a time we tested a MMIC power amplifier over temperature. Every time it hit 70C it burst into oscillation. Guess what? it was because the stabilization cap on the drain bias was dropping.
Some dielectrics like to have a DC bias across them.
The dissipated power in a ceramic capacitor is a strong function of frequency.
Always over-specify the voltage rating to a goal of 2X
Ceramic capacitor dielectric materials
Wikipedia has a good page on electro-ceramic materials. Our capacitor dielectric "master list" is on this page, but it isn't very comprehensive yet...
(used in "ceramic" capacitors)
Titanium pentoxide (Ti2O5): 41
Niobium pentoxide (Ni2O5): 26
Titanium oxide
Calcium titanate
Barium titanate (very high but not stable...) BaTiO3
EIA classes
EIA Class 1
Ultrastable
Examples: NP0, C0G
Small values (1 to 1000 pF)
Good choice for filters
Low dielectric loss (dissipation factor)
NP0 means positive negative zero, referring to the temperature coefficient
EIA Class 2
Stable
EIA Class 3
Not stable
Very high density.
Future topics:
Dissipation factor, quality factor, or ESR?
How to convert between these quantities.
Power ratings
Derate to 0% at 125C
How to calculate V or I in a microwave circuit?
In many cases the capacitor is used as a bypass element. This is the easiest case to analyze.