As Alan said, it is true that some types of dielectric materials have a dielectric constant which changes with the applied E field, so that the capacitance changes with voltage. However, that is the voltage ACROSS the cap. In this case, the cap is in series with the RF path. It should not normally see much voltage except whatever DC is being blocked. I tested both of these devices at zero volts DC. It would seem strange to me that one manufacturer would take the approach of using a cap which varied its capacitance with DC voltage and therefore had such a varying cutoff frequency as a function of DC bias, but the other one used an NP0-type material. I would be very surprised if they are not BOTH NP0 type caps. There would be too much risk of nonlinear modulation effects when the DC bias was varying. I just checked Digikey and you can get NP0, >200V rated ceramic caps for about US $3 in the 100 to 150nF capacitance range. Two or three of these in parallel would create the capacitance level I measure in the SMA DC block. Sean On Fri, Jun 27, 2014 at 10:47 AM, wrote: > > This area is way beyond me, but there was a discussion on eevblog > recently > > on how capacitor capacitance changed considerably with DC bias > > voltage. I don't recall if this was capacitor type specific. > > An explanation I have had from someone on the technical side of AVX is > that this is typical of X5R/X7R materials, also referred to as type 2 > dielectric. The effect is not as distinctive in NPO materials, also > referred to as Type 1 dielectric, but these are limited to low value > capacitors. > > There are descriptions of the effect in Microchip datasheets for devices > with internal regulators that supply the lower voltage that the core > requires. There is a graph showing the capacitance reduction against bias > voltage for various capacitors they recommend for the regulated supply > bypass cap. > -- > Scanned by iCritical. > > -- > http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > --=20 http://www.piclist.com/techref/piclist PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .