EMRFD Message Archive 13935
Message Date From Subject 13935 2017-05-30 06:39:55 Tayloe, Dan (Noki... Re: C0G cap variation?
I was trying to buy some Xicon NP0/C0G disk caps from Mouser yesterday and was very sad to see them gone.
I was looking at data sheets for some of the C0G caps that are available. Looking at them was quite enlightening. C0G is defined as +/- 30 ppm, which is really not that great for a free running VFO. However, some temperature vs. PPM variation looked like a “bow tie”. In other words the parts per million variation tended to go to zero near 25 degrees C, with the +/- ppm variation increasing above and below that point. That implies test parts are very stable at room temperature.
Here is an example for some AVX SMD caps: http://www.mouser.com/ds/2/40/C0GNP0-Dielectric-951274.pdf
However, there were other C0G caps that simply fell within the +/- 30 ppm range, but could be anywhere in that range. Here is an example that did not have the “bow tie” shape. See the chart on page 9. This is simply a sloped line with upper and lower bound limits.
If I was trying to build a VFO, this second chart seems inferior to the first, at least for room temperature. However both fall within the “C0G” designation of +/- 30 ppm. When I build a VFO, the core (T37-6) is about 30 ppm. If I assume that the C0G caps are the “bow tie” shape above and are close to 0 ppm at room temperature, then I can use polystyrene (N120, -120 ppm) for VFO temperature compensation if I set things up so that the poly caps are about 25% of the total tank C.
If my inductor is +30 ppm, and I am using 25% tank C with -120 ppm caps so that I get a net close to zero, it makes a difference if my C0G caps are ~ 0 ppm (like the AVX parts) or nominally 10 ppm but “somewhere” between -5 to +15 ppm as this second Yageo chart indicates.
It is distressing to see that most of the manufacturers don’t show the ppm vs. temperature graph. Thus you really do not know what you are getting. I can test some caps to see how they work, but given the +/- 30 ppm specification, the manufacturer could change the dielectric material used at any time. Thus nothing is guaranteed.
I miss my old Xicon disc caps as they always seemed to perform well for a very reasonable cost.
I notice that the Mouser selection of Xicon poly caps (N120) for temperature compensation are drying up. There used to be a wide range of poly caps, but now the smallest value is 150 pf. I also see that Xicon is no longer listed as a supplier in Mouser’s “supplier” list. A lack of parts certainly makes things more difficult.
- Dan, N7VE