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Old 18th Jul 2018, 7:16 am   #45
Radio Wrangler
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Location: Fife, Scotland, UK.
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Default Re: IF fault on Eddystone 770R

NP0 isn't actually the name of ceramic material. It is the code for zero temperature coefficient. It has an allowed range up to +/- 30 ppm per degree C. It follows the coding of things like N750 capacitors often used for temperature compensation.... nominally -750 ppm/C in this case.

It turns out that the ceramics used to get NP0 tempco are pretty well-behaved with high Q and low dielectric absorption, so the NP0 code might as well apply to one material. They are an extremely good choice for RF tuning applications.

Sometimes, though, wily designers pair-up magnetic core materials and capacitor types to cancel each other's tempco. Mica gives about +50ppm/C so a core material with a negative tempco is helpful, and games can be played with the amount of field path length spent in the core versus in air to dilute the core tempco to make -50ppm overall for the coil. Such foxiness in design is pretty much invisible unless you start digging.

Generally, it's safest to replace with the same materials, because the ones where it really matters aren't obvious, even if they're not that common.

For a 465kHz IFT, 50ppm/C in C is going to give 25ppm/C in frequency. 11.625Hz/C so a 50C change will give a 581Hz shift. Unlikely to be noticed!

In a 5MHz VFO, then 6.25kHz is significant, and in a well-designed circuit swapping from mica to NP0 or C0G may spoil the temperature compensation.

David
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