Dave,
We warmed up the HP impedance meter of the time (the one that had a
tunable drum as a frequency indicator and topped out at 110 MHz). Sure
enough, *EVERY* CK05 capacitor went purely resistive between 1.4 and 1.6
MHz and inductive above that.
What lead length did you use for that test?????
After my last post I grabbed my box of 100nF capacitors, fired up the NanoVNA (works well even without warming up!), and measured two dozen of them, with lead lengths typical for PCB mounting. Their resonant frequencies all fell in the range of 6 to 8.4MHz. As was to be expected, the smallest ones (leaded ceramic chips) had the highest resonance, and the largest foil capacitors had the lowest, inside that range.
I then measured with full length wires. My longest-legged one resonated at 1.85MHz. That one has 35mm long legs (each), of strongly magnetic material, which probably contributes to add lead inductance.
To get those low resonant frequencies, you must have had very long-legged capacitors, like 5cm, and you must have measured them with full lead lengths. Of course nobody would mount a bypass cap with full-length leads! So what you were getting on those boards must have been much better. Resonating around 7MHz, and producing acceptable bypassing to 30MHz or so, depending on the impedance requirements. In a great many situations that's good enough, specially with older electronics.
