[R-390] 390A IF cans/ caps

[Charles Steinmetz]

Gary wrote:

> Digi-Key has 200 PF mica caps 1% tolerance and an F tempo with is essentially no drift.

Not exactly.  "F" tempco is 0 to +70 ppm/K = "+35 ppm/K +/- 35ppm" or 
"35ppm/K +/- 100%."  This is almost exactly the same magnitude as the 
tempco of the ceramic cap (if Larry's data is correct), but of course 
they have opposite signs.  (This is not "essentially no drift.")

Also, note that the combined tempco of the pair of capacitors is NOT 
zero (if it were, we could just use a 239pF C0G/NP0 instead).  Rather, 
it is the tempco that will compensate the tempco of the coil so the 
overall tempco of the LC tuned circuit is near zero.

Best regards,

Charles