Quote:
Originally Posted by regenfreak
... I should not get hung up with trying to get input capacitance as low as possible.
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There is nothing about "as low as possible".
From the equation (1) we get the maximal allowed Cin for any tolerated max. detuning of your choice, and the example given with a typical value C of LC tank shows the range of required Cin values.
An excessive detuning when probing the shape a single LC tank is not a big problem, but trying to align coupled tanks could be challenging, and especially multi-stage filters seem to fall into category "mission impossible".
I could be wrong now ...
Quote:
Originally Posted by regenfreak
The HP and Tektronix active probes have input capacitance of 0.7pF to 2pF.
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Such probes are designed for looking at logic or other circuits with moderate or low impedances, not for probing Hi-Q LC tanks. Beside detuning, some of them will introduce measurable damping at 100MHz.
Quote:
Originally Posted by regenfreak
Interesting, how did you come up with those figures? Using Spice?
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No need for Spice. The figures come from the BF998 data, reading the plots and calculating simplified circuit models.