--- In TekScopes@..., "hankc918" <hankc918@...> wrote:
You cannot "change" the input capacitance of any Tek vertical input.
It is what it is.
Use a Tek probe.
HankC, Boston
The scope BW is tested with a 50 ohm source and 50 ohm cable, that's a 25 ohm equivalent source impedance. So 20pF and 25 ohms is a common upper testable BW at the BNC, the scope or plugin may have a lower overall BW. 9pf and 25 ohms is a much higher testable BW for a 1 meg input.
Higher BW 1meg input scopes/plugins have the lower input C. Most scope inputs have a bulk adjustable capacitance at the input to adjust to a standard 20pf input spec for example. Some at 15pf, some 47pf.
One trade off is that a higher input C requires a shorter probe cable and a lower input C could support a longer probe cable for 10x probes having the same probe tip capacitance for example. Most 10x probes use a fixed and adjustable shunt C in the comp box at the BNC end to achieve the LF probe comp flatness. A lower input C also enabled more control over peaking in the probe comp box for the mid and HF probe performance.
You can remove input capcitance in scope/plugin to make the input C smaller as long as you do so at the input side of the attenuators only. You may be able to knock it down by 50%. It is unlikely that your 9pf HP probe will perform well at the high end on a modified input C scope without tweaking the comp box HF components and the scope/plugin amp will be seting the upper BW limit, not your modification.
Hope that helps,
Dale
