TimeFox can you explain plz what does "it won't meet the 200 MHz specification" mean
Attached is the result of a Spice simulation of the AC response for a simple probe.
The circuit comprises:
1.0 megohm and 20 pF scope input (typical values at panel BNC)
6 nsec of 50 ohm coax (approximately 4 feet of RG-58/U at 66% velocity), which has 120 pF capacitance at low frequency.
9.0 megohm and 15.5555 pF (140 pF/9) between probe point and coax input.
Note that the attenuation at low frequency is 20 dB, as expected, but we get lower attenuation above about 20 MHz, and serious resonant problems above 50 MHz. These resonances are in the improperly-terminated 50 ohm coax.
With 2 feet of cable (3 nsec), the effects kick in around 100 MHz, and still mess up the circuit at 200 MHz, as shown in the second graph.
Note that Spice does not allow a frequency-dependent attenuation in the transmission-line model. In reality, the resonances won't be quite this strong, due to loss in the coax at high frequencies.