On 23/02/21 16:56, Jean-Paul wrote:
Tom is the 485 input 50 Ohm free of any capacitance? Or just lower than the others?
The 485 has two attenuators, the normal 1Mohm attenuator used with traditional "high impedance" probes, and a separate 50ohm attenuator. A relay connects the input from the BNC socket to either one or the other attenuator.
In the scanned service manual available at the usual places, have a look as the /two/ diagrams <1> for the "Ch1 attenuators (Hi Z)" and "Ch1 attenuators (Lo Z)". The relay is K151.
Other scopes' 50ohm input is merely a 50ohm resistor connected across the "Hi Z" attenuator with 1Mohm/15pF input.
