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I would like to make my 300 MHz oscilloscope behave more like a 50 MHz oscilloscope. What sort of a BNC adapter or probe would let me do this?

Specifically, I am interested in modifying how different frequencies are attenuated. With my 300 MHz oscilloscope, a 300 MHz sinewave is 3 dB lower in amplitude than the actual wave. With this adapter or probe I would like to have a 50 MHz sinewave to be 3 dB lower. I want the bandwidth vs. frequency curve to remain similar to what you normally get with an oscilloscope rather than a "hard" filter of all frequencies above a certain value.

The exact 300 MHz and 50 MHz numbers are provided as examples for this question. I might end up targeting a different bandwidth than 50 MHz.

I think another way of wording this question would be: What sort BNC adapter or probe would let me reduce the rise time of my oscilloscope?

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    \$\begingroup\$ Most high bandwidth scopes have a "bandwidth limit" function built-in. But you certainly can purchase BNC in-line low-pass filters as well. \$\endgroup\$
    – Dave Tweed
    Commented Jul 23, 2023 at 14:43
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    \$\begingroup\$ Note that if you buy a 50 MHz BNC low pass filter, it will probably be 50 ohm, meaning you must provide a 50 ohm load resistor after. If you don't have a 50 ohm setting on your scope, buy a 50 terminator (few dollars on Amazon). Otherwise the cutoff frequency will be different. \$\endgroup\$
    – user1850479
    Commented Jul 23, 2023 at 14:45
  • \$\begingroup\$ If the probe has a x1/x10 switch, the x1 position is likely to give you pretty close to the BW you require. \$\endgroup\$
    – user16324
    Commented Jul 23, 2023 at 14:46
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    \$\begingroup\$ @DaveTweed Usually, bandwidth limit functions are restricted to a few set bandwidths; I've always seen the options as 20 MHz, 200 MHz, and 1 GHz (presumably there may be a 2 GHz one etc. if you use a really high bandwidth scope). I haven't seen any that give a 50 MHz option. (I wonder if there are scopes that let you set an arbitrary custom bandwidth....) \$\endgroup\$
    – Hearth
    Commented Jul 23, 2023 at 15:14

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I think another way of wording this question would be: What sort BNC adapter or probe would let me reduce the rise time of my oscilloscope?

It sounds you need a low pass filter that has a cut-off of 50 MHz. Whether you can buy one installed insider a BNC adapter is a different think entirely. You could make one that fits between your oscilloscope probe and the oscilloscope. It depends on the probe type and your oscilloscope input impedance and, you should be able to get-away with just a parallel capacitor of maybe 100 pF to 200 pF.

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