20 Mhz switch on my old analog scope

Discussion in 'General Electronics Chat' started by Sonos, Jan 20, 2005.

  1. Sonos

    Thread Starter New Member

    Aug 23, 2004

    I have an old Tektronic CRT scope (it's huge!) military surplus. Works great for my needs.

    It has a switch on it:

    FULL <> 20 Mhz

    What does this do? I know the noise from this op amp using a 1M 1% resistor as a neg feedback is a lot less when the 20 Mhz is selected.

    How can I make a circuit to do the same thing so I get a cleaner signal before I pump it into an IC instrument amplifier?

    Thanks to any and all replies!
  2. Brandon

    Senior Member

    Dec 14, 2004
    The 20Mhz switch is to do a low pass filter on your signal that is it sampling. It basically cleans up a bunch of noise and gives you a better looking signal on the scope.

    I wouldn't worry about the noise to an amplifier, unless you using medical grade ICs.

    Any standard amp you use that isn't specifcally designed to be wideband, will have a cut off frequency usualling in the Khz range. 20Khz for audio, 60Khz in UltraSonic. Opamps are usually in the 20KHz range, but a few can be a little higher. By the time you reach 20 Mhz, thats an additional 3+ Decades and you'd probably have on the sginal, at about -60dB at worst, probably even lower than this. -60dB would make the signal 0.1% or 1/1000 of its original.

    Scopes are built with much better equipment than normally anything we would work on as a hobby project. If you're really worried about noise, a tiny caps between your signals and ground at various points in your circuit will limit noise. Use a size of 1nF.

    But I really do not think you have anything to worry about , especially since you're using an op amp.
  3. beenthere

    Retired Moderator

    Apr 20, 2004

    Technically, the switch is a bandwith limiter, and is in the vertical amp's feedback path to limit high frequency response of the o'scope.

    What is the model number?