EEVblog #453 – Mysteries of x1 Oscilloscope Probes Revealed


Dave cracks out the whiteboard and side cutters to explain two lesser known facts about x1 oscilloscope probes:
– Why the bandwidth is much lower in x1 mode than x10 mode
– Why oscilloscope probe cables aren’t like ordinary coax cables

Forum Topic HERE

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    • John

      Dear Dave,

      Hey is it possible you do a video explaining us WHY transister amplifiers are needed even thought we have op-amplifiers?
      I understand that transsisters provide more power but i know there are many other factors such as noice generated in op-amps and etc.

      Would you care to explain us?
      it would be appreciated by a lot of beginners.

      • f4eru

        Hello John,

        1) Op amplifiers are transistor amplifiers
        2) Op amplifiers are optimized for differential voltage amplification at specific impedances (low at the out, high on the input). That makes a hell of a lot of compromises on bandwidth.
        For high freq, there’s no such thing as an OP amp.

    • Christian

      I love your new Whiteboard 🙂

      • Agreed, needs a few doodles in the corner, a spiders web is traditional – although being Sydney you could put a spider sitting on a funnel…

    • ac

      So maybe this explains why those $2000 audiophile-cables “sound good”. Maybe they attenuate some frequencies? You’d only need a litte attenuation at some harsh sounding frequency for it to be audible to a golden ear person.

      • f4eru

        No, these ones have only a subjective influence based on price and a strange “religion” calles audiophoolery…

    • Paul

      Does anyone know a source for these high impedance loosy probe coax cables?

      • Can’t say I’ve ever seen a supplier of them. But then again I’ve never looked!

    • viewer

      Thanks for posting this great explanation – I had been wondering at the back of my mind what could possibly be causing x10 to be so much better than x1. As with all things, it’s easy when you know!

    • Paul

      Please check the resistance of the black layer over the white insulation layer of the center conductor.
      Is it conducting?
      (I have seen some kind of LOW NOISE coax that had that type of construction.)

    • Wartex

      TL;DR – Center wire in oscilliscope leads is a ~300 ohm resistor.

    • David

      Sometimes you want to limit your bandwidth on-purpose to see (or not see) certain signal components; so flipping the probe switch to the x1 position is a quick and dirty way to do this without having to fiddle with options or switches on the front panel. But – unless your scope automatically compensates the vertical scale by ten when you flip the probe switch (many do, low-end units don’t), you will have to do it manually 🙁

    • Worf

      Thanks for the explanation. I’ve been told about 10x probes and how they always are used to improve bandwidth, but no one ever explained why. I always though it was something stupid like it’s a lot easier to have a higher bandwidth amplifier if the input doesn’t swing as much.

    • michael

      Maybe this has got do do something with signal integrity. To have constant group velocity over frequency the transmission line has to fulfill the heaviside condition from the “telegraphers equations”

      Also using a simple coax as x1 probe will get you into trouble, because there is no 1Meg impedance coax afaik. You always end up in refelctions unless correct termination on both sides.

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