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I'm a Mechanical Engineer trying to get into electronics so please excuse my lack of understanding.

I'm feeding a 5.0 Vpp sine wave into my SDS 1202X-E scope from my signal generator (see image below). The frequency response is pretty accurate all the way to 1.0 MHz. Past 1.0 MHz, the frequency response starts to get attenuated all they way down to 1.10 Vpp(1X) at 60 MHz, which is the limit of my signal generator. However, when I switch to 10X then the opposite actually happens: the amplitude starts increasing with increasing frequency.

At 60 MHz the frequency response from the 1X and 10X probe are very similar at 1.10 Vpp and 833 mVpp respectively. I don't think this is an impedance mismatch issue since the issue is only happening at high frequencies. Also, the probes have been properly compensated at 1.0 kHz.

I understand why the signal is being attenuated at 1X due to bandwidth, but why is the oscilloscope reading 833 mVpp at 10X when it should only be reading 500 mVpp?

enter image description here

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  • \$\begingroup\$ Is the signal generator expecting to be driving a 50 ohm impedance? \$\endgroup\$ Commented Oct 1, 2023 at 13:19
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    \$\begingroup\$ Why do you think it's not an impedance mismatch issue? To a 60 MHz sine wave the arbitrary connections along the signal path definitely does not qualify as a decent transmission line. Even if your scope handles 200 MHz, what do your probes handle? Also this is the reason you should not use probes in 1x mode, unless you know what you are doing. \$\endgroup\$
    – Justme
    Commented Oct 1, 2023 at 13:20
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    \$\begingroup\$ A far better quality picture of your set-up is required. \$\endgroup\$
    – Andy aka
    Commented Oct 1, 2023 at 13:51
  • \$\begingroup\$ Correct. The output impedance of the signal generator is 50 ohm. Also, the probes came with the oscilloscope with a bandwidth of 200 MHz as well. \$\endgroup\$
    – user351001
    Commented Oct 2, 2023 at 16:03

1 Answer 1

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Your probe may be compensated at 1 kHz, but it sounds like the high-frequency compensation needs some work--there's some significant peaking there in the 10× mode. Not all probes have adjustments for high-frequency compensation--one that can get peaking this bad definitely should, but if it's a cheap probe it might not.

The fact that the amplitude falls off like that in the 1× mode is fully expected. 1× probes have horrible bandwidth, and generally should never be used if 10× is an option. Those 1×/10× switchable probes really aren't worth bothering with.

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  • \$\begingroup\$ These probes came with the oscilloscope and are rated for a 200 MHz bandwidth. These probes should be able to accurately measure a sine wave at 60 MHZ. Are you saying that I should just get a different probe? I'm thinking about buying a different probe, but I just want to make sure that its not that I'm doing anything wrong before I take that route. \$\endgroup\$
    – user351001
    Commented Oct 2, 2023 at 16:09
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    \$\begingroup\$ @user351001 They might have 200 MHz bandwidth on 10x mode, but I have never seen a 1x probe with a bandwidth better than about 10 MHz. And yes, I would recommend getting different probes; I never feel good about those switchable ones. They just seem low-quality. \$\endgroup\$
    – Hearth
    Commented Oct 2, 2023 at 18:19

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