# Difference Between Waveforms When Using An X10 Attenuator

I noticed a thing that I cannot explain on my GDS-1052-U scope: while the probe and the oscilloscope are set on 1x the noise from the LM317 power supply is about 5-6mV pp and while the probe and the oscilloscope are set on 10x, the noise from the same power supply (a LM317 power supply with PNP pass transistor) is about 50-60mVpp (see the images below, in the first image the probe and scope are set on 1x and in the second image the probe and the scope are set on 10x). Is that normal ? I tried to use another probe but it does not change the amplitude of the noise.

1x probe: https://imgur.com/tV0kUqD

10x probe: https://imgur.com/nyeucoi

• What if you don't connect the probe except to short the tip out to the earth wire? – Andy aka Jan 22 '18 at 19:00
• This is displayed on the screen with both channels shorted: 10x imgur.com/PnNIlRc 1x imgur.com/LLHXKZ6 – mike_mike Jan 22 '18 at 19:10
• Just to be safe a dumb question, do you know how the probe settings (1x and 10x) translate to the voltage output on your scope? – user8837440 Jan 22 '18 at 19:11
• If I use a 1x probe, the voltage of the input will be displayed on the scope, while using a 10x probe, the voltage will be 10x smaller than the real voltage. – mike_mike Jan 22 '18 at 19:15
• Exactly, so lets say you measure a voltage of 100V +- 10V with 1x. Using 10x you would get 10V +- 1V. -> The whole Signal is reduced by a factor of 10, amplitude and noise. Isn't this exactly what you are getting? Your noise moves up and down by a factor of 10, would be a strange coincidence i think ;) – user8837440 Jan 22 '18 at 19:20

It must be scope noise and/or cable pickup. I have a Tektronix TDS2024 just back from calibration and I see the same thing with the tip grounded to its ground clip - 1.6mv p-p on 1x and 16 mv p-p on 10x.

This would make sense since 10x probes only attenuate the signal, and the scope just changes the scale accordingly. A millivolt or two of noise is pretty good, and you can use 1x to measure small signals for this reason. So, the answer to your question must be: yes, this is normal.

• I have a question @John Birckhead: did you measured 1.6mV with your scope set on 1x and your probe set on 1x ? And you measured 16mV with your scope set on 10x and your probe set on 10x ? – – mike_mike Jan 23 '18 at 19:07
• Yes, I did exactly as you did. It is a 4-channel scope, and I had the same result on all 4 channels with 4 different probes. – John Birckhead Jan 23 '18 at 19:13

This means the noise is the oscilloscope input noise. The noise voltage is the same at the oscilloscope inputs in both cases, but the displayed value is multiplied by 10x when set up to use a 10x probe.

Usually probes set to x1 have much lower bandwidth than when set to x10.

It is possible that you are getting more peak-to-peak noise at oscilloscope input because the probe set to x10 has a greater bandwidth, hence it lets more noise reach the oscilloscope input.

More or less it goes like this:

$$V_{noise(rms)}= (noise~voltage~spectral~density) \times BW_{probe}$$

assuming a flat spectral density and a flat bandwidth (otherwise you'd need an integral).

Hence the more the bandwidth of the probe, the higher the RMS value of the noise voltage, hence the higher the peaks of the noise.

This is an excerpt from GWinstek GDS1052-U manual (p.126 - emphasis in yellow is mine):

So, assuming you are using one of those probes, in x1 position you have a BW which is more than ten times smaller.

This is a result of the attenuation ratio you are using. At a 1:1 probe ratio, you only get 1x the oscilloscope's front end noise. At 10:1, you get 10x the oscilloscope's front end noise.

The 10:1 probe attenuates the signal by 10x when it comes to the scope, and the scope then has to add in a x10 multiplier to get an accurate reading. It gives you better isolation than the 1:1 probe, but more noise.