Scope and Pk-Pk Values 1x vs 10x

[Dreded]

I am confused... I have never used a scope to check anything other than basic Blink type crap and mainly Serial communication(my usual job is to pass fail based on a serial output.. I dont fix)
so I have a 12v power supply signal that seems quite noisy it comes from an industrial VFD
if I use a 1x probe with scope set to 1x the Pk - Pk on the noise is 168mV
if I use a 10x probe with scope set to 10x the Pk - Pk on the noise is 1.60V

clearly this is 10x of difference.. however since the signal is sitting where its supposed to be shouldn't the noise be relative too?

the noisy signal repeats at ~ 8.8us

[David Hess]

The x10 probe has much higher bandwidth than the x1 probe so more high frequency noise is included in the measurement.

[schratterulrich]

In addition to the higher bandwidth of a x10 probe, there may be another effect.
A x10 probe does not attenuate the shield voltage effect caused by common mode currents.

Try to connect your scope ground and tip together... now touch the shorted probe to ground of your power supply. You should only see the shield voltage. And this voltage is not influenced by the x1 or x10 setting.

See "High-Speed digital design, A Handbook of Black Magic" from Howard Johnson for details.

[dzseki]

Also make sure your head in 10x position is properly compensated using the scope's square wave generator.

[Dreded]

The x10 probe has much higher bandwidth than the x1 probe so more high frequency noise is included in the measurement.

still confused... so the noise is +/- 100mV or +/- 1V? with +/- 1V of noise how can I figure out if my PSU is dropping 1volt for real?

[Dreded]

In addition to the higher bandwidth of a x10 probe, there may be another effect.
A x10 probe does not attenuate the shield voltage effect caused by common mode currents.

Try to connect your scope ground and tip together... now touch the shorted probe to ground of your power supply. You should only see the shield voltage. And this voltage is not influenced by the x1 or x10 setting.

See "High-Speed digital design, A Handbook of Black Magic" from Howard Johnson for details.

ok did that... and yes the same noise is still there its +/- ~ 100mV on x1 reading says +/- 1Volt on 10x both tip and ground connected to scope ground/shorted together

this still leaves me wondering how id possible measure a 1Volt drop from my PSU(which is what I think im experiencing on occasion causing a reset in my circuit) when the Noise on the scope is over 1V drop.

[Dreded]

Also make sure your head in 10x position is properly compensated using the scope's square wave generator.

it is/was, thanks for the suggestion

[bdunham7]

Take 4 readings as follows, using 10ns/div on all, and post the photos if you can.

Disconnect your scope probe ground lead for all 4 measurements.  Then take readings where you previously were using both 10X and 1X (change the scope setting as needed), then take readings again at 10X and 1X where you previously were connecting the ground.

[Dreded]

Take 4 readings as follows, using 10ns/div on all, and post the photos if you can.

Disconnect your scope probe ground lead for all 4 measurements.  Then take readings where you previously were using both 10X and 1X (change the scope setting as needed), then take readings again at 10X and 1X where you previously were connecting the ground.

ok done.. 19/20 are ground and 21/22 are +13V... the noise on 1X is at 8v for some reason(can be seen in trigger level) hopefully this tells you something that is beyond my smarts.

also please not the 1x positive rail reading is at 200mV/div for limit reasons

[radiolistener]

1x probe has 1 MOhm input, while 10x probe has 10 MOhm input.
It may affect your measurements

[Dreded]

so while I have a super noisy environment I think I was hitting the bandwidth of my scope probes(100Mhz probes on 200Mhz scope im sure you can guess why)... putting bandwidth down to 20Mhz solved the noise not matching at 10x issue...(both read ~160mV now)

also solved my other mystery that started all this... here is a shot of my PSU output when drawing a sudden 240mA load..I am grabbing ground off an assertion level pin(that is always asserted at GND with a physical selector switch)(from VFD).. apparently bad idea... if I use the other GND pin thats always GND this doesnt happen :p Woops.

image 27 = 1/100x of pulling power

[TK]

Try AC coupled input.

[tautech]

@ Dreded
Any scope can show nice and not so nice squiggly lines but really they are meaningless unless the scope is set correctly to capture a real measurement.

Simple features for you to practice with are Single, Trigger levels and AC coupling.

Unless you are really switched on and doing this stuff all the time settings are likely to be incorrect. Capturing these for another thread yesterday took a few attempts to get something that looked reasonable and portrayed all the info intended. These are power ON shots of a PSU and ripple checks when driving a 24V 5W bulb that drew 170mA.
Ripple and noise spec for this PSU is max 350uV so any mains ripple is very hard to identify.

Single trigger


AC coupling and Averaging


Having the appropriate menu in the screenshot helps give the reader info to accept the sceenshot to see that something might be wrong and suggest more appropriate settings.
Understand first what you are looking for instead of blindly accepting what's on the display helps immeasurably with scope use.
In my early scope days my mentor said that before you even connect understand first what you are looking for and what should be there ! When it's not then ask yourself why.....so often it's user error. 

[Electro Fan]

@ Dreded
Any scope can show nice and not so nice squiggly lines but really they are meaningless unless the scope is set correctly to capture a real measurement.

Simple features for you to practice with are Single, Trigger levels and AC coupling.

Unless you are really switched on and doing this stuff all the time settings are likely to be incorrect. Capturing these for another thread yesterday took a few attempts to get something that looked reasonable and portrayed all the info intended. These are power ON shots of a PSU and ripple checks when driving a 24V 5W bulb that drew 170mA.
Ripple and noise spec for this PSU is max 350uV so any mains ripple is very hard to identify.

Single trigger


AC coupling and Averaging


Having the appropriate menu in the screenshot helps give the reader info to accept the sceenshot to see that something might be wrong and suggest more appropriate settings.
Understand first what you are looking for instead of blindly accepting what's on the display helps immeasurably with scope use.
In my early scope days my mentor said that before you even connect understand first what you are looking for and what should be there ! When it's not then ask yourself why.....so often it's user error. 

Really good post / advice tautech !

[tautech]

Really good post / advice tautech !

Why thank you EF. 

Well I like to think I have a good handle on the basics but yesterday when delivering a 4 ch X-E to a customer his young EE kept showing me up ! 
But really it was great to see a young technician with such good understanding of the 1202X-E that he had been using for some 18 months and the guy was quite excited that his boss had bought him a 4ch X-E and all the additional advanced functionality they offer.
Only a small company but doing some really nice power electronics development particularly with wireless power transmission.

6KW over 200mm distance is pretty impressive with a goal of reaching 10KW in the near future. 

They wanted to connect the 2 DSO's using Trig Out for timing correlation so to have a total of 6 channels to better observe their power switching behavior.
Now they have a total of 3 Siglent DSO's and a Tek TDS2012C that is long retired to office not bench duties.

[schratterulrich]

For measurements on the power supply a simple and effective way is to use a coax cable instead of a probe. This is possible due to the low impedance of the power supply. A short GND connection (shield of caoax) is essential.

You will get less noise with this method.

For very low frequency measurements you can use the standard 1 Meg input impedance of the scope. With AC coupling you can optimize the vertical setting to see the ripple in more detail. With AC coupling you will see frequencies down to about 10 Hz I think.

For high frequency measurements you have to use a 50 Ohm termination at the scope input. Be aware to not overload the 50 Ohm termination. If your scope has an internal 50 Ohm termination you can use AC coupling. Then your lower frequency limit is about 200 kHz. (At least with my scope)

[Dreded]

just in case anyone runs across this post... dave did a video all about this.. its common mode noise.