Oscilloscope Probes.... What the hell am I doing wrong?

[ptp170]

Ok, been trying to figure out why my voltage readings on screen with a new set of eBay probes is so wrong.  Have watched Dave's EEV Blogg on YouTube which describes the extra inline resistance in the coax which was a help but even so I'm at a bit of a loss so I decided to take some resistance measurements of certain parts of the probe.


It's a x1/x10 switchable probe:

From probe tip to BNC tip settles at 8.97Mohms on X10 and 250Kohms on X1.

From probe tip to the point where the coaxial cable enters the small circuit trimmer board is also 8.97Mohms on X10 and 121kohms on X1.

The only visible SMD resistor is on the trim board and is 130ohms....

Shouldn't these values be 1Mohm and 10Mohm?

If I'm simply being a idiot over this then please don't hold back!
TIA

Chris

[c4757p]

If I'm simply being a idiot over this then please don't hold back!

Careful what you wish for.....

Your resistance measurements look good. 121k is a bit high IMHO, but to be honest I've never measured the resistance of a switchable probe on X1 mode. 8.97Meg is spot on, it should be 9Meg*. Are you sure it's the probe and not the scope? Stick a wire gently straight into the BNC jack and feed a voltage in that, see if it displays right.

*Consider that the scope has a 1M input impedance, so probe + scope = 10M. If you apply 10V to that system, 1uA will flow, giving 1V across the 1M resistor.

[ptp170]

Ok... I've fed a 10v DC feed directly into the BNC. And the scope is displaying circa 20v....

[ptp170]

If I add a 1Mohm resistor to the end I get just a smidge under  10v displayed.....

[c4757p]

Wow. That's impressively bad. Have you double-checked every single control on the scope? What kind of scope is it?

[Andy Watson]

If I add a 1Mohm resistor to the end I get just a smidge under  10v displayed.....

Have to ask, ... have you tweaked the compensation to match the probes to your scope?

[ptp170]

It's an ISR-Tech ISR622 20MHz dual channel scope. V/Div set to 1V. Voltage displays 4 'boxes' high ( 20 divs high).  There is also a VAR. Control per channel which alters from 1 to 2.5x.... Currently it is left turned down to '1'...

[ptp170]

If I add a 1Mohm resistor to the end I get just a smidge under  10v displayed.....

Have to ask, ... have you tweaked the compensation to match the probes to your scope?

Yes. In x10 mode I compensated the probes using the inbuilt 1KHz 2Vp-p square wave...

[c4757p]

It's an ISR-Tech ISR622 20MHz dual channel scope. V/Div set to 1V. Voltage displays 4 'boxes' high ( 20 divs high).  There is also a VAR. Control per channel which alters from 1 to 2.5x.... Currently it is left turned down to '1'...

Each "box" is a "div" - not the little tick marks. You're counting them wrong.

[ptp170]

It's an ISR-Tech ISR622 20MHz dual channel scope. V/Div set to 1V. Voltage displays 4 'boxes' high ( 20 divs high).  There is also a VAR. Control per channel which alters from 1 to 2.5x.... Currently it is left turned down to '1'...

Each "box" is a "div" - not the little tick marks. You're counting them wrong.

Noobie mistake number 1.... In that case then it's displaying 4v for a 10v DC feed?

[c4757p]

It would appear so. That's bizarre as well, but a bit more easily explained perhaps, since it's reading low... are you sure that the VAR control is set correctly? Consider that the knob on the front could be misaligned with the actual pot and switch inside - actually rotate it.

[grumpydoc]

It's an ISR-Tech ISR622 20MHz dual channel scope. V/Div set to 1V. Voltage displays 4 'boxes' high ( 20 divs high).  There is also a VAR. Control per channel which alters from 1 to 2.5x.... Currently it is left turned down to '1'...

OK, there's one thing - "1 Div" on the 'scope is always 1cm so at 1V/div with a 10V input really the trace should be off screen - i.e it's still wrong but the other way.

Are you sure the VAR control is set to the CAL mark, since it appears to be under-reading by exactly 2.5x which is the maximum value for the VAR control.

[ptp170]

Partial declaration of stupidity 

I've had the scope placed on the bench at such an angle that the 'CAL' markings aren't visible   
This coupled with the misinterpretation of actual 'divides' has led me on a self fuelled mission of idiocy 

So.... it appears that the VAR. controls attenuate by >x2.5 and not the other way around??

[grumpydoc]

So.... it appears that the VAR. controls attenuate by >x2.5 and not the other way around??

Yes, the vertical VAR controls always attenuate the signal, never amplify it.

[ptp170]

So.... it appears that the VAR. controls attenuate by >x2.5 and not the other way around??

Yes, the vertical VAR controls always attenuate the signal, never amplify it.

Noobie mistake number 2 then....


I'll just pop in to a field and shoot myself   

And nobody called me an idiot.... most disappointing 

Thanks for pointing the Ape towards the stone tools 

[Fraser]

Everyone has to learn by their odd mistake or two 

At least you knew something was wrong ! The scary people are those who use test equipment without questioning results, considering the equipment infallible. As in this case, always have an idea on what you expect the test equipment to indicate in terms of measured values before trusting such.

You have done well to work through this problem using a known input voltage. Some newbies might have just dived into their project and become totally confused by strange readings.