## Re: Proper Understanding of Oscilloscope Probe Voltage Measurements

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• Alan, Been watching your videos. Good teaching! Stan
Message 1 of 9 , Apr 8, 2013
Alan,

Good teaching!
Stan

--- In Electronics_101@yahoogroups.com, "w2aew" <alan.wolke@...> wrote:
>
> The scope/probe combination will show you whatever voltage is present at that point (provided it doesn't load the circuit down). So, you can probe low impedance nodes as was as high impedance nodes (noting the loading comment). The reason your voltage doubled is because the signal generator was no longer seeing the 50 ohm load it expected. If you terminated the output into 50 ohms, and then probed that with the scope probe, it should show you the correct amplitude.
>
> You may want to check out a few of my videos on scopes and probes, etc. Here are a few links:
>
> Basics of 1x/10x Probes:
>
> AC/DC Coupling on an Oscilloscope:
>
> Analog Oscilloscope Triggering Controls:
>
> I've got >80 videos on my channel that present topics on basic electronics, ham radio, test & measurement, etc.
>
> Enjoy!
>
> Alan
>
>
> --- In Electronics_101@yahoogroups.com, "stan_katz" <stan_katz@> wrote:
> >
> > Hi John,
> >
> > I followed your instructions, and the 475 registers exactly 3 div at the calibrator with a 1x probe, attenuator at .1v/div, or with 10x probe, at 10mv/div. Therefore, the 475 can be trusted. However, I did learn something new. I never engaged DC output on the signal generator. With DC output mode, the output of the signal generator doesn't double on the scope display when I remove the 50ohm termination at the scope. Does that mean the signal generator (3325A) switches out its 50ohm output impedance to 0 ohms output impedance for the DC mode? I understand the voltage divider condition that occurs when the scope is 50ohm terminated and the signal generator is 50ohm output impedance. I still don't have a satisfactory understanding of what happens when I'm using the scope on a test point on one side of a capacitor (ac coupled) and the test point is at an unknown low ac impedance (e.g. 50ohms). I'll get a wrong (too high) voltage there, won't I, just like with the signal generator? This is where I remain confused. It's silly to think that you can only use a scope probe on high impedance test points?!
> >
> > Stan
> >
> > --- In Electronics_101@yahoogroups.com, "John" <jferrell13@> wrote:
> > >
> > > We are missing something here!
> > > With the 475:
> > > power on, switch to channel 1, DC mode, horiz display=A Lock Knobs, Trig mode=Auto, Coupling = DC, Source ch 1, time/div= .5ms, Channel 1 probe to calibrator loop (don't worry about ground, it is all in the same box). Chan 1 volts div = 10m. Use the A trigger level knob to lock in a stable trace. The trig indicator should come on when the trace stabilizes. You can set the displayed trace with the position control knob for chan 1. Since the sync is set to dc the position will affect the setting of the trigger level. If you are using a x1 probe or a straight wire the volts/div of .1 should show a square wave that is 3 divisions high. A x10 probe will require a volts/div setting of 10m for the same display. The horizontal width of the pulses is 1 division.
> > >
> > > Tek has some excellent instruction manuals & videos on their site. They do ask that you register. YouTube is another good source of information. I like to start with youtube because of the variety of presentations. I can make the TEK 400 series work for me, but I don't consider myself an expert!
> > > There may be some flaws in your scope but if you can get through this procedure OK it will be useful. If it all works, you have my pick for the best all around scope. I have a newer TEK storage scope but this is the one I always seem to use.
> > >
> > > As to the signal generator and student scope, believe the 475. It tells the truth.
> > > BTW, the reason for using the X10 probes is to minimize loading or otherwise interfering with the DUT (device under test).
> > >
> > > de W8CCW John
> > >
> > > PS: I have found that totally ignoring rude people works great. Their behavior seems to reflect the value of their responses.
> > >
> > > --- In Electronics_101@yahoogroups.com, "stan_katz" <stan_katz@> wrote:
> > > >
> > > > I've used a student o-scope for years, and then I acquired a Tek 475 and an HP 3325A signal generator. I was thrilled that now I could calibrate the student scope (3325A not good for wideband 475 cal.). I followed the cal. procedure which required that the o-scope end be terminated with signal gen. impedance (50ohms). I performed the cal., and then I removed the 50ohm terminator, put on a 10x probe, and measured the 3325A square wave p-p voltage at the end of the coax cable. It was twice what the 3325A was putting out. The same results with the 475...50ohm terminator..correct voltage..10x Tek probe, double voltage. I'm now confused about what I witnessed. If I want to use my newly calibrated scope with a probe, and I'm measuring a point on a pc board that has a low impedance (e.g. 50ohms) then I get double the voltage on the scope? Or is this just a phenomenon that occurs at the end of a coaxial cable, and any other point on a circuit board that has low impedance will register the right voltage? After all these years I don't know if I've been measuring voltage with a scope properly. I'm fearful that although this group is targeted to assisting hobbyists and neophytes, that this question may be downright stupid. You can understand why I've avoided asking this question in test equipment groups I belong to. I'd be too embarrassed. Hope someone will take pity on me here;-)
> > > >
> > > > Stan
> > > >
> > >
> >
>
• Alan, your videos are great. I started with the Tek 453 49 years ago and I have never seen a better presentation on probes. I am going to watch more of them
Message 2 of 9 , Apr 9, 2013
Alan, your videos are great. I started with the Tek 453 49 years ago and I have never seen a better presentation on probes. I am going to watch more of them because the better I know my tools the more valuable they are. The biggest problem most users have when they first look at the Tek scopes is that they are intimidated by the complexity. If you don't have a simple setup as I offered Stan in your collection it would be a worthwhile addition. A little early satisfaction helps with motivation.

de John W8CCW

--- In Electronics_101@yahoogroups.com, "w2aew" <alan.wolke@...> wrote:
>
> The scope/probe combination will show you whatever voltage is present at that point (provided it doesn't load the circuit down). So, you can probe low impedance nodes as was as high impedance nodes (noting the loading comment). The reason your voltage doubled is because the signal generator was no longer seeing the 50 ohm load it expected. If you terminated the output into 50 ohms, and then probed that with the scope probe, it should show you the correct amplitude.
>
> You may want to check out a few of my videos on scopes and probes, etc. Here are a few links:
>
> Basics of 1x/10x Probes:
>
> AC/DC Coupling on an Oscilloscope:
>
> Analog Oscilloscope Triggering Controls:
>
> I've got >80 videos on my channel that present topics on basic electronics, ham radio, test & measurement, etc.
>
> Enjoy!
>
> Alan
>
>
> --- In Electronics_101@yahoogroups.com, "stan_katz" <stan_katz@> wrote:
> >
> > Hi John,
> >
> > I followed your instructions, and the 475 registers exactly 3 div at the calibrator with a 1x probe, attenuator at .1v/div, or with 10x probe, at 10mv/div. Therefore, the 475 can be trusted. However, I did learn something new. I never engaged DC output on the signal generator. With DC output mode, the output of the signal generator doesn't double on the scope display when I remove the 50ohm termination at the scope. Does that mean the signal generator (3325A) switches out its 50ohm output impedance to 0 ohms output impedance for the DC mode? I understand the voltage divider condition that occurs when the scope is 50ohm terminated and the signal generator is 50ohm output impedance. I still don't have a satisfactory understanding of what happens when I'm using the scope on a test point on one side of a capacitor (ac coupled) and the test point is at an unknown low ac impedance (e.g. 50ohms). I'll get a wrong (too high) voltage there, won't I, just like with the signal generator? This is where I remain confused. It's silly to think that you can only use a scope probe on high impedance test points?!
> >
> > Stan
> >
> > --- In Electronics_101@yahoogroups.com, "John" <jferrell13@> wrote:
> > >
> > > We are missing something here!
> > > With the 475:
> > > power on, switch to channel 1, DC mode, horiz display=A Lock Knobs, Trig mode=Auto, Coupling = DC, Source ch 1, time/div= .5ms, Channel 1 probe to calibrator loop (don't worry about ground, it is all in the same box). Chan 1 volts div = 10m. Use the A trigger level knob to lock in a stable trace. The trig indicator should come on when the trace stabilizes. You can set the displayed trace with the position control knob for chan 1. Since the sync is set to dc the position will affect the setting of the trigger level. If you are using a x1 probe or a straight wire the volts/div of .1 should show a square wave that is 3 divisions high. A x10 probe will require a volts/div setting of 10m for the same display. The horizontal width of the pulses is 1 division.
> > >
> > > Tek has some excellent instruction manuals & videos on their site. They do ask that you register. YouTube is another good source of information. I like to start with youtube because of the variety of presentations. I can make the TEK 400 series work for me, but I don't consider myself an expert!
> > > There may be some flaws in your scope but if you can get through this procedure OK it will be useful. If it all works, you have my pick for the best all around scope. I have a newer TEK storage scope but this is the one I always seem to use.
> > >
> > > As to the signal generator and student scope, believe the 475. It tells the truth.
> > > BTW, the reason for using the X10 probes is to minimize loading or otherwise interfering with the DUT (device under test).
> > >
> > > de W8CCW John
> > >
> > > PS: I have found that totally ignoring rude people works great. Their behavior seems to reflect the value of their responses.
> > >
> > > --- In Electronics_101@yahoogroups.com, "stan_katz" <stan_katz@> wrote:
> > > >
> > > > I've used a student o-scope for years, and then I acquired a Tek 475 and an HP 3325A signal generator. I was thrilled that now I could calibrate the student scope (3325A not good for wideband 475 cal.). I followed the cal. procedure which required that the o-scope end be terminated with signal gen. impedance (50ohms). I performed the cal., and then I removed the 50ohm terminator, put on a 10x probe, and measured the 3325A square wave p-p voltage at the end of the coax cable. It was twice what the 3325A was putting out. The same results with the 475...50ohm terminator..correct voltage..10x Tek probe, double voltage. I'm now confused about what I witnessed. If I want to use my newly calibrated scope with a probe, and I'm measuring a point on a pc board that has a low impedance (e.g. 50ohms) then I get double the voltage on the scope? Or is this just a phenomenon that occurs at the end of a coaxial cable, and any other point on a circuit board that has low impedance will register the right voltage? After all these years I don't know if I've been measuring voltage with a scope properly. I'm fearful that although this group is targeted to assisting hobbyists and neophytes, that this question may be downright stupid. You can understand why I've avoided asking this question in test equipment groups I belong to. I'd be too embarrassed. Hope someone will take pity on me here;-)
> > > >
> > > > Stan
> > > >
> > >
> >
>
• Stan, No matter what you do, the scope, when calibrated and when used within its range, always shows you the correct actual voltage present on its probe tip.
Message 3 of 9 , Apr 9, 2013
Stan,

No matter what you do, the scope, when calibrated and when used within
its range, always shows you the correct actual voltage present on its
probe tip.

The action of attaching/removing a 50 ohm load on an actual 50 ohm
source, will cause the signal to go down or up by 2:1. If the 3325A
is designed (and is itself calibrated) to produce the specified output
level when a 50 ohm load is attached, then that's how IT happens to
work. That is, IT gives you the specified output level only when you
connect the 50 ohm load, because that was how they designed it. I
don't know why switching it to DC output mode apparently causes its
output impedance to change, but apparently it does. So be it.

The point is, the scope measures the voltage on a wire, period.
Whether you attach a load or not, it may affect the circuit by
changing the voltage on that wire, which the scope will show you. The
load affects the circuit, not the scope or its ability to correctly
measure voltages. The scope always shows you the actual voltage
present at the time the probe was connected.

If you are randomly probing points in a circuit, you probably want to
use a probe with a much higher impedance than that of the points being
probed, so that the action of connecting the probe does not alter the
voltage there.

On the other hand, if you are using the scope to measure a signal
source that normally is supposed to be loaded, but its normal load is
disconnected while you are making the measurement, then you want the
scope (which takes the place of the normal load) to have that 50 ohm