O'scope Back to Basics for you guys... a learning game...

[John_P_WI]

A little head scratcher, learning game for you guys with new scopes (and why you have been injecting a too hot of signal into your amps using your DMM).

Playing around with my signal generator and DMM's at my temporary dining room bench, it came to me that a fundamental of scopes is not ever talked about and is a source of confusion.

Why is that my scope shows nearly a 3 times larger signal than the DMM? If I set my signal generator to show a 1 volt signal on the DMM... isn't that what I am supposed to use (I read it somewhere that is what I am supposed to do. )

OK, let's get started, for you seasoned guys hold off a little, let's use this for education...

First of all, my scope is calibrated, 0.5v displays as 1 division at 0.5/div

[IMG576]http://i1148.photobucket.com/albums/o579/john_p_wi/IMG_2015041421771_zpstyce9rbm.jpg[/img]


Second of all, a signal of approx 4 v peak to peak was injected into the scope's channel 1 using a BNC cable and using a Tee a bnc coax was connected to channel 2 scope probe set at 1x. Scope is set at 1v per div for ease. The 2nd channel was inverted showing two distinct waveforms:

[img576]http://i1148.photobucket.com/albums/o57 ... 4zkqtl.jpg[/img]


Huh??? Wait, I have a 4 volt signal, why are the DMM's only showing approximately 1/3 of the voltage??? Certainly 2 DMMs both can't be wrong???

Think about it.... and discuss.

[img576]http://i1148.photobucket.com/albums/o57 ... n3nc01.jpg[/img]

[sluckey]

4V peak to peak = 1.414V RMS

[cbass]

Is it because the dmm is measuring RMS voltage not peak to peak?
I have k ow clue how to use an oscope

[John_P_WI]

Well both DMM's are true RMS, guess that kind of gives is away...

The easiest way to think about it is the way the both traces are shown simultaneously above. Think about 1 wave with the bottom half flipped up to the top half. In the pictures above, simply remove the bottom half of the wave forms (that is why I inverted the second channel) leaving a series of peaks next to each other and touching at the 0 volt line.

Now we have peaks that measure from 0v to 2v peak.

Yes, Sluckey and Cbass gets the attaboy, 2v peak x 0.707 = 1.4 v RMS.

Sooo, IF you are using your DMM to measure a 1v RMS injected signal into your amp you are waaaayy overdriving the front end by nearly 3x compared to a 1 v p to p signal.

[JazzGuitarGimp]

So, let's make this thread even more educational by discussing what real-world pickup output voltages are. I don't know the answer to this and would like to: if we were designing an amp from the ground up, and wanted to know what the signal voltage is at the output of each stage, we'd need to know what the expected voltage at the input is, right? It seems the cathode voltage at the first 12AX7 stage is typically 1.2VDC. This would imply a maximum 2.4V p-p input signal before clipping, but aren't there pickups out there that exceed 2.4V p-p?

[C Moore]

Not that I ever, really, considered it.....but when I (rarely...wish I was better with it) use a scope, I connect it to my handheld Sig Gen. It has a Fine Adjust on it. I turn that pot until I see 1 Volt P to P on the scope. I figured 1 Volt was a "normal" guitar amplitude for some reason.
Kind of glad this was posted, because I have another question.
Speaking of scope basics.....how "important" is the frequency that is used for the test signal.?
I use 520 Hz.
Is it better to use a higher frequency.?
Thank You

[Structo]

I think the 1K frequency is used mostly because it's a good reference point.

[John_P_WI]

I will usually use a 0.5v p-p to 1v p-p input signal. The 1v p-p is a "hot" signal but is easy to do the math in my head when using my analog scope.

I generally tend to use 1khz as this is not attenuated by "treble peaking" circuits and is a clear tone, but of course it is easy to sweep the range, especially IF I want to torment my teenage daughters.

[dorrisant]

Keep going John. I'm glued to this!

Tony

[John_P_WI]

OK, let's have a part 2.

Diodes and their forward voltage. Diodes can be used to clamp voltage, or for clipping. Anyone who has built a distortion foot pedal knows this, let's look at the clamping voltage difference between a DMM and an applied sine wave using a scope with the LED in circuit.

First we measure the forward voltage of a red LED using a DMM and note 1.8v

[img:450:800]http://i1148.photobucket.com/albums/o57 ... x6krok.jpg[/img]

Next we build a little circuit with a 10k resistor in series with the LED. Inject a signal (1khz 6 v p to p) to 10k resistor, put the scope probe between the resistor and LED anode and ground the cathode of the LED.

[img:800:450]http://i1148.photobucket.com/albums/o57 ... 3n1u9i.jpg[/img]

When the sine wave and clipped signal traces are superimposed we get the following. Note how it is clamped to approximately 2v only on the positive side due to the fact that we only used one LED and the orientation of LED anode connected to the resistor. (scope set at 1v/div)

[img:800:450]http://i1148.photobucket.com/albums/o57 ... c3eb5r.jpg[/img]

Now let's look at what happens when we throw a large signal at the LED, something we might see in an amp. Below is a 22v p to p signal from a Wavetek function generator into the same circuit. (scope set at 2v/div)
Note how the clamping is now nearly 5v and the edges are more squared off! Wait, the Fluke says 1.8v Fv

FYI, there will be no change in the nearly 5v clamping if a 470k r is placed parallel to the LED simulating a potentiometer, grid leak or other real world impedances down stream.

[img:800:450]http://i1148.photobucket.com/albums/o57 ... vdmvel.jpg[/img]

Finally, to add a little scope learning, if I'm lazy I like to use the XY function to look at the knee of the diode. The following demonstrates the sine wave of the two traces with the channel display set to XY. Note how the knee is at the same 2 volt clamping level that was shown a couple of images above:

[img:800:450]http://i1148.photobucket.com/albums/o57 ... zlbpp5.jpg[/img]

edit: We can clearly see from the scope shots that we need s signal at least 2x peak to peak the forward voltage of the diode to have clipping. Now think about foot pedals with a single nine volt rail split for opamps -it doesn't leave a lot of working room to throw a "relatively" large signal.

I hope that some good can come from these posts. I would encourage others to join with their tips and tricks. In closing, know your test equipment and experiment, experiment, experiment.

[JazzGuitarGimp]

Sorry y'all. I guess my post was seen ad a hijack, though I thought it fit right in. Whatever....

[John_P_WI]

Hi Lou, your question was perfect. Guitar waveforms are very messy on a scope and hard to interpret, especially on an analog scope. I was rather vague in my answer above, my humbucker based strat (bridge position Rio Grand BBQ) puts out 1 v ptp droning the low E string, then quickly settles down to .5v ptp or less. Of course hammering a chord, the impulse can be several volts or higher.

My strat with callahams, bridge position, is around 0.5v ptp when droning the low E string, then settles down from there to approx .25v to .3v ptp.

Of course sweep pickers like Yngwie will have lower output than a thrasher with high gain humbuckers.

Maybe one of the DSO guys will chime in, I can't play and take a picture at the same time.

I guess this tread is too boring for most, I'm hoping to get a "lunch and learn" thing going here and encourage others to join in.

John

[C Moore]

I wish there were more posts about scopes. Mine does way more than I know.....like deciphering the shape of the wave.
I know what a distorted wave LOOKS like, but I do not know what it means.
Some of our members are Real Good at reading the wave and judging what is going on with an amp, based on what part of a the wave is affected.

I have a "cheap" BK Precision 2120B 30 MHz.
So what exactly is the difference in bandwidth.?
That is to say.....working in Audio (20Hz to 20kHz)...what would be the difference if my scope was 50kHz instead of 30MHz.?

And again, working in just AF, what would be the advantage of having a digital scope.?
Thank You

[John_P_WI]

30Mhz is more than enough bandwidth for audio. I used a Hitachi V212 20 Mhz for years until the sweep messed up. The only advantage of a higher bandwidth scope for audio use is in determining if there are oscillations outside of the audio range - which virtually any scope is capable of displaying.

To answer your question, if your scope was rated at 50 kHz and an oscillation was in effect at 100 kHz your scope would not trigger or display the oscillation as it is at twice the frequency of the scope's bandwidth.

[schaublin65]

Hi

oscillation above the rated bandwidth might well produce a "fuzzy trace" on a good analogue scope.

The rated bandwidth is not a hard cut off for information

take care...

John