what an oscilloscope recommended for a woman passionate about electronics?

[rstofer]

In audio there is a measurement called 'total harmonic distortion - THD' and those offending harmonics show up on the FFT of the signal.  About half way down the page:

https://www.gamry.com/application-notes/EIS/total-harmonic-distortion/

[tautech]

Oh and another thing concerning fundamentals and harmonics, a little thing but important all the same for the sake of correctness.

The fundamental frequency IS the 1st harmonic, subsequent are 2nd, 3rd and so on.

^^ something learnt very recently here on the blog from a wise old tech and confirmed on other websites, even Wikipedia. 

[CharlotteSwiss]

thanks, you are very kind 
tomorrow afternoon I read carefully what you advised me, the work is looming ..
 

[borjam]

it is always useful to know how to understand an FFT spectrum, so I wanted to understand how it works, at least the basic things.
I imagine that harmonics can be a source of disturbance in many signals

But beware, a harmonic is not always a disturbance.

If your intended signal is a quare wave, for example, those N frequency multiples with a 1/n amplitude are not disturbances!

[rstofer]

With Tautech's comment re: time/div, I created a much better looking FFT of the 1 kHz fundamental at 1Vp-p + 3 kHz at 0.3Vp-p + 5 kHz at 0.2Vp-p, identical to what I posted earlier.  Given more cycles on the screen, the FFT is much more useful.  The trace itself isn't all that useful at the time/div setting but that's ok.  Look at the waveform, then look at the spectrum.

The x-axis scale for the FFT is 500 Hz/div so the first spike is at 1 kHz, the next at 3 kHz and the last at 5 kHz - exactly what I created with the AWG. 

[dietert1]

For real measurements, i mean when you don't know the result before, these FFT calculators are of limited use. You will see a very high noise level and random results. What i missed in our Rigol scope is a way of averaging multiple FFTs to get the noise down. Until now i diid not discover anything like that in the menus.
Our old Lecroy DSOs 9314 and 9354 support averaging spectra, so the results are more useful.

Regards, Dieter

[rstofer]

I don't know if it does the same thing but the Rigol DS1054Z has an Acquire->Mode->Average along with a couple of others.

One of the other features is some kind of ''Windowing' and this tends to clean up the display.  For no particularly good reason, I chose 'Hanning' and I have attached the screen image

[tautech]

With Tautech's comment re: time/div, I created a much better looking FFT of the 1 kHz fundamental at 1Vp-p + 3 kHz at 0.3Vp-p + 5 kHz at 0.2Vp-p, identical to what I posted earlier.  Given more cycles on the screen, the FFT is much more useful. 

Certainly and you can give the FFT lots more data points to work with yet. Unlike an analog scope DSO's are only as good as the amount of info they have to work with.
Some simple examples with my SDS1104X-E that has exactly the same FFT as Charlotte's.
1 KHz 1V sine wave ex SDG6202X AWG.
Looking at the fundamental and harmonics it was decided to set CF to 4 KHz and use a 1 KHz/div horizontal scale and reduce FFT points to 256K for faster refreshes.

For real measurements, i mean when you don't know the result before, these FFT calculators are of limited use. You will see a very high noise level and random results. What i missed in our Rigol scope is a way of averaging multiple FFTs to get the noise down. Until now i diid not discover anything like that in the menus.
Our old Lecroy DSOs 9314 and 9354 support averaging spectra, so the results are more useful.

Regards, Dieter

Certainly, and in these little X-E Siglents we can eliminate the noise floor with either Max Hold or Averaging as required.


Now with the noise floor eliminated we can change the vertical scale to see more of the signal.


And enable markers to Peaks.


And finally inspect the fundamental in better detail.



FFT exercise for the willing.
From your stash find an AC low voltage wallwart (for safety) and do all the above....fundamental frequency will be whatever your local mains frequency is and harmonics for course multiples of it.
If the widely experienced mains sine wave distortion is prevalent in your area it should make for some interesting FFT's.
Tips.
Adjustment of parameters using the multi function encoder can be made easier by pressing it and using the virtual keypad that appears. Don't forget to select the unit designator and press the encoder to enter/exit.
 

[CharlotteSwiss]

FFT pro tip for Siglents: use a slower timebase setting to get many cycles of the waveform on the display then enter a new world of FFT results. 

With Tautech's comment re: time/div, I created a much better looking FFT of the 1 kHz fundamental at 1Vp-p + 3 kHz at 0.3Vp-p + 5 kHz at 0.2Vp-p, identical to what I posted earlier.  Given more cycles on the screen, the FFT is much more useful.  The trace itself isn't all that useful at the time/div setting but that's ok.  Look at the waveform, then look at the spectrum.
The x-axis scale for the FFT is 500 Hz/div so the first spike is at 1 kHz, the next at 3 kHz and the last at 5 kHz - exactly what I created with the AWG. 

I did the test too: usually 1khz compensation signal fundamental, with three different sec / div = fast 500us / medium 2ms / slow 10ms.
I must say that the FFT chart I prefer is not the one with slow time, but medium 2ms ..

[dietert1]

I don't know if it does the same thing but the Rigol DS1054Z has an Acquire->Mode->Average along with a couple of others.
..

Thanks for your hint. Yes, averaging in the time domain does reduce noise as well if you have a clean signal in the time domain that makes for a good trigger.
But again that's not the "broadband receiver" you want for detection of small signals. Last week i tried to measure the output noise of a linear lab power supply. No valid result yet, except i saw the DSOs own power supply and some prominent lines at multiples of 32 MHz.

Regards, Dieter

[CharlotteSwiss]

I created a much better looking FFT of the 1 kHz fundamental at 1Vp-p + 3 kHz at 0.3Vp-p + 5 kHz at 0.2Vp-p

rstofer, not having a generator yet, I would have a curiosity: but in practice they are 3 separate signals, which then the generator adds into a single signal? do you have a settings screen on the siglent when you create this signal? so I understand better then what we see in your FFT .. maybe it would be useful to widen the vision band and then see the harmonics going down to the right ...
thank you 

[CharlotteSwiss]

But beware, a harmonic is not always a disturbance.
If your intended signal is a quare wave, for example, those N frequency multiples with a 1/n amplitude are not disturbances!

yes of course, I think those harmonics are used to have a square wave. I was talking about harmonics maybe with very high peaks that maybe create disturbances 

[CharlotteSwiss]

By convention, the first harmonic IS the fundamental, 220 Hz.  The second harmonic is 440 Hz, etc.

ok, I expressed myself wrong: the first harmonic is the fundamental, indicated with f (or f1), then the second harmonic f2 etc .. (always with multiple frequency)

Those coefficients are correct (after scaling) for a square wave but may not be anywhere close for some other waveform.  Here is a video discussing the FFT of a sawtooth.  Note that it has both even and odd terms for the sin() while the cos() terms drop out.

the video is terribly into math 
But I understand, the operation f / 2- f / 3 etc .. is valid for the square wave, it may not be accurate for other waveforms ..
But let's see the thing in general, and therefore I can say: in the FFT graph, the harmonics to the right of the fundamental will have an amplitude that from time to time will become less and less ..
Returning to the square wave of my example, the reference value for the amplitude is db .. but it can be said that if the fundamental harmonic has an amplitude of 2v, the second harmonic will have an amplitude of 1v, the third of 666mv etc ...

Engineers are a lazy bunch and they don't like carrying a lot of digits.  Assuming a voltage ratio, the dB value is 20 log (voltage divided by reference).  Let's assume a reference of 1V.  In this case, if the voltage is 10V, the db is 20 log 10 or 20 dB.  If the voltage is 100 the dB is 20 log 100 or 40 dB.  200 dB is a HUGE number:  200 = 20 log (V) so V is 10 to the 10th or 10_000_000_000 or 10 billion.  I'm not sure what the scale is all about on that FFT but it probably isn't voltage.  A sound level makes more sense. We can get 175 dB from a gunshot of a rifle or pistol.
Here is a table of sound levels.  I jet engine at 25 meters is about 150 db
Logarithms are also used because many of our physical responses are non-linear.  By log(), I mean log base 10 of some value.  The 'common' logarithm, not the natural logarithm which is base e.

ok, so the Y db scale would be the noise level of the signal; obvious that more noise = more signal amplitude. So I forget in the FFT chart to think in volts, and think about the amplitude in db.
Positive noise levels are already very high: for example when I listen to my receiver, I normally set the volume to -40db, if I want to exaggerate I go to -15; but I would never dream of going to positive decibel values ..

Since the spike on the FFT shows the amplitude at a frequency, I'm not sure they can be negative.  I certainly haven't see a series that has that result but it would take more math than I'm up for to prove it one way or another.

is that maybe I get confused by the graph in the time scale, where the signal has high and low peaks if alternated; but the FFT graph in the frequency scale must be seen in another way, the peaks represent the Vpp amplitude of the signal, it is right that the peaks are only upwards to have a clear view of the signal.

thanks 

[CharlotteSwiss]

In audio there is a measurement called 'total harmonic distortion - THD' and those offending harmonics show up on the FFT of the signal.  About half way down the page:

https://www.gamry.com/application-notes/EIS/total-harmonic-distortion/

of course, a perfect sinusoid has no harmonics with noticeable peaks. So maybe we can say that by analyzing an audio output with FFT, we should have a fundamental, but then a linear descent without high harmonic peaks

[CharlotteSwiss]

Some simple examples with my SDS1104X-E that has exactly the same FFT as Charlotte's.

the menus, however, are different, I don't have config, but I have 3 menu pages.
I did a test with my compensation signal, but setting Vrms instead of db as the unit.
We have a more linear graph, where only the peaks stand out. (and as a reference on the y axis we have a scale in mv)
Then I have no idea if the references are exact: in theory a square wave like this should have a Vrms of about 1.5 / 1.6v; I see that it does not correspond to any peak, but I do not think it is important, the purpose of FFt is to see if there are anomalous peaks, certainly not to detect the precise amplitude .

[rstofer]

You're getting messed up on the harmonic numbers.  A square wave has only odd harmonics so there is no 2*f and, therefore, no 1/2 amplitude.  It starts out as f(1) = 1, f(3) = 1/3, f(5) = 1/5 for the relative amplitudes.


If I type this right, the square wave looks like 4/pi * (f(1)/1 + f(3)/3 + f(5)/5 ... out to infinity).  Clearly, there is a point where the relative amplitude is no longer significant.  It is never 0 but it may not contribute much to the waveform.

Big hint:  These harmonics are why a 100 MHz scope can't display a 100 MHz square wave.  The 100 MHz fundamental may make it through the scope front end but even the 3rd harmonic is at 300 MHz and if we want to include the 9th harmonic, we need a 1 GHz scope!

It doesn't appear that I can get a screen image directly from the SDG2082 so I took a crappy photo.  This particular view shows the information about the fundamental (frequency, amplitude, offset and phase) plus the info for the odd, 3rd harmonic with amplitude 1/3V at 0 deg phase shift.  With a button push I can get to the next screen which will display the fundamental and 5th harmonic.

It's not clear to me from the manual but I think I can stack up 8 different harmonics anywhere within the 80 MHz capability of the signal generator.  That Valentines Heart uses quite a few.

You can also see over on the left what the waveform should look like as a function of time along with the FFT.  These Siglents are very nice AWGs.

This might be a good time to visit tautech's link to the Valentine Heart given a few pages back. 
https://www.eevblog.com/forum/chat/a-valentine_s-day-activity-for-your-scope-and-function-generator/

The generator puts out two signals each different but containing the fundamental plus the 2d 4th 6th and 8th harmonics at vatious amplitudes and phases.  The generator is creating the X and Y signals and the scope will be set to XY mode rather than YT (Y is amplitude, T is time - the normal display mode).  In X Y mode, signals drive both axis and the internal timing signals are not used.

[rstofer]

the menus, however, are different, I don't have config, but I have 3 menu pages.
I did a test with my compensation signal, but setting Vrms instead of db as the unit.
We have a more linear graph, where only the peaks stand out. (and as a reference on the y axis we have a scale in mv)
Then I have no idea if the references are exact: in theory a square wave like this should have a Vrms of about 1.5 / 1.6v; I see that it does not correspond to any peak, but I do not think it is important, the purpose of FFt is to see if there are anomalous peaks, certainly not to detect the precise amplitude .

The original signal has a DC component and it shows up at zero frequency at the left edge of the FFT.  The first harmonic (fundamental) is a short distance to the right and we'll call that amplitude whatever you want.  Note that the 3rd harmonic is about 1/3 as tall and the 5th harmonic is about 1/5 as tall.  See if you can make the DC spike disappear by switching to AC coupling.

[borjam]

But beware, a harmonic is not always a disturbance.
If your intended signal is a quare wave, for example, those N frequency multiples with a 1/n amplitude are not disturbances!

yes of course, I think those harmonics are used to have a square wave. I was talking about harmonics maybe with very high peaks that maybe create disturbances 

Not really, a disturbance would be something you don't want, period. So, imagine you have a circuit that should produce sinusoids and instead you find out it's making square waves. In that case there is a "disturbance" (distortion).

However, is it a problem if your calibration output produces square waves? Not at all, because that's what it was designed for.

Maybe it helps to think in terms of color. Imagine that you buy two buckets of blue paint and a bucket of green paint.

You open the first bucket of blue paint and the contents are indeed blue. So that's fine.

Now you open the second bucket of blue paint and turns out it's green. So the blue paint inside was contaminated with yellow paint.

You open the third bucket and you find out it contains green paint as advertised. So, in this case, the yellow paint is not "contamination" nor a "disturbance", but an intended ingredient.

So, what constitutes a disturbance depends on what you are expecting. If the scope calibration signal was producing sinusoids it would also be a malfunction.

[CharlotteSwiss]

Not really, a disturbance would be something you don't want, period. So, imagine you have a circuit that should produce sinusoids and instead you find out it's making square waves. In that case there is a "disturbance" (distortion).

However, is it a problem if your calibration output produces square waves? Not at all, because that's what it was designed for.

Maybe it helps to think in terms of color. Imagine that you buy two buckets of blue paint and a bucket of green paint.

You open the first bucket of blue paint and the contents are indeed blue. So that's fine.

Now you open the second bucket of blue paint and turns out it's green. So the blue paint inside was contaminated with yellow paint.

You open the third bucket and you find out it contains green paint as advertised. So, in this case, the yellow paint is not "contamination" nor a "disturbance", but an intended ingredient.

So, what constitutes a disturbance depends on what you are expecting. If the scope calibration signal was producing sinusoids it would also be a malfunction.

thanks for the example borjam, very clear. Yes, a disturbance is always a disturbance. But the siglent engineers wanted to have a square wave signal, and not a sinusoidal one: so in this case the disturbance (odd harmonics laws) is wanted.
Different would be an audio output where I should maybe have a sine wave, and instead I have a wave that looks like a square; ok there we have some unwanted disturbance
 

[CharlotteSwiss]

You're getting messed up on the harmonic numbers.  A square wave has only odd harmonics so there is no 2*f and, therefore, no 1/2 amplitude.  It starts out as f(1) = 1, f(3) = 1/3, f(5) = 1/5 for the relative amplitudes.

if I wrote f / 2 indicating a square wave it was my mistake, it has no even harmonics; those are for example in a sawtooth wave (in addition to the odd ones).

Big hint:  These harmonics are why a 100 MHz scope can't display a 100 MHz square wave.  The 100 MHz fundamental may make it through the scope front end but even the 3rd harmonic is at 300 MHz and if we want to include the 9th harmonic, we need a 1 GHz scope!

I had read this speech as soon as I started, now I understand why. But then a 50Mhz square has the third harmonic at 150Mhz, and therefore it too cannot be measured correctly by a 100Mhz oscilloscope? (but also 30Mhz, the fifth will be 150Mhz ... and so on)

It doesn't appear that I can get a screen image directly from the SDG2082 so I took a crappy photo.  This particular view shows the information about the fundamental (frequency, amplitude, offset and phase) plus the info for the odd, 3rd harmonic with amplitude 1/3V at 0 deg phase shift.  With a button push I can get to the next screen which will display the fundamental and 5th harmonic.

It's not clear to me from the manual but I think I can stack up 8 different harmonics anywhere within the 80 MHz capability of the signal generator.  That Valentines Heart uses quite a few.

You can also see over on the left what the waveform should look like as a function of time along with the FFT.  These Siglents are very nice AWGs.

This might be a good time to visit tautech's link to the Valentine Heart given a few pages back. 
https://www.eevblog.com/forum/chat/a-valentine_s-day-activity-for-your-scope-and-function-generator/

The generator puts out two signals each different but containing the fundamental plus the 2d 4th 6th and 8th harmonics at vatious amplitudes and phases.  The generator is creating the X and Y signals and the scope will be set to XY mode rather than YT (Y is amplitude, T is time - the normal display mode).  In X Y mode, signals drive both axis and the internal timing signals are not used.

the image is fine, now I understand: you used ch1 (you have two channels for two different signals at the same time) and you created a sine wave but also setting a value for the harmonics (in this case odd); doing so begins to obtain a square wave.
With these generators you can get very customizable signals out, great.
Yes, I looked at Valentine again, that topic is always nice.
Thanks rstofer
 

[CharlotteSwiss]

The original signal has a DC component and it shows up at zero frequency at the left edge of the FFT.  The first harmonic (fundamental) is a short distance to the right and we'll call that amplitude whatever you want.  Note that the 3rd harmonic is about 1/3 as tall and the 5th harmonic is about 1/5 as tall.  See if you can make the DC spike disappear by switching to AC coupling.

ok, i hadn't thought enough with my head, obviously if the signal has a DC component, it has zero frequency and will be at the beginning of the FFT graph.
Here is the Ac coupling signal, in fact we do not have the DC peak.
For the amplitude of the f and the harmonics f3, f5 etc .. I would have liked metetre tools like tautc, but with the new firmware version they have removed that tools-peak submenu.
However I can see that about the fundamental has a value on the y scale of 1200 (we call 1200 balls :-)), the f3 has a value of 400 (so exactly, about 1200/3), the f5 has an amplitude value of about 220 then about 1200/5) and so on.
/ the references aren't exactly 1/3 - 1/5, but that's not what matters, we're talking about an obvious economic spectrum)

[rstofer]

Look at the 7th harmonic, it has too much amplitude for an actual square wave.  That's because your calibration signal has a very slow rise time, nowhere near 0 nanoseconds but much closer to several microseconds.  You can have the scope tell you the rise time.  That's part of the magic of the modern DSO.

Siglent, and everybody else, uses a square wave for calibration because it has high frequency components.  We're trying to adjust the capacitor in the 10x portion of the probe to eliminate overshoot and undershoot at higher frequencies,  frequencies that a square wave has,

A 100 MHz scope doesn't suddenly quit displaying a trace at 101 MHz.  In fact, the Rigol DS1054Z, after being unlocked for 100 MHz, has a bandwidth of 130 MHz, give or take.  The thing is, we need to understand that the 'bandwidth' is considered to be a point where the signal has lost 3dB or the value displayed is 0.707 times the actual value.  If I stuff a 130 MHz 1V signal into the Rigol, the display will read about 0.707V.  The displayed voltage has been declining from around 100 MHz and will continue to decline down to essentially 0V at some really high frequency.  That's what the Bode' plot is all about.  Attenuation versus frequency.  But the signal still shows up!  The problem is that the high frequency components of a square wave disappear and the square wave ultimately displays as a normal sine wave because the harmonics are all attenuated.  What you see on a scope is just a squiggly line.  Interpreting it takes knowledge about the actual nature of the signal.

Here is a typical Bode' plot of an RC filter and the front end of a scope works exactly the same way.  I have a cursor at the -3dB frequency where we also expect a 45 degree phase shift

https://www.eevblog.com/forum/testgear/starter-scope/msg2969998/#msg2969998

There are some more experiments a few replies down.  I was having fun with my Analog Discovery 2 that day!

[rstofer]

The scope discussion was settled when you made an excellent purchase.  The signal generator hasn't really gotten serious and that's fine.  In fact, it's for the best because the more recent discussions have centered on the capabilities of the modern AWG.  I'm using the Siglent SDG2082X but I think the less expensive SDG1032X will do the same thing but with a lower frequency range.

The things we're talking about in this thread go well beyond what the average hobbyist will need to know.  We're covered a lot of ground!

What we need to do is get Tautech to discuss the differences between the SDG1032X and the SDG2082X other than frequency.  It's important to realize that the maximum frequency is also the highest harmonic frequency that can be included in an arbitrary waveform.  In other words, a square wave created by adding harmonics to a fundamental will have an upper limit on the frequency of the harmonic.   If we really want a fast rising square wave, an AWG isn't the way to get one.  There are several projects here on eevblog discussing such a tool and it is usually made with a high speed logic gate.  We're talking sub nanosecond rise times.

My brief reading of the SDG1032X User Manual didn't turn up any obvious differences in features between it and the SDG2082X (other than upper limit on frequency) and it's features that matter.  The SDG1032X may be just right.  I suppose there are reviews over in the Test Equipment forum.

[tautech]

SDG2000X models vs SDG1000X models.
16 bit, faster sampling engine, more waveform memory and touch display for SDG2000X. Touch is occasionally handy but you can live without it.
SDG1000X, square wave to max frequency whereas SDG2kX top out at 25 MHz for square wave.

That's about all the key differences as they have the same waveform suite and other functionality.
For the hobbyist SDG1032X is a fine choice and our best seller and was my main AWG until upgrading.

I spoilt myself and got a SDG6202X a while back after grumbling at the price for a year or 2 but I needed one for a demo model anyways. 200 MHz sine, 80 MHz square wave and 1ns risetime is very nice and so it should be for the price. 

[rstofer]

Touch is occasionally handy but you can live without it.

I didn't even know I had a touch screen until a couple of days ago.  I guess I'm not big of reading User Manuals...
I have been using the button menus and getting along just fine without the touch screen.


14 bit resolution versus 16 bit should be a non-issue for most hobbyists.

The SDG1032X is $333 USD at Amazon with very fast delivery.

https://www.amazon.com/Siglent-Technologies-SDG1032X-Arbitrary-Waveform/dp/B01LECZU98

When these discussions turn technical, it's nice to have the equipment required to put together an experiment reinforcing the learning.  My learning...