Well, I Got My First Oscilloscope?

[The Ballzz]

Hey Folks,
As per an earlier thread, I recently acquired a Tektronix 2215A oscilloscope. I have been through several "primers/tutorials" with some "hands on" exercises and am at least confident in my understanding of what the controls do and how to take at least simple measurements. What these measurements mean is a different thing altogether! I've at least discovered that it gets much easier when you have a "ballpark" idea of what voltages should be present at the test points. I've measured many things, heck I even tried measuring the cat, he was less than thrilled! Just joking!

As a novice, I'm curious as to whether or not an oscilloscope can be set up to be used as a frequency spectrum analyzer, say in the 10hz to 30khz range to view the response curve and look for any "out of the audible range" oscillations and/or peaks or phase cancellations? I'm going to make a guess that a more elaborate scope with more advanced features would likely be required for this purpose.

Any other tips for getting me "scoped out" friendly would certainly be appreciated.
TIA,
Gene

[jelle]

This may be of interest:

http://r-type.org/articles/art-125.htm

[The Ballzz]

jelle,
WOW, thanks for that link! After reading the article, I returned to the "Articles Content Page" and can see that A LOT of research and reading is in my near future! I just hope I can wade through what is pertinent to me and what is superfluous for my needs and skills. Rarely do good things come easy.
Thanks Again,
Gene

[Garthhog]

You are correct about needing a more modern scope. The FFT (the algorithm that converts the time domain measurements to the frequency domain) requires substantial computing power to perform the calculation in near real-time. That being said, I picked a new Chinese junk scope that does a reasonable FFT from Amazon: http://www.amazon.com/gp/product/B007T6 ... UTF8&psc=1

[Structo]

Thanks Jelle,

That will be a good refresher for me and the oscilloscope.

[jelle]

You are welcome!

[teemuk]

Geat link Jelle. Should be "stickied" in some form, IMO. In knowing hands, a lot of information can be extracted with plain scope and square and sinusoidal wave inputs. This is good stuff to know even if you're never going to buy/use a scope.

e.g.
Perfect sinusoidal waveform consists of only a single (fundamental) frequency. Thus, if there is distortion in the waveshape using sinusoidal input we know the signal is now also carrying other frequencies ("harmonics"). Waveform shape will indicate what other frequencies and in what amplitudes are conveyed. So, we learn plenty about amplitude distortion using sinusoidal inputs.
Perfect square wave consists of several frequencies and features very steep rising and falling edges. Thus, if there is difference in the waveshape using a square wave input signal (and we know it isn't caused by amplitude distortion) then we know that the frequency response must be non-linear. (Frequencies within the wave were not conveyed in equal amplitude, thus waveform got distorted). The waveform shape reveals lack of bandwidth towards both high and low frequencies, as well as all non-linearity within the bandwidth. Additionally the quick rising and falling edges are great for revealing "ringing" oscillations and alike.

...I'm curious as to whether or not an oscilloscope can be set up to be used as a frequency spectrum analyzer, say in the 10hz to 30khz range to view the response curve and look for any "out of the audible range" oscillations and/or peaks or phase cancellations?

I'm not really sure what you mean by "frequency spectrum analyzer"....

If you want to hunt for oscillations above or below audible frequencies even most cheap scopes should be capable of that. In practice you only need to probe nodes of interest and see if the waveform in them carries signs of oscillation.

But plotting frequency response is a different matter. You can - again - do it with an ordinary scope but it's a somewhat complex setup:
You basically probe the signal as usual, but now you calculate gain from known amplitudes of input and scoped signal. You naturally need to do such measurement at various frequencies (in practice a little bit more than throughout the effective bandwidth), and preferably with ample "sample rate" to catch all narrow peaks and notches in the response . Throw the results to Excel spreadsheet (or something alike) and the software will draw a nice graph automatically. Some use "sweep" inputs, meaning a "burst" sinusoidal signal with constant amplitude but frequency sweeping from, say, 10Hz to 30kHz. Plot that "burst" waveform in complete and its envelope is directly proportional to gain changes, hence frequency response. So, in practice this stuff begins to require either decent signal generator or plenty of patience to do several measurements manually.

Spectrum analyzer is again a different deal... That device analyses signal distortion at frequency of interest and plots the magnitude of each harmonic component. And yes, unless you want to try to do the analysis manually from that waveform in the scope screen, you need a spectrum analyzer in some form for this. Either as an external unit or as a feature in the scope.

Again, it's about individual measurements at frequencies of interest. If you want to plot distortion characteristics throughout effective bandwidth then it again means several measurments at different frequencies and with decent "sample rate".

[Merlinb]

As a novice, I'm curious as to whether or not an oscilloscope can be set up to... view the response curve

ANother option is to make your own sweep generator gadget which allows you to 'draw' the frequency response on a scope:
http://freestompboxes.org/viewtopic.php ... ld#p241412

[The Ballzz]

As a novice, I'm curious as to whether or not an oscilloscope can be set up to... view the response curve

ANother option is to make your own sweep generator gadget which allows you to 'draw' the frequency response on a scope:
http://freestompboxes.org/viewtopic.php ... ld#p241412

Kind Sir,
I couldn't open any file on the linked page,
Thanks,
Gene

[Merlinb]

I couldn't open any file on the linked page,

I'll put it here temporarily:
https://dl.dropboxusercontent.com/u/578 ... enfold.pdf

[The Ballzz]

Thank You Sir!
Gene

[xtian]

I got a new digital 'scope: Hantek DSO5072P. It is, in a word, pretty totally fucking awesome. This is their entry-level model, and can be had for just over $250 from Hong Kong, shipping took less than 10 days. Main drawbacks to entry-level model are 70MHz sample rate (can be hacked for 200MHz), low 40K memory buffer, and slow screen refresh on some detailed analyses. But seems like it will be way more powerful than I'll ever need for guitar amps.

On a related note, I just read a head-smacking fact about adjusting probes to match your scope. There is a little adjustable capacitor that much be matched to the scope's internal capacitance.

Starting on p.222:

http://knob.planet.ee/kirjandus/raamatu ... ifiers.pdf

Lots of other great stuff in this book.

[David Root]

Several years ago I bought an entry level Chinese scope (new) and despite having the manual I have never been able to get it to do anything useful.

I can't even get it to display properly.

I'll read Jelle's piece, hopefully that will help, but my experience to date has been dismal.

[lord preset]

I just bought Xtian's old analog scope and signal generator. Unfortunately no mad scope skills came in the box with the gear, which was disappointing, but while waiting for a signal generator probe to arrive I have managed to feed the scope tones from my iPhone and look at pretty wavy lines.

I'm sure the pretty wavy lines are overstuffed with deep meaning and revelation but for now they are mum. Regrettably I may have to read some stuff first.

[xtian]

Well, you guys know there's plenty of stuff lying around you can read. But to get you started, get your signal generator (iPhone is fine) set for 1KHz sine wave, and measure that signal with your scope. Set the horizontal time scale so you can see one or three cycles, and the vertical scale so the wave fits comfortably in the window. Now figure out the amplitude of the sine wave in volts, peak-to-peak (p-p). For example, if the vertical scale is set to 0.5v/div, and you count 4.5 divisions on the display, then your wave is 2.25v p-p. Adjust the volume on your iPhone so the wave is 1v p-p. This is a good baseline test signal to send into the input of your guitar amp.

If you get that far, you can start to measure what happens to the sine wave as it passes thru the amp, and you can see the effects of distortion on the wave, and begin to recognize various common amp issues like crossover distortion, asymmetric clipping, etc.