Author Topic: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions  (Read 1309 times)

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Offline JohnnyMalaria

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Re: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions
« Reply #25 on: March 22, 2021, 09:38:48 pm »
It would be interesting if you can put a microphone near the strings and look at the signal. It should look dramatically different than what's coming from the pickup. Also, it would be interesting to compare the different pickups.

BTW, if you have a function generator, you could use the scope as a visual tuner. Set the generator to the target frequency and trigger the scope with it. When your string is close but not exactly in tune, its signal will appear to roll to the left or right. You could also use the XY mode and enjoy the watching how that changes as you tune the string.
 

Online ve7xen

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Re: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions
« Reply #26 on: March 22, 2021, 09:59:24 pm »
Many scopes do have a separate counter that measures the frequency of the trigger completely separate from the acquisition system. It seems that you're using the 'software' measurements for this, and might get better results from the counter with a carefully set trigger value.

Not sure about this scope, but it would make sense for the Frequency measurement (based on acquisition data) to be based on either a frequency domain analysis or a count of zero crossings with DC elimintation. If it's measuring zero crossings that definitely seems like it would explain what you're seeing. Really an FFT is necessary to do what you want here, why don't you use that?
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Offline alsetalokin4017

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Re: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions
« Reply #27 on: March 22, 2021, 10:51:30 pm »
That's a great set of screenshots/experiments. Looks like your Rigols and Agilents are working just fine for this.

I tried a few more times, single shot mode in every case. I also limited bandwidth to 20Mhz in one case, the rest are full bandwidth. This 'scope runs at 300 Mhz, so maybe it is just picking up more fluctuations. Anyway, could not lock onto the fundamental frequency in any of these attemps. The auto set for the cursor also chooses the 2 closest waveforms and shows a much higher frequency. The only way to get the right fundamental frequency is to use cursors - manually.

These are stock single coil strat bridge pickup btw. No preamp - straight from guitar into 'scope - via 1/4" to bnc cable. This time I made sure volume was all the way up to get max signal.

There is something wrong somewhere. Your signal is only showing +/- 50 mV, unless I am badly misreading your scope display. Look at how strong our signals are. (mine and SilverSolder's). Your first scopeshots only show +/- 2 or 3 mV ! 
« Last Edit: March 22, 2021, 10:54:38 pm by alsetalokin4017 »
The easiest person to fool is yourself. -- Richard Feynman
 

Offline javadesigner

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Re: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions
« Reply #28 on: March 23, 2021, 01:31:21 am »
There is something wrong somewhere. Your signal is only showing +/- 50 mV, unless I am badly misreading your scope display. Look at how strong our signals are. (mine and SilverSolder's). Your first scopeshots only show +/- 2 or 3 mV !

Great catch about the earlier screenshots - they were lesser because my volume wasn't all the way up :-[

However, with the volume all the way up, +/- 50 mV  is actually at the upper bound of the expected voltage. This is a single coil stock strat pickup - not a humbucker (which will be 2-3x hotter comparatively). Also, to be clear, my scope channel is not attenuated and running 1:1.

See this article I found, for example:
http://tomsguitarprojects.blogspot.com/2014/12/electric-guitar-output-voltage-levels.html
"picking the open A string softly I got about 10mV, medium-hard 20mV and hard 30mV (which you'll find under 'A' in the table, above)"
« Last Edit: March 23, 2021, 01:54:57 am by javadesigner »
 

Offline alsetalokin4017

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Re: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions
« Reply #29 on: March 23, 2021, 02:30:37 am »
Thanks, I did not know that humbucking pickups were so much hotter than single-coil. Learn something every day around here!

I still think there must be some way to get your scope to focus on the frequencies of interest. Do you have alternate triggers? For example the Rigol has a "pulse" trigger where one can set the width of a pulse or peak to trigger on.

Or maybe you do need a preamp! Maybe with a stronger signal the scope will concentrate on the larger peaks and ignore the "noise" higher harmonics.
The easiest person to fool is yourself. -- Richard Feynman
 

Offline javadesigner

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Re: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions
« Reply #30 on: March 23, 2021, 04:58:52 am »
I still think there must be some way to get your scope to focus on the frequencies of interest. Do you have alternate triggers? For example the Rigol has a "pulse" trigger where one can set the width of a pulse or peak to trigger on.

Or maybe you do need a preamp! Maybe with a stronger signal the scope will concentrate on the larger peaks and ignore the "noise" higher harmonics.

Hi - thanks again for the great suggestions!

However (and being a n00b I'm probably wrong about this, but anyway here goes) - I don't think this is trigger type issue. The existing rising/falling slope trigger works great and in single shot mode already captures the waveform for the entire horizontal duration properly.

I think the issue is what happens after the capture - there are a lot of harmonics which confuse the automated measurements.

Some scopes apparently make is easy to ignore certain parts of the captured waveforms and measure an arbitrary part of the captured wave. One would think the R&S RTB2004 would do the same but I guess one would be wrong (this is a $3-4K scope mind you).

This review of another R&S scope on eevblog shows this gating feature (search for "gating"):
https://www.eevblog.com/forum/testgear/rohde-schwarz-rtm3000-review/

I also remember reading that the keysight InfiniiVision series (the lower end of which are same price as RTB2004) also do have the gating function for various measurements (among various other features).

Edit: this is available on LeCroy scopes as well, from one of their manuals:
The default starting positions of the measurement gate posts are 0 div and 10 div, which coincide with the left and right edges of the grid, and the First and Last points. Therefore, the measurement gates initially enclose the entire visible acquisition. By moving the measurement gates, you can focus the measurement on the section of the acquisition of greatest interest. For example, if you "gate" six rising edges of a waveform, calculations are performed only on the six pulses bounded by the gate posts.

Anyone from R&S reading that can shed a light on this?
« Last Edit: March 23, 2021, 02:16:02 pm by javadesigner »
 

Offline SilverSolder

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Re: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions
« Reply #31 on: March 23, 2021, 02:20:17 pm »

What happens if you capture less waves on the screen?  - e.g. a little more than one cycle of the fundamental.   Does it work then?
 

Offline cheater

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Re: Guitar plugged into RTB 2004 Oscilloscope - frequency display questions
« Reply #32 on: March 23, 2021, 03:40:18 pm »
People have already mentioned that you're seeing a different frequency due to the oscilloscope expecting a sine wave like wave with a clearly defined cycle (or square or triangle etc), meanwhile you're putting in a very complex wave. If you'd like to see how pitch tracking can be done better, look at the schematics for the Korg MS-20 pitch tracking input, or for the Roland GR-30 guitar synth.
 


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