Author Topic: DB with oscilloscope  (Read 1821 times)

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Offline Adrian_Arg.

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DB with oscilloscope
« on: March 09, 2018, 05:13:52 pm »
hello, I'm a novice with my rigol ds1054z, I have several doubts but the main hory is how you measure decibles with an oscilloscope, if you can measure it.
 

Online TimFox

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Re: DB with oscilloscope
« Reply #1 on: March 09, 2018, 05:39:21 pm »
Decibels (dB) are merely the ratio between two values.  In specific contexts, such as dBm (dB over 1 mW), the second value is specified.
If you measure two voltages, the usual assumption is that the impedance/resistance where the two voltages were measured are equal, so the two power values are proportional to the voltage squared, and the impedance value cancels out when taking the ratio.
Therefore, the equation is
dB = 20 x log10 (V2 / V1).
A 2:1 ratio in voltage therefore corresponds to a 6 dB ratio.
 
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Offline Adrian_Arg.

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Re: DB with oscilloscope
« Reply #2 on: March 10, 2018, 01:45:19 pm »
So there is no quick way to measure decibels with an oscilloscope, I mean connect the output of a microphone to the probe and based on that measure?
 

Offline Ice-Tea

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Re: DB with oscilloscope
« Reply #3 on: March 10, 2018, 03:05:37 pm »
Depends on the scope. Some have extensive math functionality that may support it.

Online drussell

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Re: DB with oscilloscope
« Reply #4 on: March 10, 2018, 03:09:06 pm »
If you are trying to measure sound pressure level, you need a microphone with a known output level that increases linearly with increasing sound pressure and has a flat frequency response.  Calibrated microphones for this purpose are available and will come with specifications for the particular unit from the test lab.
 

Offline Adrian_Arg.

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Re: DB with oscilloscope
« Reply #5 on: March 10, 2018, 03:13:45 pm »
In the case of a rigol ds1054z does it have that range? It's the one I have
 

Offline rstofer

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Re: DB with oscilloscope
« Reply #6 on: March 10, 2018, 08:46:33 pm »
In the case of a rigol ds1054z does it have that range? It's the one I have

Check the User Manual.  The Math button pulls up a menu that includes things like dBV and dB, but only for the FFT operator AFAICT.
 

Online TimFox

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Re: DB with oscilloscope
« Reply #7 on: March 11, 2018, 11:43:09 pm »
Again, dB are a ratio.  You need to specify the 0 dB point for your microphone, or equivalently the output voltage for a specified reference level to get meaningful dBs.
 

Offline Brumby

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Re: DB with oscilloscope
« Reply #8 on: March 12, 2018, 03:07:50 am »
Repeating what has been said above - dB is a comparative measurement.  You are comparing one absolute value (such as power) against another absolute value of the same type.

It's the same as asking "How many times is X louder than Y" - it's just that "Y" is a reference point that must be defined somehow because there is no absolute standards laboratory reference for it, unlike volts or amps.
 

Offline Audioguru

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Re: DB with oscilloscope
« Reply #9 on: March 12, 2018, 03:11:39 am »
A 'scope is used to see waveforms. Waveforms must be displayed linearly, not in dB's.
db's are ratios of steady levels that can be seen on an audio or RF multimeter.
 

Offline Brumby

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Re: DB with oscilloscope
« Reply #10 on: March 12, 2018, 03:16:00 am »
There are a number of scopes which can do that math.
 

Offline Calaverasgrande

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Re: DB with oscilloscope
« Reply #11 on: April 22, 2021, 07:54:32 pm »
Repeating what has been said above - dB is a comparative measurement.  You are comparing one absolute value (such as power) against another absolute value of the same type.

It's the same as asking "How many times is X louder than Y" - it's just that "Y" is a reference point that must be defined somehow because there is no absolute standards laboratory reference for it, unlike volts or amps.

Not entirely true. In audio engineering we have AES and EBU reference levels. These are defined in the context of a specified load, say 600 ohms, a reference voltage .775 volts and one milliamp of current.  This gives you a point of reference for 0Dbu in the standard +4Db operating level used in professional analog broadcast, public address or recording. +4dB itself is 1.23 volts in this context.
Of course in the digital audio age this gets more complex, as we reference the 0DbU level as -20Dbfs or 18Dbfs or -14Dbfs depending on the audio system manufacturer reference levels.
If the OP or someone else is looking for something similar, the Rigol 1054Z does have an FFT mode which scales in Dbm, which should be adequate for audio work. (though Dave who seems to love the 1054Z for an entry level scope dinged it's FFT mode, he was looking at beyond audio range performance).
You can also measure the voltage from the screen and do the math to get Db for the load and voltage. But as has been said frequently elsewhere. A scope is not a DMM.
 

Offline rstofer

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Re: DB with oscilloscope
« Reply #12 on: April 23, 2021, 03:04:38 pm »
Short answer:  No, the DS1054Z does not have dB as a vertical scale in any function other than FFT.
 

Offline Electro Fan

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Re: DB with oscilloscope
« Reply #13 on: April 23, 2021, 05:03:11 pm »
A 'scope is used to see waveforms. Waveforms must be displayed linearly, not in dB's.
db's are ratios of steady levels that can be seen on an audio or RF multimeter.

I think this ^ is very good point (about linear vs logarithmic measurements) in a great thread about an important topic (dBs).

Here is another attempt at helping the OP - others should feel to correct whatever I get wrong.

dBs are useful in making measurements that have relatively large dynamic range, ie lots of difference between the largest and smallest values being measured or considered.  dBs become most valuable when the dynamic range being examined needs to be expressed in orders of magnitude (factors of 10, ie 10x, 100x, 1000x, etc).  To make dBs conveniently useful, as stated in other posts in this thread, dBs are often/generally (but not always explicitly) referenced to some specific value such 0dBm = 1mW and therefore we can quickly recognize that 30dBm = 1 watt.  (The point made above about impedance assumptions in the use of dB for power is a great example of some assumptions being present but not always explicit, which can lead to confusion when first getting a grip on the various uses of dB; just something to be aware of.)

Back to oscilloscopes.  When using an oscilloscope we might be looking at somewhat small values such as 1uV or somewhat large values such a 100V but we typically don’t use a scope for a task such as looking see if something is 100.000001V, or if one signal is a 100V vs another at 1uV.  In fact we generally use probes that are specified within an order of magnitude such as 10x (or sometimes 1x or 100x).

So while an oscilloscope is a fantastically useful piece of test equipment (occasionally even referred to as the king of test equipment) it is not one of the types of test equipment most likely to be used for measurements of values frequently designated with the dB nomenclature.

To be sure an oscilloscope might be used to measure in ranges from, for example, seconds to picoseconds on the (horizontal) time axis, but in terms of the (vertical) amplitude scale dBs don’t get used much (hardly at all?).

In contrast, in the case of spectrum analyzers we can look also at the horizontal scale in terms of sweeps of full seconds to small fractions of a seconds as we look at relatively larger or smaller swaths of bandwidth, and we might greatly reduce the span of frequency to see with more detailed resolution.  So again, time might range from seconds to billionths or maybe trillionths of seconds.  However on a spectrum analyzer we might look at 10 orders of magnitude in amplitude (or more depending on how the reference graticule is set).  So on a spectrum analyzer dBs get used regularly on the vertical scale.  With a spectrum analyzer we might be measuring and comparing amplitudes in the range of +30dB to -160dB.

In summary, a key concept to help make dBs come into better focus is “dynamic range”, ie the need to not only measure but also readily compare values (of whatever is being measured) across multiple orders of magnitude.

The more you get your head around dBs the more profoundly you can get your head around the enormous potential for dynamic range and scale for various realms in various systems.  Kinda metaphorically (so similar to but not exactly) like trying to quantify and compare how big is an electron relative to an atom or an atom compared to a solar system or a solar system compared to the Universe.
 

Online ebastler

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Re: DB with oscilloscope
« Reply #14 on: April 23, 2021, 05:07:47 pm »
Short answer:  No, the DS1054Z does not have dB as a vertical scale in any function other than FFT.

Right. Software features aside, let's not forget that it only has an 8-bit AD converter. Not much fun to be had there with a logarithmic scale...
 


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