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Measuring Distortions with the Scope:What you see is not what you really have..
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elecdonia:
If you have a harmonic distortion analyzer (or tuneable notch filter), then it is incredibly useful to use the oscilloscope to view the output signal from the THD analyzer, which contains the distortion and noise components alone, without the fundamental. Today many digital 'scopes can perform FFT which will clearly display the amplitude of each harmonic. The first time I saw an FFT of THD analyzer output (in 1978!) I thought "this explains why some amplifiers sound better than others". But at that time FFT analyzers were strictly unaffordable for ordinary people like me. Times have changed!

In terms of perceived audio quality: The best sounding amplifiers have mostly 3rd harmonic (3x fundamental frequency), with equal or lower 2nd harmonic. Higher order harmonics should drop off rapidly. Ideally the levels of 4th, 5th, and 6th order harmonics are -60dB (or lower) compared to the fundamental. The best amplifier circuits have unmeasurably low levels of 7th and higher order harmonics, at least -80dB below the fundamental. There should never be any high order harmonics with levels above -20dB compared to amplitude of 2nd or 3rd harmonic.

Another "THD analyzer with oscilloscope" view which I have used for many years is to put the oscilloscope into X-Y mode.
This is extremely useful even when using only a modest quality THD analyzer (Examples: HP 331A, 332A, 333A, 334A, Heathkit IM-5258).
Apply the output signal from the amplifier under test to X axis.
Apply output signal from THD analyzer to Y axis.

Ideally the X-Y display will be a flat line until the amplifier under test begins to clip.
The onset of clipping will immediately raise large vertical components at the extreme left and right edges of the X-Y display.
More important, "crossover notch distortion" will appear as a single vertical line at the middle (0V) of the X axis. Crossover notch distortion is extremely audible, even in tiny amounts. Many early transistor amplifiers had plenty of crossover notch distortion. Especially Crown and Phase Linear. But improved solid-state circuit designs largely eliminated this issue after 1980. In contrast, tube (valve) amplifiers rarely have significant crossover notch distortion. In my opinion this is a big reason why people have been saying (for many years) that tube amps sound better than solid-state amps.

Rough diagram of X-Y displays for different conditions:

No significant distortion:               ---------------------

Clipping:                                     |--------------------|

Crossover notch:                          ----------|----------

I'll post images from actual measurements if anyone here is interested.
elecdonia:
Why does this topic have two names?
     1) Re: Measuring Distortions with the Scope:What you see is not what you really have
     2) Re: Comparison between Siglent SDG1000X and 2000X
tautech:

--- Quote from: elecdonia on January 05, 2023, 07:28:22 pm ---Why does this topic have two names?
     1) Re: Measuring Distortions with the Scope:What you see is not what you really have
     2) Re: Comparison between Siglent SDG1000X and 2000X

--- End quote ---
See reply #63, the OP chose to change it to better reflect the content.
elecdonia:
Several posts in this topic display the FFT harmonics from the output signal of a "function generator." Typically the ratio of fundamental/harmonics shown is no better than -50dB for function generators. This isn't suitable for testing audio gear. It corresponds to THD of .5% to 1%. This is far higher than the THD of any decent audio amplifier circuit. In order to make useful measurements of distortion, the "residual" distortion of the signal source (oscillator or generator) should be at least 10 times lower than the expected THD of the amplifier under test.

Function generators always produce plenty of harmonics, especially higher order harmonics. The reason for this is that function generators internally start out by generating either a ramp or triangle wave shape. This is then filtered and/or processed with non-linear diode clipping circuits to "approximate" a sine wave. The best function generators have harmonics of -40dB to -60dB below the level of their fundamental. Function generators also produce extensive high-order harmonics up to and above 10th order. Adding up all these harmonics corresponds to about 1% THD, with the very best function generators approaching .25% THD.

In contrast, "sine wave audio oscillator" circuits actually generate a sine wave as their starting point. Some generators may offer a square-wave output also, but that is produced by running the pure sine signal through a comparator. The best sine-wave oscillators have THD as low as .0001% (after background noise is filtered out).
Pure digital generation of sine waves by high-grade 24-bit D-A converters can also achieve very low THD.

I recommend that future FFT data shown in this topic should use an "ultra-low distortion" sine wave oscillator (or 24-bit D-A) for the signal source.
elecdonia:
Theoretically a very high-quality ultra-low-distortion sine wave oscillator could be phase-locked to the output signal from a function generator. But this isn't how most function generators are made.

Function generator:
     Available output signals: Sine, square, ramp, triangle, pulses of controllable width
     Output frequency range can be as large as .01Hz to 10MHz
     Output signal amplitude is tightly defined (changing the frequency doesn't alter output amplitude)
     Distortion for sine-wave output: .25% to 2% THD
     Some models offer voltage-control of output frequency

Low-distortion sine wave oscillator:
     Available output signals: Sine (and optionally square)
     Typical output frequency range is 20Hz to 100kHz or sometimes 1MHz
     Output signal amplitude may vary by +/- 1-3dB as frequency changes
     Distortion for sine-wave output: .0001% to .1% except older tube (valve) sine wave oscillators may have distortion spec of .5-1% THD
     Some sine wave oscillator circuits provide only a single fixed output frequency (typically 1kHz), or a limited number of fixed output frequencies
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