How to measure distortion with oscilloscope

how to measure distortion with oscilloscope

Welcome, Guest. Please login or register. Home Help Search Login Register. I'm stumbling along the learning curve with my collection of test equipment. Got a decent signal generator and a dual trace Tecktronix oscilloscope. Using a t-connector, I've got the signal generator going to the input jack and to CH 1 of the 'scope.

I'm using the probe to connect to CH 2, starting at the grid of the second triode and checking subsequent grids in the main signal path. Anybody know offhand what a typical single coil output is in terms of voltage swing? How about vintage and hot humbuckers?

So here are some things I've observed so far.

how to measure distortion with oscilloscope

First, the Treble and Bass controls seem to induce a certain amount of phase shift for a portion of their range. Second, for some frequencies, voltage swing maxes out at about "8" on Volume. Third, the onset of distortion is different in terms of the Volume setting for different frequencies, and the shape of the distorted wave can differ as well.

Is that a reasonable sample set? From what I've read, it seems like some guys just look at a single frequency like 1KHz, but if my observations are typical I'd think we should look at a spectrum of frequencies. It's kind of fun at least for me to play with the tone controls too. Just for reference, the low E string is 82Hz, high E string is Hz, and 21st fret on high E is about 1.

Cheers, Chip. Quote from: jjasilli. You won't see much just diddling the knob of a fixed-freqeuncy audio generator. Because the performance at any one freq is not especially useful. You'd need some type of sweep to see the freq response of a particular section of an amp. Unless, of course, you were willing to make, log, and graph painstaking measurements at different frequencies.THERE is a need to observe small signal details in many modern applications.

Transducers, biomedical sensors, high energy physics, power integrity, and high-speed digital designs are examples of situations where details can be obscured by measurement system noise. Measurement system noise is the noise of the oscilloscope, probes and connection method that is superimposed on the signal of interest. When the signal being observed is small, like the ac ripple and noise on a power supply, the signal presented on the screen of the oscilloscope may only vaguely represent what is real if care is not taken to reduce measurement system noise.

The complete elimination of measurement system noise is not a realistic goal though there are some practical steps that can be taken to substantially reduce it.

There are two primary sources of noise in an oscilloscope-and-probe system. The input amplifier and buffer circuits in the scope contribute some noise, and the probe amplifier of an active probe has its own noise. Scopes use an attenuator to vary the vertical scale factor. The same thing happens when a probe with attenuation is attached to the scope. The scope noise appears larger relative to the signal at the input to the probe by the amount of the attenuation.

One obvious suggestion is to choose the low-noise path. Unfortunately, many users stumble here, not knowing they may have even better options available to them. A measurement of the baseline noise of the oscilloscope measurement system is a sanity check similar to shorting the leads together on a DMM before making a continuity or resistance measurement.

If the results from the null measurement are not acceptable, it may mean that a different oscilloscope, probe, or connection method needs to be used.

More bandwidth is better, right? Not always. The noise voltage of an oscilloscope, probe, and connection accessory are a function of frequency.

Limiting the used bandwidth to only the amount necessary for the given measurement will reduce the amount of oscilloscope, probe and connection noise that shows up in the measurement. Noise comparison of a and probe measuring a 50 mV pp sine wave.

Using a distortion meter or oscilloscope to see distortion?

Both a probe and a probe measure the same signal, simultaneously—a MHz 50 mV pp sine wave. The only difference between the two measurements is the attenuation ratio. The measurement is 52 mV pp while the measurement is 65 mV pp. This illustrates that with small signals where oscilloscope and probe noise can problematic, it is best to use as small an attenuation ratio as possible to minimize noise.

Oscilloscope manufacturers recognize the need to adjust bandwidth to make different measurements and have provided a variety of bandwidth limit presets. Some manufacturers also provide the ability to set any bandwidth limit to further tailor the limits to the measurement. If a desired preset or adjustment is not available a math function can be implemented to filter the signal, though this can reduce throughput because there are calculations performed on each acquisition.

To use this filtering technique, one must know the amount of bandwidth necessary for the signal of interest. There are resources available on oscilloscope manufacturer websites that explain, in depth, how to determine the needed bandwidth.

In summary, for digital signals, the necessary oscilloscope bandwidth is 2X the signal bandwidth.If you are using an oscilloscope, make sure you are using the right bandwidth!

Choosing the wrong amount could adversely affect your measurement results. Bandwidth is often regarded as the single most important characteristic of an oscilloscope. Measured in Hertz, the bandwidth of your oscilloscope is the range of frequencies that your oscilloscope can accurately measure. Without enough bandwidth, the amplitude of your signal will be incorrect and details of your waveform might be lost.

With too much bandwidth, you will capture excessive noise, providing you with an inaccurate measurement.

how to measure distortion with oscilloscope

You can think of an oscilloscope like a low pass filter, meaning it will only pass frequencies from 0 Hz up to a specified frequency. What the heck is a 3 dB down point? Read on. Low pass filters allow signals to pass through them at full amplitude until the signal frequency approaches the high end of the frequencies that the filter can pass.

Look at the diagram below to visualize the frequency response of a low pass filter, depicted in blue. Figure 1. Frequency response of a low pass filter, depicting the 3 dB down point and cutoff frequency. Why does this matter for your measurements? Even measuring a signal as fast as the bandwidth of the scope is not a good idea. Measuring a MHz signal on a MHz oscilloscope will not provide you with the best representation of your signal, as the filter has already begun to roll off and distort your input.

Measuring with too little bandwidth will provide distorted results. Here is the rule of thumb for choosing the right bandwidth:. Oscilloscopes can capture environmental noise. Oscilloscopes also add noise to your signal from filtering, processing, and digitizing though a high-quality oscilloscope will do all of this properly and add less noise than a poorly-designed scope. And noise occurs at all frequencies. So if you have a MHz oscilloscope, that scope is only going to show noise up to MHz.

But, if you have a 33 GHz oscilloscope, it will add noise to your measurement through its entire measurement range up to 33 GHz, regardless of the frequency of your signal. If you want to measure a 50 MHz signal, a MHz oscilloscope will give you plenty of bandwidth to clearly display your signal without attenuation and filter distortion but not so much that it adds high frequency noise content to your measurement. Insider tip: If all you have access to is a high bandwidth oscilloscope, but you are measuring low frequencies, turn on hardware filters in the oscilloscope to eliminate that high frequency noise and get a cleaner measurement.

The higher the bandwidth, the higher the price. If you are worried the bandwidth you need today will not be enough for future measurements, look for an oscilloscope that lets you upgrade the bandwidth with a software license.

That way you can buy the bandwidth you need now and upgrade later without having to purchase a new oscilloscope or send it in to the factory for a hardware update. Most Keysight oscilloscopes can be bandwidth upgraded with a software license for this very reason. Did she settle for the porridge that was too hot or too cold? She went for the one that was just right. Here is an example of how even a simple sine wave can be falsely represented on an oscilloscope without the right bandwidth.

In this demonstration, I am measuring a sine wave oscillating with a frequency of 80 MHz and a peak-peak voltage of about 2 volts. I am using an 8 GHz oscilloscope. This is an excessive amount of bandwidth for an 80 MHz signal. The rule of thumb for analog signals is to use about 3 times the frequency of the signal.

If I just want a quick check on the basics like voltage and frequency, the difference might not be crucial. The signal will be passing right through the 3 dB down point of the filter.

You can see that the voltage is decreased from 1. This is The effects of harmonics on a sine wave are best understood by performing a short, but extremely profitable, experiment. Actually, it is not necessary to have a variable frequency oscillator; fixed frequencies of 2 times, 3 times, etc.

The type of oscillator used should be of the sine wave variety. Any oscillator described as 'phase shift' or 'wien bridge' will be suitable - many other types of audio oscillator do not give a pure sine wave and are therefore unsuitable.

We now come to the experiment itself. A voltage of 6. The audio oscillator is applied to the slider and earthy end of this potentiometer, with the result that the signal level it injects into the circuit is adjustable. If this is a perfect sine wave it will have the appearance shown in waveform a below, but there may be slight irregularities due to the presence of harmonics. The result will be a waveform similar to that shown in b. If the second harmonic input is varied slightly in phase, the irregularity it introduces can be made to travel along the fundamental.

Sine Wave distortion measurements

See c. Note the basic difference between even and odd harmonics. Odd harmonics do not alter the symmetry of the fundamental, whereas even harmonics introduce marked asymmetry.

The effect of injecting higher harmonics can also be checked with the aid of this circuit. It is possible, simply by connecting a second audio generator in series with the first, to study the effects of two harmonics on the fundamental. AF amplifiers can be tested with an oscilloscope by making sequential stage checks, using a sine wave input.

The circuit of a typical 10 Watt high quality valve amplifier is given above, and the circled numbers in this diagram indicate the test points to which the oscilloscope may be applied, and the order in which the checks should be made.

It will be seen that tests commence at the speaker transformer secondary, and next proceed back to the input. Points 11 to 16 are bypassed, and little signal should be present at these points. A large signal amplitude indicates a faulty bypass capacitor, which must be replaced before further tests can be made. Points 17 and 18 are also bypassed, but it has to be remembered that a small level of signal will be present here due to the negative feedback from the speaker transformer secondary.

The final test is at point 19, which is the positive terminal of the reservoir capacitor for the HT rectifier. Some ripple voltage will be evident at this point, and also at point Experience with serviceable amplifiers will assist in determining the acceptable level for such ripple and will also give a good working idea of the waveform amplitudes to be expected at other points in the amplifier. If the amplifier is faulty it may be necessary to adjust the volume control to the level where distortion is most apparent.

Be careful not to overload the amplifier, or this will result in distortion, such as is shown in diagrams a and b below.Latest Projects Education. JavaScript is disabled. For a better experience, please enable JavaScript in your browser before proceeding. Sine Wave distortion measurements. Thread starter Wendy Start date Nov 12, Search Forums New Posts. Thread Starter Wendy Joined Mar 24, 21, Way back when I used a piece of HP equipment to measure distortion of a sine wave.

It was fun to use, but long gone. How would a hobbiest measure distortion of a sine wave? It can look good on a oscope and still be pretty bad.

One thought is to take the waveform in question, filter it though several low pass filters to create a reference without phase shifting itinvert it, and null it with op amps. What is left is the distortion. Anyone seen a home brew setup to do this?

With all the audiophiles out there I suspect it exists. Scroll to continue with content. SgtWookie Joined Jul 17, 22, I don't know how you could run a signal through several low pass filters and not get at least SOME phase shift, just due to propagation delays alone.

Use the output of the transformer to drive the y-axis of your o-scope, and the input signal for the x-axis. If there is no distortion and no losses, you should get a perfect circle on the 'scope.

Last edited: Nov 12, Any imperfections in the circle being distortion? It is often overlooked that to get meaningful measurements on low value distortion you require a very low distortion signal. The "poor man's" way is to use spot frequencies. Both the generator and the filter can be finely tuned, twinTEE being the frequency defining network of choice, although I have seen the Wein used as well. The old HP equipment don't remember it's model number you adjusted for a in the meter, then flipped the switch to automatic, and it would tune itself to the waveform.

My shop used it to check the quality of amplifiers in milspec equipment. I think it was a precursor to a selective voltmeter. Externet Joined Nov 29, 1, For a signal generator, I don't know. Adjust channel gain for minimum amplitude reading at maximum magnification. It may need adjusting delay setting. The thing to do to eyeball it in a scope is to use a dual channel. Input the signal input to the amp into one channel Take the output from the amp through a sharp filter to remove this frequency and amplify what's left or 10, times in the second channel.

You can see the distortion this way. Sorry, no dual channels here. X and Y maybe, but have I mentioned how old my oscope is? Another thought occurs, the PC sound card oscope can also be used as a spectrum analyzer, so I've heard.

Audioguru Joined Dec 20, 11,By definition, power factor is a dimensionless quantity ranging from -1 to 1. It is the ratio between real power dissipated in the load and apparent power, which oscillates in the circuit but does not dissipate in the load and consequently does not perform useful work. The value of the power factor depends upon the nature of the load. In a purely resistive load such as an incandescent light bulb or electric heater, the power factor is close to 1.

There is always some inductance in any conductive body. Reactive loads, either inductive or capacitive, lower the power factor, necessitating use of larger conductors and devices though the reactive component performs no useful work.

Also, there is wasted power. This is costly and undesirable for the utility. When the power factor is less than one, the cost usually gets passed on to the customer in the form of a power factor penalty added to the utility bill.

Most premises loads, especially in industrial or large commercial facilities, have a significant inductive component. Motors, transformers, and other magnetics — as well as fluorescent light ballasts and nonlinear electrical equipment — contribute to this imbalance. Capacitive loading is less common. When there is capacitive reactance, it is algebraically subtracted from the inductive load to find the net reactance. This is in contrast to the way resistive and reactive loads are added, which is vectorial, because they are out-of-phase.

Capacitive loads can be shunted across the line to intentionally correct unwanted inductive power factor. Similarly, more expensive synchronous motors can be used to counteract the power factor contribution of induction motors.

Power factor correction in the form of large capacitors in metal enclosures gets deployed at the generating station, at any point along the distribution line, or at the customer facility. Power factor, caused by a net inductive or possibly capacitive loading, arises from the fact that the applied voltage and measured current waveforms are out of phase.

Their peaks occur at different points in time, as shown along the Y-axis of and oscilloscope when it is operating in the time domain. It is easy to see why this happens. The amount of current in a reactive load relates not to the amplitude of the applied voltage, but to the rate of change of that amplitude.

Examine the graph of a sine wave and you will see that the slope is near vertical as it crosses the Y-axis and near horizontal at its peaks.

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When the amplitude of the applied voltage is high, the rate of change is low, and when the amplitude is low, the rate of change is high. In an inductive load, the current waveform lags the voltage waveform. Energy needed to build the magnetic field where it is stored requires a definite amount of time to do so, and another time interval elapses as the energy is returned to the electrical circuit.

If the load is capacitive, the current waveform is said to lead the voltage waveform, where electrical energy is stored in the dielectric medium in the form of an electrostatic charge.

Current does not actually flow through a capacitor, but the effect in an electrical circuit is equivalent. The amount of temporal separation between the voltage waveform and the current waveform — in other words the degree to which they are out of phase — is expressed as power factor.

What is Bandwidth? How Much Do You Need?

The oscilloscope is the preferred instrument for visualizing and measuring power factor in a real-world electrical system and also to ascertain the contribution of an individual motor or other device. An oscilloscope with conventional probing is a simple voltmeter but one that displays a graphic representation of the voltage plotted with respect to time.

In addition and simultaneously, an oscilloscope having at least two analog input channels is capable of displaying the current waveform of that signal, plotted against the same amplitude Y and time X axes. The two aspects of the signal may be shown superimposed or in split-screen format. Voltmeter leads are attached so the instrument is in parallel with the power source or load. An ammeter current reading, in contrast, uses leads connected in series with the power source or load.

Using a conventional ammeter usually a milliammeter requires cutting into one of the circuit wires or device leads and later re-soldering or re-terminating it.

how to measure distortion with oscilloscope

In making a voltage reading, the meter hookup is not problematic as long as CAT voltage restrictions are observedsince — the memter has a high impedance and is shunted across the circuit. In either low-impedance ammeter or high-impedance voltmeter readings, the circuit should not be loaded significantly.By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.

Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up. Suppose I have a signal that consists of a sine plus noise. Is it possible to measure the SNR with an oscilloscope? The SNR is the ratio between the power of all the signal and the power of all the noise.

In the rest of this answer, every time I say dB, it's in voltage. The power of the noise floor will be the sum of all the RMS values, squared.

The total power of the noise floor can be approximated, which I assume you want to do. So let's say you are doing a point FFT, you measure the noise floor and find out that it is about dB, this is your green graph. Then let's say you do the red graph and measure your sine wave and find out that the maximum point is 6 dB. If you have spectrum analyser settings, then you can adjust the bandwidth or number of fft bins.

Guest Video - Karl Adams - Audio Distortion Measurement

As you adjust this narrower, the noise floor drops. This is of course why we have filters, FFT's and spectrum analysers, and why we can make radios when the SNR at the antenna terminal is dB i. If the SNR is -ve then you may have to use a spectrum analyser, as the signal cannot be accurately measured in all the nosie.

Signal-to-noise ratio is a measure of useful signal amplitude compared to noise floor. Both quantities are usually defined in the power spectrum domain. And measurements in power spectrum domain heavily rely of instrumentation bandwidth and frequency resolution. Technically you can turn your oscilloscope into a simple spectrum analyzer if you have proper software installed, and this is a bit more than just FFT.

I would strongly suggest to read up on literature on fundamentals of measurements in frequency domain. For example, Keysight Technologies offers a lot of literature on the subject, like this Appnote. You might want to explore their offerings. Here is another word of wisdom from Keysight on the subject of SNR measurements. In your particular case the height of sine peak will depend on processing window and spectral resolution of your simple FFT routine.

So it is not simple, there are much more to proper SNR analysis.


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