Passive intermodulation distortion pim is a phenomenon that occurs when 2 signals present on a transmission line mix in a nonlinear manner. Analysis and measurement of two tone intermodulation. The multitone stimulus is distributed over the full audio band generating smaller voice coil peak displacement and less intermodulation than the critical twotone signal, but the distortion level is still. Human hearing distortion audibility part 3 audioholics. A method for measuring transient intermodulation distortion. One cd player, for example, might have higher measured noise than another cd player when measured with a rms method, or even an aweighted rms.
In order to measure the effects of intermodulation distortion, a black box was built which was capable of generating a known and controllable percentage of pure intermodulation distortion, and then listening tests were held at different sound pressure levels with different kinds of music with several speakers and listeners. When distortion is discussed, many think only of harmonic distortion. Measurement of intermodulation distortion in highlinearity. The problems from compression arise as a result of the distortion which occurs to the signal when the amplifier runs into compression. The amount of intermodulation distortion generated depends on the density of the input spectra. Aug 11, 2017 an attempt to measure imd in three vacuum tube amplifiers. Intermodulation distortion is caused by nonlinear movement of the cone e. Bridge method for measuring amplitude intermodulation. Im distortion and harmonic distortion are two pairs of shoes and must be defined individually as already shown in the short essay about myth and facts about aliasing but more on this later on.
Compression and receiver intermodulation distortion. To measure harmonic distortion with a spectrum analyzer the procedure illustrated in figure. The high isolation allows us to measure very high ip3 values without distortion from the synthesizers affecting. Harmonic distortion when flashad users talk about distortion they are generally concerned with the spurs introduced into the spectrum of interest. Jul 04, 2016 170710 valve studio guitar pentode preamp part 1b analog discovery 2 generates thd and imd duration. Understanding intermodulation distortion measurements. Distortion is a relative measurement, usually referenced to the linear por tion of the output signal both in ampli tude and frequency. These additional signals intermodulation products occur mainly in devices such as amplifiers and mixers, but to a lesser extent they also occur in passive devices such as those found in many transmission. As with harmonic distortion measurement, this is done with a spectrum analyzer or a dedicated intermodulation distortion analyzer. Sc amp is the worlds only dedicated audio test amplifier, with features such as integrated impedance measurement, fixed gain to reduce calibration errors, and dc voltage offset support. Nonlinear elements generate harmonic signals, or in more detail. Intermodulation distortion behavior in ldmos transistor.
The stimulus is a strong lowfrequency interfering signal f1 combined with a weaker high frequency signal of interest f2. In order to measure the effects of intermodulation distortion, a black box was built which was capable of generating a known and controllable percentage of pure intermodulation distortion, and then listening tests were held at different sound pressure levels with different kinds of. The practical measuring procedure is described, and measurement results of commercial audio power amplifiers and integrated operational amplifiers are. Introduction the theories and operations of intermodulation distortion imd are well understood. Imd adds components to the sound that are not found in the original signal. As with other tests described, two audio signals at different frequencies are fed through the device to be tested and the output is monitored. Introduction although twotone measurements still represent the industry standard in intermodulation distortion imd characterization, nowadays, engineers seek for alternative test procedures closer to the systems final operation regime. Intermodulation distortion is one of the most interesting signal analyzer measurements. The intermodulation distortion imd application makes it very easy to set up and calibrate sweptimd measurements of both amplifiers and frequency converters. Intermodulation distortion is nonlinear with level, in that an increase of 10 db in the test signal typically results in a far greater increase in level in the intermodulation distortion.
Bridge method for measuring amplitude intermodulation distortion matrix test equipment inc. The figure above illustrates the interpretation of the intermodulation distortion measurement using a fixed low frequency tone f 1 and a high frequency tone f 2 which is varied over the audio band voice tone sweep. Intermodulation distortion is the result of two or more signals interacting in a non lineardevice to produce additional unwanted signals. Intermodulation distortion measurements on modern spectrum analyzers this application note describes the differences in intermodulation distortion measurements between traditional spectrum analyzers with analog narrowband if signal path and modern spectrum analyzers using a wideband if signal path and digital rbw filters. It has also been found that an interaction between third and fifth order nonlinearities can give rise to an imd sweetspot if they intermodulation distortion behavior in ldmos transistor amplifiers. Measurement of intermodulation distortion at audio frequencies. Imd intermodulation distortion measurements provide useful information. Intermodulation distortion in audio is usually specified as the root mean square rms value of the various sumanddifference signals as a percentage of the original signals rms voltage, although it may be specified in terms of individual component strengths, in. Imd measurement with e5072a ena series network analyzer. A twotone stimulus with a constant bass tone at 50 hz generates significant intermodulation of the second tone anywhere in the audio band.
This method utilizes a bridge technique to cancel the test signals, leaving the di8 tortion products of the device under test. Close3rd order intermodulation distortion measurement. For measurements of this type, the test waveform is a nominally pure singlefrequency sinusoid and the measure is the summed level of any components produced at harmonics of the chosen frequency. The tektronix cookbook of standard audio tests 1975. This effect results from nonlinearities in your audio system. If separate test tones are fed to two separate loudspeakers, the loudspeakers harmonic distortion will have no influence on the measured intermodulation frequency components.
Ratio measurement procedures are optimistic in up to 7db when evaluating real cochannel distortion. Accurately characterizing third order intermodulation distortion imd3 in highlinearity photodiodes is challenging. Apr 14, 2016 for example, if a 2 khz audio tone and a 7 khz audio tone pass through a nonlinear device and undergo am intermodulation distortion, the output signal will contain new tones modulation products at 9 khz the sum of the tones and at 5 khz the difference of the tones. Nonlinear distortion characterized by the appearance of output frequencies equal to the sums and differences of integral multiples of the input frequency components.
Apr 22, 2005 harmonic and intermodulation im distortion are well known examples of nonlinear distortion. The existence of intermodulation components can affect the performance of an audio production in various ways. The most common method for indicating the amount of nonlinear distortion introduced by an audio amplifier is the total harmonic distortion thd. Radio receiver intermodulation performance radioelectronics. Rf amplifiers engineering notes, rf amplifiers technical reference material. Multitone intermodulation distortion performance of 3rd. Harmonic and intermodulation im distortion are well known examples of nonlinear distortion. Two measurement techniques are evaluateda standard twotone measurement and a more complicated threetone measurement technique to measure imd3. Aug, 2015 the amount of intermodulation distortion generated depends on the density of the input spectra.
Put another way, audio quality isnt just about total. Improve twotone, thirdorder intermodulation testing. As the input signal becomes more complex the intermodulation distortion becomes greater, and this can be seen of a speaker measurement in figure 15, where hundreds if not thousands of spectral distortion components are generated from a handful of. Nov 24, 2017 how does intermodulation distortion occur november 24, 2017 by rajiv leave a comment intermodulation is a phenomenon of combining undesired frequency components within an active system or from an external source. A method for measuring transient intermodulation distortion tim. Dec 16, 2011 im distortion and harmonic distortion are two pairs of shoes and must be defined individually as already shown in the short essay about myth and facts about aliasing but more on this later on. Intermodulation distortion in vacuum tube audio amplifiers elpaso tubeamps. For applications from cellular to wifi, and pmr to other professional radio applications, the receiver intermodulation performance is of paramount importance. Intermodulation interference is the undesired combining of several signals in a nonlinear device, producing new, unwanted frequencies, which can cause interference in adjacent receivers located at repeater sites. Audio system measurements are made for several purposes. Microphone intermodulation distortion measurement listen, inc. Aes elibrary intermodulation distortion listening tests.
Intermodulation causes spurious emissions that can create minor to severe interference to other operations on the signal. A model of the measurement system is developed and used to analyze the limitations of the two techniques in determining the. Here we have two signals, signal 1 at 595 hertz, and signal 2 at 600 hertz. An attempt to measure imd in three vacuum tube amplifiers. How to avoid aliasing intermodulation distortion aid the. Intermodulation distortion imd application option 087. Then, details on the measurement technique will be given. Put another way, audio quality isnt just about total distortion level, but the type of distortion too. Bridge method for measuring amplitude intermodulation distortion.
It is a side effect of the use of a too strong negative feedback, which makes the modern transistor amplifiers, in particular, very susceptible to it. Intermodulation distortion imd measures the level of unwanted combinations of different frequencies found in the input signal. Aes elibrary transient intermodulation distortion in. Consequently, only the distortion of the microphones will be measured fig. Its useful to keep in mind that evenorder distortion products are caused by asymmetrical driver system nonlinearities and conversely, oddorder distortion products are caused by symmetrical driver system nonlinearities. The din intermodulation distortion technique uses a similar method. A more meaningful test than thd, intermodulation distortion gives a measure of. How to test for intermodulation distortion marki microwave. Pdf a comparison of nonlinear distortion measurement methods. Intermodulation distortion article about intermodulation. This is the result of an interaction between two signals of different frequencies. Such harmonically related distortion is called harmonic distortion.
Transmit intermodulation is a measure of a transmitter to inhibit the generation of intermodulation products in the presence of the modulated signal and the tone signal. Intermodulation distortion is an artifact of nonlinear systems, so technically can occur either in the analog or digital domains. It controls the frequency and power of internal and external sources and tunes the receivers at main tones as well as imd tones in the single measurement channel. Intermodulation distortion professional monitoring. Such systems include cd players, portable media players and. The additional frequencies of a singlefrequency burst cause particular trouble to some types of amplifiers, and the resulting distortion appears more noticable to human listeners than more general distortion. Configuration of imd measurement with vna dut combiner attenuator optional usbgpib interface i. Intermodulation distortion in audio is usually specified as the root mean square rms value of the various sumanddifference signals as a percentage of the original signals rms voltage, although it may be specified in terms of individual component strengths, in decibels, as is common with rf work. This is also why we developed the pbr series of power combiners. Several techniques are currently in use for measuring distortion of audio equipment. Although imd may affect many types of signal data, this definition focuses on sound. Im designing an audio amplifier and im trying to simulate and measure all the characteristic features of it output power, offset, bandwidth, inputoutput impedances, etc.
Intermodulation distortion measurements on modern spectrum. Welcome to tis basics, to audio measurements and results, part 2, discussing intermodulation distortion, frequency response, and phase. It is one of the key metrics in categorizing the linearity of active and passive electronic components and devices. Interaction of intermodulation products between dut and. Since a distortion measurement on an audio effects device usually consists of. Intermodulation distortion is a tough thing to measure. Intermodulation should not be confused with general harmonic distortion which does have widespread use in audio effects processing. Filter for intermodulation distortion measurement of audio. If a modern spectrum analyser is available, the relative amplitudes in decibels from all components at the output can be displayed. However, i dont know how to measure intermodulation distortion imd or transient intermodulation distortion tim. Distortion analysis toolkita software tool for easy analysis of. Audio intermodulation distortion and frequency response.
Twotone, thirdorder intermodulation distortion im expresses the degree of nonlinearity of an electronic device that has a definable dynamic range, such as. Intermodulation distortion diagnostics in audio systems. Projectnotes engineering riefs b transient intermodulation distortion in commercial audio amplifiers matti otala university ooulu, sf90100 and raimoensomaa kajaani oy electronics, sf90100 oulu, finland oulu, finland they are therefore clipped, and the feedback mechanism 2, 6 then tends to lengthen up to a hundred times the natural cut. The intermodulation measurement measures the third order and. While the stiffness nonlinearity can not produce significant intermodulation, the other nonlinearities generate wideband. The concept of harmonic distortion is easy to understand you apply a clean sine wave, with very low content at its harmonic frequencies, and then analyze the output from the device you are testing for increased relative levels at those harmonic frequencies. As a result one should check the test conditions before comparing measurements of different loudspeakers. This article describes distortion measurement method that greatly ces contributions from the s source and the spectrum analyzer. Often abbreviated imd, it is an important metric of linearity for a wide range of rf and microwave components. Imd measurement measurement example sweep delta power levels of main tones and im products in swept frequencies can be monitored with the vnas absolute measurements. How to avoid aliasing intermodulation distortion aid the invisible distortion in digital audio if an adc the components used for the analog to digital conversion receives frequencies at more than the half samplerate, it will mirror the frequencies around the half of the sample frequency, and therefore create new aharmonic frequencies. The stimulus signal is the sum of the two sine waves. Traditionally, the imd of distorting systems has been measured by feeding a sum of. We encounter this problem every day trying to measure the ip3 of the t3, the worlds highest linearity mixer.
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