RU2247995C2 - Harmonic distortion digital meter - Google Patents

Abstract

FIELD: electric and radio measurement technology.

SUBSTANCE: device allows estimating presence and degree of harmonic distortions in amplifying sections. Estimation of harmonic distortions is based upon measurement of value showing difference of duration of positive and negative half waves of signal after non-linear transformation under condition that the initial process has to harmonic process with permanent parameters. Harmonic distortions meter has sinusoidal signal generator, comparator, two units for measuring time intervals and inverter. Input of inverter is connected with output of comparator which has signal input to be test input of digital meter. Reference voltage input of comparator is connected with common bus. Data inputs of first and second units for measuring time intervals are connected with output of comparator and output of inverter correspondingly. Admission inputs of units for measuring time intervals are integrated to compose gating input of digital meter. Output of units for measuring time intervals have to be data outputs of digital meter.

EFFECT: simplified design.

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Description

The invention relates to the field of electro-radio measurements and allows to assess the presence and degree of harmonic distortion in the amplification paths.

As a prototype, a classical device for measuring harmonic distortion is selected, which contains a sinusoidal signal generator, a notch filter, two RMS measurement units and a ratio measurement unit, the output of which is the information output of the device, the test output of which is the output of the sinusoidal signal generator, and the input is the input a notch filter, the output of which is connected to the input of the first unit for measuring rms values, the output of which is connected with the first input of the relationship measuring unit, the second input of which is connected to the output of the second rms value measuring unit, the input of which is combined with the input of the notch filter [Meizda F. Electronic measuring instruments and measurement methods: Trans. from English - M .: Mir, 1990, p. 384, fig. 14.146].

The prototype meter is used for approximate calculation of the harmonic coefficient K _g , expressed in this case as the ratio of the rms value of the higher harmonics to the rms value of the entire signal

where U ₁ , U ₂ , ... U _l are the harmonics amplitudes.

It is known that the calculation of the rms values of non-sinusoidal signals in a relatively wide dynamic range leads to the construction of fairly complex converters, regardless of whether the meter is implemented in an analog or digital basis. Of course, this feature of the prototype is its significant drawback, since hardware complication reduces reliability and increases the cost of meters. Moreover, the latter factor limits their widespread use. Another disadvantage of the prototype is the need to remove the fundamental harmonic. The accuracy of calculating the harmonic coefficient K _g depends on the efficiency of this operation, as it is easy to verify from the above formula. It is important that the notch filter used for these purposes does not attenuate neighboring harmonics, which are usually much smaller in amplitude than the fundamental.

The technical result achieved by using the present invention consists mainly in simplifying the meter.

The technical result is achieved in that a digital harmonic distortion meter containing a sinusoidal signal generator, the output of which is the test output of the meter, according to the invention further comprises a comparator, two time interval measurement units and an inverter, the input of which is connected to the output of the comparator, the signal input of which is a test input meter, the input of the reference voltage of the comparator is connected to a common bus, the information inputs of the first and second blocks for measuring time int vomited respectively connected to the output of the comparator and to the output of the inverter, permitting inputs timeslots measuring units incorporated in and constitute a strobe input of the meter, the outputs of the measurement unit time slots are information meter outputs.

The invention is illustrated by drawings. Figure 1 shows graphs illustrating the principle of harmonic distortion estimation, and figure 2 is a functional diagram of a digital harmonic distortion meter.

The graphs of Figure 1 contain a variable component _Rin u (t) of the input sinusoidal signal, performing a function test, and a variable component u _O (t) signal output from the nonlinear path. The time parameters Δ t ₁ and Δ t ₂ are the durations of the positive and negative half-waves of the signal u _out (t), respectively.

Functional diagram (figure 2) of a digital harmonic distortion meter consists of a sinusoidal signal generator 1, a comparator 2, time interval measurement blocks 3 and 4, an inverter 5, and a test amplifier 6 with a load R _L. The output of the generator 1 is the test output of the meter, the test input of which is the signal input of the comparator 2, the output of which is connected to the input of the inverter 5, the input of the reference voltage of the comparator is connected to a common bus, the information inputs of blocks 3 and 4 are connected respectively to the output of the comparator 2 and the output of the inverter 5 , the enable inputs of blocks 3 and 4 are combined to form the gate input E of the meter, and the clock inputs of blocks 3 and 4 form the clock input CLK of the meter, the outputs of blocks 3 and 4 are information outputs of the meter body.

In turn, block 3 (4) measuring time intervals consists of an element And 3-1 (4-1) and a counter 3-2 (4-2), the counting input “+1” of which is connected to the output of the element And 3-1 (4-1).

The operation of the meter is based on a method, the essence of which is as follows.

When a sinusoidal signal acts on a nonlinear four-terminal network, various sections of the signal u _in (t) amplify differently depending on how the steepness of the transfer characteristic of the four-terminal network changes. In particular, the positive and negative half-waves of the input action u _in (t) will be amplified differently if the transfer characteristic has the form of a monotonic nonlinear function. After isolating the variable component, the reaction u _out (t) of the four-terminal network will be a periodic signal, the positive and negative half-waves of which have different durations, depending on the nature of the nonlinearity of the four-terminal network. Figure 1 shows an example of the passage of a sinusoidal signal with a frequency of 1 kHz and an amplitude of 0.15 V device, the transfer characteristic u = f (u _I ) which can be represented by expanding in a Taylor series relative to the operating point u ₀ and limited by the first three members

u (t) = 0.4 + 4 (u _in (t) -u ₀ ) +11.55 (u _in (t) -u ₀ ) ²

According to the result u _in (t), the graphical form of which is shown in FIG. 1, it is easy to understand that the durations of the positive Δ t ₁ and negative Δ t ₂ components of the quadripole reactions really differ. Moreover, modeling various characteristics of non-linearities by changing the coefficient in front of the quadratic term of the above polynomial showed a strict relationship between the degree of non-linearity of the transfer characteristic and the difference | Δ t ₁ -Δ t ₂ |, which becomes the greater, the more pronounced the non-linearity of the transfer characteristic.

From the foregoing, it follows that a numerical indicator of harmonic distortions can be a value that shows how the durations of the positive Δ t ₁ and negative Δ t ₂ half-waves of the signal after non-linear conversion differ from each other, provided that the harmonic process with a constant amplitude, frequency and phase. Hence, as a practically applicable parameter, it can be as the modulus of the difference δ _s = | Δ t ₁ -Δ t ₂ | (1) and the ratio

,

,

either relative deviation

Thus, the harmonic distortion meter should measure Δ t ₁ and Δ t ₂ as the determining variables for subsequent simple calculations (1) - (3).

From the functional diagram (figure 2) of the harmonic distortion meter it is seen that the test sinusoidal signal from the output of the generator 1, passing the tested amplifier 6, is fed to the signal input of the comparator 2, intended for the formation of rectangular pulses with an amplitude corresponding to a logical level of “1”. The durations of the received pulses will be equal to the durations Δ t _{1 of the} positive components of the output process, and the pauses between them - the durations Δ t _{2 of the} negative components. An inverter 5 is used to isolate pauses. As a result, pulses of Δ t ₁ duration are supplied to the information input of block 3, and Δ t _{2 of} duration to a similar input of block 4. The beginning and duration of the measurement process are specified by the gate input E. The duration τ of the strobe pulse at input E must be a multiple of the period T of the test signal u _in (t), i.e., selected from the condition

τ = nT, n = 1, 2, ..., N.

In this case, an equal number of positive and negative half-waves will be recorded.

To reduce the measurement error, the estimation of the parameters Δ t ₁ and Δ t ₂ in practice should be carried out not for one period T, but for a time equal to many periods. The sums obtained at the same time at the outputs of counters 3-2 and 4-2 (outputs of blocks 3 and 4)

can be used both to calculate averages

(вместо Δ t₁ и Δ t₂), а во втором - непосредственно суммы S₁, S₂.and for the direct calculation of the parameters δ _s , δ _D or δ. In the first case, the values are substituted into formulas (1) - (3)

(instead of Δ t ₁ and Δ t ₂ ), and in the second - directly the sum of S ₁ , S ₂ .

The introduction of the total durations into formulas (1) - (3) allows one to reduce the relative measurement error caused by discretization due to lengthening the measured time interval. However, it is easy to understand that in this case the application of formulas (1) - (3) will allow us to calculate not the difference in durations of the bipolar components, their ratio or relative deviation, but the difference in total durations, the ratio of total durations and, accordingly, the relative deviation of the total durations. Moreover, for comparability of the results obtained on various devices that implement the above-described principle, it will be necessary to introduce a single initial parameter: the total half-wave duration of one sign obtained for a strictly fixed number of periods of the test signal, the frequency of which does not change and is also taken as a single one.

Returning to the functional diagram of the meter (figure 2), we note that the connection of the tested amplifier 6 with the signal input of the comparator 2 should be only for alternating current. Therefore, if the load of the device through the isolation capacitor is not provided in the circuit of the device under test, it must be included in the circuit between the output of the device under test (for example, amplifier 6) and the signal input of comparator 2.

The presented device (figure 2) has the following advantages over the prototype:

- the meter is functionally simple and does not require large hardware costs during construction;

- operations on the output signal containing informative components (higher harmonics) are performed only in the time domain, which allows the use of purely digital processing;

- improving the accuracy of measurements is achieved by well-known and fairly simple methods, providing for an increase in the sampling frequency (if the elemental base allows) and a decrease in the relative error.

Claims (1)

A digital harmonic distortion meter containing a sinusoidal signal generator, the output of which is a test output of the meter, characterized in that a comparator, two time interval measuring units and an inverter are connected to it, the input of which is connected to the output of the comparator, the signal input of which is the test input of the meter, input the reference voltage of the comparator is connected to a common bus, the information inputs of the first and second blocks for measuring time intervals are connected respectively to the output of the comparator pa and to the inverter output, allowing block inputs measurement time slots incorporated in and constitute a strobe meter input, outputs measuring units timeslots are information meter outputs.

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