RU2037159C1 - Method of measuring signal nonlinearity - Google Patents

Abstract

FIELD: measurement technology. SUBSTANCE: output tested signal is compared with reference sinusoidal signal. Modules of relations of instant values of these two signals are measured at specific time moments for different phase shifts between these signals. Result achieved have to be averaged. Maximal absolute deviation of the modules is determined. Value of nonlinearity is determined from maximal absolute deviation in relation to averaged one relatively averaged value itself. Method is useful at infralow frequencies. Measurements may be carried out in real time scale. EFFECT: high speed of operation; high precision of measurement: simplified realization. 2 dwg

Description

 The invention relates to specialized measuring equipment and is intended to determine the nonlinearity of the output signal, for predominant use at infra-low frequencies in the study of the magnitude of the nonlinearity of elements and devices when high speed, measurement accuracy and ease of implementation are required.

 Figure 1 shows an example of definitions of measurement times for an arbitrary phase shift between signals; figure 2 is a structural diagram of a device that implements the method.

 For each phase shift (Fig. 1), four intervals are obtained on the period of one of the signals and, accordingly, four values of the ratio modules, which do not differ from each other taking into account the minimum error of the used comparison method for a signal without linear distortions. With an increase in nonlinear distortion, the deviations of the moduli of relations between themselves at different phase shifts will increase, and the ratio of the maximum absolute value of the deviation from the average value to the average value itself will determine the nonlinearity.

Indeed, if an output sinusoidal voltage is applied to the device under study, then the nonlinearity n can be determined by the following expression
n [(U _out U _l )] / U _l ] 100% (1), where n is non-linearity expressed as a percentage;
U _{o the} amplitude of the output voltage of the investigated device;
U _{l the} amplitude of the output voltage of the ideal (linear) device.

Expression (1) can be represented as follows
n [U _Rin (k + _and K _i) (U _{Bx and} By)] / (U K _{and Rin)}} 100%
(2) where U _{in is the} amplitude of the input sine wave,
For _a transmission ratio of an ideal device,
(K _a + K _i ) the transmission coefficient of the investigated device.

After transforming the expression (2), we obtain the expression for determining the nonlinearity in percent
n (K _m / K _a ) 100% (3), where K _{m is the} maximum deviation of the transmission coefficient from its nominal value.

 When using this method, the averaged value of the obtained amplitude ratios for many different phase shifts tend to a certain value.

The device (figure 2) contains the test unit 1, the phase shifter 2 and the two-beam oscilloscope 3. The input of the test unit 1 receives from the output of the reference generator (not shown in the diagram) a sinusoidal voltage signal
U _o (t) A _o sin ω t, (4), which also enters the input of the phase shifter 2, which can change the phase of the output voltage relative to the input phase. The output of the phase shifter 2 also receive a sinusoidal voltage
U ₂ (t) A _o sin (ω t + F _i ) (5) of the same amplitude, the value of which does not change with changes in phase shifts. At the output of the investigated device receive the output voltage
U ₁ (t) (K _a + K) A _o sin ω t (6).

Thus, the studied signals U ₁ (t) and the reference sinusoidal voltage signals U ₂ (t) having a phase shift F _o between themselves, for example, as shown in FIG. 1, are fed to the two inputs of the two-beam oscilloscope 3.

For each phase shift F _o at each of the four time points corresponding to the middle of the selected intervals, the amplitude ratio modules (K _a + K) are determined, the result obtained is averaged, and the maximum absolute value of the deviation of the current values of the ratio modules Km from the averaged result and with respect to To _m / K _a determine the nonlinearity in the signal under study. To increase the resolution, the number of phase shifts for analysis should be increased; if necessary, studies can be carried out in the large signal mode and in the small shift mode. It should be noted that the amplitude of the reference generator does not affect the measurement error, since the relative deviations of the values of the ratio modules do not depend on the obtained values of K _a .

Claims (1)

 A METHOD FOR DETERMINING SIGNALS NONLINEARITY, based on the conversion of the input signal and measuring the result on an indicator, according to which certain time intervals are allocated for analysis, characterized in that a reference sinusoidal signal with a frequency lying in the passband of the device under study is formed, is repeatedly shifted in phase by one the signal relative to the other, each time highlighting the time intervals inside the half-periods of two signals, when both signals do not change their signs, determine the modules with respect of instantaneous values of signals at time instants corresponding to the middle of each of the selected time intervals average the obtained values of the relationship moduli and determine nonlinearity as the ratio of the maximum absolute value of the deviation of the value of the ratio moduli from the average value to the average value itself.