Loudspeaker impedance meter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Measuring technology

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This device measures the modulus and phase of the loudspeaker's electrical impedance in the audio frequency band and is very useful for audio enthusiasts who build or modify their own loudspeakers. Knowing these parameters allows you to correctly configure the phase inverter, select and calculate the crossover filters of the loudspeaker, and improve its phase response.

The frequency dependence of the resistance modulus, as well as the phase shift between current and voltage on the coil of a typical low-frequency loudspeaker head, is shown in fig. 1. The impedance below the natural resonance frequency is inductive, at resonance it is active, and above it it is first capacitive and then becomes inductive again with increasing signal frequency. The phase-frequency characteristic of the impedance allows you to obtain additional information necessary for the calculation and analysis of the loudspeaker.

Using the device proposed here, it is possible to determine the specified characteristics in the frequency range of 17,4 Hz ... 29,4 kHz. The measurement limits of the impedance modulus and phase angle are respectively |Z|= 0...200 Ohm and f=+90°. The measurement results are reflected in the form of DC voltages 0...200 mV and 0...+900 mV, numerically coinciding with the corresponding parameter values. Two general purpose digital voltmeters or multimeters can be connected to the device for faster measurements. Recorders may be used.

The principle of operation of the meter, the scheme of which is shown in Fig. 2 is as follows. In two frequency ranges covering the entire audio frequency band, the generator generates two sinusoidal voltages that differ in phase by 90 ° (quadrature signals). One of them in the form of a stable current is supplied to the load under study - a loudspeaker or head, and the other, leading in phase by 90 °, is converted into a rectangular signal - a meander. The square wave phase is the reference for measuring the phase shift between the sinusoidal current and the head voltage. Provided that the current through the coil is stable, the voltage across it is proportional to the modulus of the impedance.

(click to enlarge)

The generator in the meter is built using an op-amp and a voltage-controlled current amplifier (ITUN). To ensure the necessary frequency setting accuracy, the sound frequency range of the generator is divided into two. Dual variable tuning resistors (R6 and R8) are connected in series with the limiting resistors. For them, an exponential characteristic of the change in resistance is required (group B). Using switch SA1, the generator frequency range is selected: in one position - 17,4 ... 1000 Hz, in the other - 530 Hz ... 29,4 kHz.

In the DA2.4 op-amp generator, the frequency-setting elements are a tunable phase filter and an inverting integrator on ITUN DA1 and DA2.3 op-amp, covered by feedback. The integrator has a phase shift of 90°, so the oscillator phase balance condition is met when the phase filter produces a phase shift of -90°. In sum, the phase rotation is 0°.

The operating frequency fG of the generator is determined by the elements R8, R9, C10 (or C9):

To maintain the oscillation amplitude at the output of the integrator in the range of operating frequencies, its input current must change in proportion to the frequency. The corresponding change in the output current DA1 is achieved by adjusting the control current ITUN (on pin 5) with a variable resistor R6, combined with another frequency-setting resistor R8.

The imperfect matching of the resistances of the resistors R6 and R8 in the frequency band leads to a change in the amplitude of the generated voltage, but the auto-regulation circuit restores its required value. The current rectified by the diode VD1, proportional to the amplitude of the oscillations, is algebraically summed through the resistor R12 at the input of the integrator DA2.2 with the current through the resistors R13, R14. With an increase in the signal, the output voltage of the integrator DA2.2 decreases, and the current ITUN DA1 also decreases. As a result, a stable oscillation amplitude is established, equal to 2,14 V.

The corrective integrator on DA2.1 performs the function of stabilizing the DC mode, forming a tracking feedback circuit, and maintains the voltage at the DA2.4 output with an accuracy of several millivolts.

The voltage generated by the generator is converted by resistor R15 into the corresponding load current. Due to the relatively low resistance of this resistor compared to the load (Zn max = 200 Ohm), accuracy in the measurement range of the parameter is ensured by a special voltage-to-current converter: the AC rectifier on DA3, together with R15, acts as a current generator in relation to the tested head.

For clarification in Fig. 3 shows a diagram of the Howland current source, which is formed from a negative resistance converter (for more information about it, see the book by V. L. Shilo "Linear integrated circuits". - M: Radio and communication, 1979. - Approx. ed.).

If

the internal resistance Ri of the source and the current IL flowing through the load from the voltage source Ue are determined from the relationships:

If

the internal resistance Ri reaches a very high value.

Note that the described properties of the current generator are also preserved when elements of a full-wave rectifier are introduced into it. Thus, the effective internal resistance increases to approximately 36 kOhm. Resistors R16-R20 must be used accurate (deviation no more than 1%). When independently calculating the resistance of resistors, R22 must also be taken into account, focusing on the values ​​​​of the coefficients

For DA3, an op-amp with a high gain cutoff frequency was used, while the frequency-dependent rectification error can be neglected. This broadband open-loop op amp has a DC gain of about 1500, so diodes VD2 and VD3 are chosen with a low forward voltage. Capacitors C11 and C13 separate DA3 from the diodes in the OOS circuit, and the op-amp bias voltage does not affect the measurement result. Its input stage on pnp transistors has a typical base current value IB = 2,8 μA, which provides a voltage drop across the resistor R22 relative to the output of the DA3 op-amp of about 0,9 V, sufficient to polarize the tantalum capacitor C13.

Rectified for measuring |ZН| voltage is removed from the cathode of the diode VD2. It consists of two components: the negative half-wave corresponds to the voltage at the load ZH, the positive half-wave of the voltage is amplified by alpha times. The integrating circuit R21C14 forms from this asymmetric AC voltage the average value UC14, which is the output rectified voltage (in millivolts), numerically equal to the impedance modulus (in ohms):

The magnitude of the phase shift between the measured current and the voltage acting on the load is determined using two comparators DA4 and DA5 and the DD1 microcircuit. Regardless of the load resistance, an alternating voltage acts on the resistor R23, the double amplitude of which is greater than the sum of the voltages acting on the diodes VD2, VD3, so the DA4 comparator switches clearly even with a low-resistance load. The sinusoidal voltage acting at the DA2.3 output is converted by the DA5 comparator into a rectangular voltage.

After the comparators, both signals are processed by four parallel-connected XOR elements of the DD1 microcircuit, the supply voltages of which are equal in magnitude relative to the common wire. As a result, after integrating the voltage pulses from the outputs DD1 by the elements R28-R33, C19 and C20, its average value corresponds to the phase shift (numerically in degrees) between the measured current and the alternating voltage falling on the resistance ZH.

The device is powered by a separate unit with integrated voltage stabilizers. It provides a bipolar supply voltage of +6,7 V relative to the common wire with a total value adjustment within + 15%.

A 200 ohm precision resistor is suitable for calibrating the impedance meter. Then, at a signal frequency of, for example, 100 Hz, resistor R14 sets a voltage UZ = 200 mV at the load. The voltage Uf must be set only by adjusting the voltage in the power supply. The R24C16 circuit compensates for some phase shift caused by the active rectifier on DA3. As a result, the setting of the tuning resistor R24 ​​at high frequencies is carried out so that there is no phase shift for the non-inductive load dummy resistor (f=0°).

To calibrate the phase meter, the outputs of both comparators are temporarily connected to the -6,7 V power bus, and the trimming resistor R33 slider is set to the position at which Uf = -900 mV is obtained.

About the possibility of replacing the elements of the device.

It is permissible to replace the TL084 op-amp microcircuit with TL074, TL082 or domestic K574UD2 (the last two microcircuits contain two op-amps in the package). As amplifiers and comparators DA3-DA5, you can use the K1401UD6 chip, which contains one op-amp and a comparator each. However, the LM311 comparators are replaceable by others that have an open collector output - LM306, LM393, K554CA3, KR521CA3. EL2044CN op amp can be replaced with another broadband one; the input stage of most of these op-amps is made on npn structure transistors and therefore it will be necessary to change the polarity of switching on the capacitor C13.

Diodes VD1-VD3 (with a Schottky barrier) have a reduced voltage in direct connection; they are replaced by KD922(A-B), KD523A. However, if the broadband op amp DA3 has a gain of more than 5000, it is permissible to use diodes of the KD503, KD518, KD520 series.

CD4030 has a domestic counterpart K561LP2. In the PSU rectifier, it is possible to use diodes KD521, KD522 with any index and a microcircuit of an adjustable bipolar voltage regulator KR142EN6 (NE5554).

We also note that almost any functional generator is suitable as a quadrature signal generator, containing in its structure an integrator and a triangular-to-sinusoidal signal converter with an output impedance of no more than 50 ohms.

Author: Kuhle H. Messchaltung fur Lautsprecher. - Radio Fernsehen

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