AUDIO UNIVERSAL METER


     Having the capability of the whole measuring laboratory, this Universal Meter allows to carry out comprehensive testing both commercial and professional electronic audio equipment.
     The work with the Meter is very simple, there is no need to resort to programming and at all to read the Meter's manual - its front panel tells the user what can be measured and what should be done for that. Such routine procedures as range selecting, calibration, nulling, tuning and others are performed fully automatically.
     Constructively, the Universal Meter consists of two parts (UM1 and UM2) which cope with different tasks of measurement; the UM1 Meter being fully independent and therefore can be manufactured, sold and used separately, while the UM2 Meter in most applications must be added by the UM1.

     The UM1 Meter comprises:
     - an ultra-low distortion oscillator producing two sine- and two corresponding square-wave outputs in a 16Hz to 160kHz frequency range with manually variable level of each output, phase shift (within ±180°) between them and manually fine or automatically step-by-step (1/3 or 1 octave) variable frequency, both sine outputs having less than 0,0002%THD and better than ±0,3% amplitude stability at any audio frequency;
     - a precision autoranging two-channel millivoltmeter having basic accuracy of ±0,3% and functioning in the mean-rectifying or RMS mode, in these cases the frequency range being correspondingly 20Hz to 200kHz and 10Hz to 1MHz, dynamic range - 0,03mV to 100V and 0,7µV RMS to 100V RMS and measurement time - 0,5s and 1,5s;
     - an accurate self-calibrating analogue-to-digital converter handling two DC voltages and measuring their levels and ratio in lin or log scales; in combination with the previous unit it allows to obtain two AC signals' ratio within ±120dB with a 0,01dB resolution;
     - a simple auxiliary circuit for phase and RLC-measurements, providing a 0,1° accuracy of phase measurements within ±180° in a frequency range of 20Hz to 20kHz and ±0,3% accuracy of RLC-measurements in the next ranges: 100mOhm to 10MOhm for R, 0,01mH to 1000H for L and 0,1pF to 100µF for C, the measuring frequency being 1kHz;
     - an autoranging frequency meter operating in a 10Hz to 1MHz range, having a 1s measurement time and providing a 0,01Hz resolution at the lowest frequencies;
     - two digital displays, first of them giving the value of frequency and the second - any other measured function, the latter display being also duplicated by the traditional pointing indicator.
     The UM1 Meter has two inputs (Y1, Y2) to which external audio signals can be applied and two outputs (X1, X2) from which the signals generated by its internal oscillator can be taken. There are also additional RLC-terminals for connecting the resistor, coil or capacitor to be examined.
     After making all the necessary connections, the user should select the required measurement function - voltage mean-rectified V, voltage VRMS , ratio K (level, gain, attenuation), ratio K(D) (distortion, in combination with UM2), phase, R, L, C - and then choose where the measurement is to be carried out (one of the inputs/outputs Y1, Y2, X1, X2 or a pair of them in the case of ratio and phase shift). In one second after that the measurement result will appear on the FUNCTION display.
     Frequency measurement is switchable, it can be performed either on the UM1 Meter outputs or on its inputs, the result being represented on the FREQUENCY display.
     Noise measurements can be carried out with the desired spectrum limitation - the Meter includes selectable A-weighting and audio 20Hz-20kHz band filters.
     To obtain the amplitude- or phase-frequency response, the UM1 Meter's oscillator should be used in the automatic, one- or third-octave scanning mode with a 2-20s duration of one step oscillation. The results of running the chosen sequence can be viewed on the two displays (FREQUENCY and FUNCTION), noted or printed on a standard printer. The FUNCTION display gives in this case the measured voltage V, ratio K or phase at each current frequency.

     The UM2 Meter, being the second part of the Universal Meter, includes:
     - an automatic active rejection filter accepting an audio signal within 0,1-30V RMS in a frequency range of 20Hz to 20kHz, normalizing it to a 1V level and then performing a 135dB suppression of the fundamental frequency with variable up to +80dB gain of the residuals in a 100kHz bandwidth;
     - a selective filter performing automatic tuning to the frequency of a reference signal in a 32Hz to 160kHz range, keeping within ±2% the analyzed signal’s level at this frequency and providing a maximum 40dB stop-band attenuation and variable up to 30 Q-factor;
     - a voltage-controlled oscillator producing two quadrature sine- and two corresponding square-wave outputs with manually, by a single knob, variable frequency in two ranges - 20Hz to 20kHz or 200Hz to 200kHz and also having the mode of automatic, provided with marks smooth and one- or third-octave stepped frequency sweep between two preliminarily set limits in the entire frequency range or its narrow band with a period of 0,1s to 1000s, the accuracy of the established 1V/decade voltage-to-frequency relationship and the sine-wave output amplitude stability being correspondingly within ±1% and ±0,5%;
     - a fast AC millivoltmeter acting in a wide range of accepted voltages (up to 1000:1 or 60dB) and frequencies (from 20Hz to 200kHz), providing in these conditions better than ±1% accuracy and having a 0,1s response time;
     - a one-channel oscilloscope with the long-persistance phosphor screen.
     Similar to the previous device, the UM2 Meter has two inputs (Y1, Y2) and outputs (X1, X2) too. The selectable measurement functions here are distortion, selection, oscillation and the mode of using the UM2 as an oscilloscope. The first two functions are usually realized jointly with the UM1, the two Meters must be therefore connected with each other.
     To conduct distortion measurements, the user should apply the signal to be investigated to the UM2 Meter Y1 input, select the DISTORTION function and then to choose THD or the desired harmonic's number. In some seconds after that the THD+noise residue or the chosen harmonic will appear on the UM2 Meter's screen and simultaneously the UM1 Meter's FUNCTION display will represent the measurement result (in %).
     THD resolution is better than 0,0003% at any audio frequency, the measurement here being carried out in a 100kHz bandwidth. When measuring the separated harmonics of distortion, the selective filter, being included in this case to distortion measurement chain, brings the resolution down to 0,00002%.
     By the same way, the selective filter improves to 20nV the sensitivity of voltage measurements conducted by the UM1 Meter, the necessary condition for that is originating the investigated voltage from the Meters' oscillators.
     Usually, the tuning signal for the selective filter is derived from the voltage-controlled oscillator, so if the latter runs in its stepped or smooth sweeping mode, the filter, set in corresponding Q-factor, can perform a coarse (one- or third-octave) or fine spectrum analysis of the signal passing through it. The measurement function, as in the previous case, is here SELECTION.
     The next function, OSCILLATION, means using the voltage-controlled oscillator as a flexible signal source for various purposes, one of them being dynamic plotting of the amplitude-frequency response on the UM2 Meter's screen. For that, the device under test should be fed from the sweeping X1 output and its response signal - applied to the Y1 input from which the AC millivoltmeter performs fast continuous measurement of this signal and sends the result (in V or dB) to the scope or dumps it to an external printer.
     The Universal Meter's two parts are depicted in the enclosed drawing.




Fig.1. Audio Universal Meter.


     The below are simplified circuit diagrams of the main parts of the Universal Meter.




Fig.2. Low distortion oscillator.




Fig.3. Voltage-controlled oscillator.




Fig.4. Rejection filter.




Fig.5. Selective filter.




Fig.6. RMS converter.

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