SU1636782A1 - Compensation method of alternating voltage amplitude measurement - Google Patents

Abstract

This invention relates to an electrical measurement technique. The purpose of the invention is to improve the accuracy by reducing the frequency reliability of converting the amplitude of the alternating voltage into a constant voltage. The method is implemented in an amplitude detector. The input signal through the coupling capacitor 4 is fed to the gate of the field-effect transistor 1, where the signal from the output of the DC amplifier 3 is also connected via the resistor 5. Field-effect transistor 1 serves for thermal compensation, and current sources 6 and 7 for specifying operating conditions. The capacitor 8 is designed for frequency correction of the DC amplifier 3 so that the frequency of the input signal is higher than the limiting frequency of the gain. 3 il. with s

Description

O5 SO OE J 00 1чE

Fig.Z

The invention relates to the field of detecting an amplitude value of an alternating voltage, namely, measuring the amplitude of an alternating voltage of medium and high frequency, and may find application in which an accurate measurement of the amplitude of an alternating voltage of stable or varying frequency is required, for example, in creating masses - spectrometers quadrupole to omegatron types.

The aim of the invention is to improve the accuracy by reducing the frequency error of the conversion.

FIG. Figure 1 shows a graph (characteristic) of the dependence of the current-drain current on the gate-source voltage of a field-effect transistor; FIG. 2 shows the dependence of the constant voltage applied to the gate on the value of i alternating voltage; on fig.Z - Ischem amplitude detector.

FIG. 1, the operating point a of the field-effect transistor, selected near the cutoff, corresponds to the voltage U and the current d; When an alternating voltage is applied to the gate, the voltage is converted to a current that is asymmetric due to the nonlinearity of the characteristic; the positive half-wave is amplified to a greater extent, and the negative is less or cut off. In this case, the average value of the drain-source current increases. If simultaneously with the alternating voltage a constant voltage is applied to the gate, adjustable so that the average value of the drain-source current is always 0, then the value of this constant voltage can serve as a measure of the amplitude of the alternating voltage (Fig. ). For sufficiently large values of the amplitude, the dependence is close to linear.

The implementation of the method is illustrated by the operation of the amplitude detector

The amplitude detector (FIG. 1) contains a balanced pair of field-effect transistors 1 and 2 ,. DC amplifier 3, coupling capacitor 4, resistor 5, two current sources 6 and 7 and a capacitor. The output of the amplifier 3 through a resistor 5 is connected to the gate of the transistor 1, which is connected via a separating condenser

the detector 4 is connected to the detector input, the gate of transistor 2 is connected to the common wire, and the inputs of the operational amplifier are third sources of transistors 1 and 2, current sources 6 and 7 are included in the source circuits of field-effect transistors 1 and 2, the drains of which are combined. The capacitor 8 is connected between the inverting input and the output of the DC 3 amplifier. In the detector, a variable voltage is supplied to the input through the coupling capacitor 4, which is not amplified, but converted into a DC voltage at the output of the DC amplifier.

Conversion is possible subject to the conditions

  f gr.ot)

0 5

0. „5

,-P

five

where f- .. is the frequency of the converted

BH,

input voltage, ffh - limiting frequency of the amplification - ° nor the amplifier of the constant

current f - limiting gain frequency

field effect transistors. This mode is not an input voltage boosting mode, as this does not increase the AC or DC voltage. Capacitor 8, connected between the inverting input and the output of the operational amplifier, reduces the limiting frequency of the DC amplifier, thereby expanding the frequency range voltage in the direction of small values.

The detector works as follows.

i

If there is no detector at the entrance

AC voltage at the output of amplifier 3 sets the voltage corresponding to the balance of the circuit. Since the currents of sources 6 and 7 are equal, the characteristics of transistors 1 and 2 are the same, and the gate voltage of transistor 2 is zero, this voltage, which corresponds to the balance of the circuit, is also close to zero.

When an alternating voltage arrives at the detector input, the drain-source of transistor 1 becomes pulsed; tension Equality is broken at low amplitudes. The error that arises is greater, the more linear the characteristic of the transistor. This error is fundamentally unavoidable. However, for applications in which the absence of frequency losses is more significant, the range of amplitudes detected with a sufficiently small error may be sufficient. In the amplitude detector there is no valve element. The pn junction gate of the field-effect transistor is not a valve, since during operation the reverse voltage is always applied to it. Therefore, there is no loss of charge type switching. The frequency properties of the detector are determined by the process of modulating the current in the channel of the field-effect transistor, which is principleless in principle. The alternating voltage at the outlets of the source and drain is absent. Therefore, the frequency loss is minimal. They can occur only at the detector input. Moreover, all parasitic capacitances can be considered connected in parallel and forming a divider

Voltage with isolating capacitor. To reduce their effect, capacitor 2 should have a sufficiently large capacitance.

Claims (1)

Invention Formula A compensation method for measuring the amplitude of the alternating voltage, which consists in that the input of the reference element is supplied with the sum of the measured alternating voltage and constant voltage, characterized in that, in order to improve the accuracy by reducing the frequency error of the conversion, as an element of the comparison is use an element with a nonlinear transfer characteristic, fix the average value of the output signal of this element and change the DC voltage, keeping the average value of the output signal of the element with a nonlinear transfer characteristic, measure the current value of the constant voltage and take the measurement result as a measure of amplitude measured ac voltage  cm In / №Ср , t 1 Phys. 1 On