CZ301389B6 - Circuit arrangement for measuring extreme impedances of microwave circuit elements - Google Patents

Abstract

In the present invention, there is disclosed a circuit arrangement for measuring extreme impedances of microwave circuit elements comprising a circuit vector analyzer (1), which measures transmission coefficient from an excitation port (6) to a receiving port (7), a 3dB 180 degree hybrid element (2), which has its input port (8) connected to said excitation port (6), two internally non-insulated output ports (10) and (11). One of these output ports (10, 11) is connected to measured impedance (3) while the other is connected to reference impedance (4). A third output port (9), internally insulated from the input port (8), is connected via an amplifier (5) with the receiving port (7) of the circuit vector analyzer (1). The amplifier (5) amplifies a differential signal leaving the hybrid element (2). The vector analyzer (1) has a circuit arrangement for measuring transmission coefficient.

Description

Wiring for measuring extreme impedance of microwave circuit elements

Technical field

The present invention relates to a new circuit designed to measure the impedance of microwave circuit elements whose impedance differs considerably from the usual reference impedance values of 50 or 75 Ω.

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BACKGROUND OF THE INVENTION

Vector circuit analyzers are used to measure the impedance of circuit elements in the microwave region of the spectrum, which measure the reflection coefficient of the measured element relative to the reference impedance of the vector analyzer, which is typically 50 or 75 Ω. The reflection coefficient is given by the ratio of the incident and reflected voltage waves and is given by the relation

Z _x - Zo r \ = - (1),

Z _x + Zfl where Z _x is the measured impedance value and Zo is the reference impedance value of the vector circuit analyzer. When measuring, on the other hand, the measured reflection coefficient Γ _{χ is} calculated by the relation

Ι + Γ,, (1)

1-r, yo obtains the measured impedance value Z *. For impedance values close to the reference impedance value, this method provides a good resolution of the impedance values, but for impedances that are considerably smaller or considerably greater than the reference impedance, the magnitude of the reflection coefficient Γ _{χ is} close to one and relative to (2) even a very small change in the reflection coefficient value, caused for example by the measurement uncertainty, will result in a large relative change in the calculated impedance value. This also means that a relatively large impedance change causes only a hardly distinguishable change in the measured reflection coefficient.

This implies that only impedances ranging from about 0.1 to about 100 can be measured relatively accurately using the known vector circuit analyzers with a reference impedance of 50 or 75 Ω.

10 kΩ. In view of the rapidly evolving technology of producing carbon nanotubes based microwave components whose impedances range from tens to hundreds of kΩ, current vector analyzers are unusable to measure the values of such impedances.

SUMMARY OF THE INVENTION

This undesirable feature allows to eliminate the developed circuitry for measuring the extreme impedances of microwave circuit elements that are modified by an existing vector circuit analyzer. The wiring includes terminals for connecting the measured impedance and reference impedance. The essence of the new solution is that it comprises a 3dB 180-degree hybrid member having a first to fourth gate, any of which is an input gate and is connected to the drive gate of the vector circuit analyzer. Any remaining are the first through third exit gate. Two of these output gates are internally not isolated from the input gate and one of these output gates is connected to the measured impedance and the other of these output gates is connected to the reference impedance. The third of these gateways outputs is internally isolated from the gateway and is connected via an amplifier to the receiving gateway of the vector circuit analyzer. The vector analyzer is set to measure the transmission coefficient.

Using a 180-degree hybrid element, a reflection coefficient value that is selected approximately equal to the measured reflection coefficient is subtracted from the reflection coefficient corresponding to the measured impedance. In this way, a very small differential signal is obtained, which can be significantly amplified by means of an amplifier. The amplified difference signal is then measured using a vector circuit analyzer as a transmission coefficient.

The vector circuit analyzer then shows an enlarged section of the area around the reference impedance in the Smith impedance diagram. This cutout is magnified G / 2 times, where G is the gain of the amplifier, making it possible to better utilize the range of A / D converters used in the vector circuit analyzer to increase the sensitivity of the measured transmission coefficient value to change the measured impedance.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a schematic diagram of the measurement circuitry implementing the developed method.

Fig. 2 shows equation (3) as a representation of the Smith plot of the measured impedance values Ζχ and the Smith plot of the reference impedance values Z _ref to the complex plane of the measured transmission coefficient values S ₂ i. principle of the developed method. The results of the experimental measurement are shown in Fig. 4.

DETAILED DESCRIPTION OF THE INVENTION

Example of a schematic of a developed measuring circuit for measuring extreme microwave impedances

The circuit is formed by the measured impedance 3 indicated for better orientation in the drawing also as Z _x , the reference impedance 4 also indicated on the drawing also for Z _re f, the vector circuit analyzer T by amplifier 5 and 3dB by 180-degree Any of the four gates of the 180-degree hybrid member 2 can be considered as the gateway 8 and is then connected to the vector analyzer driver gate 6, and the remaining three gateways 9, 10, 11 can be laid while driving to the gateway 8 behind the output gate. Two of these exit gates, for example the first exit gate 10 and the second exit gate 11, are not internally insulated from the entrance gate 8. One of the two exit gate 10 or 11. is connected to the measured impedance 3 and the other of the two output gates is JJ. or 10 coupled to a reference impedance 4. The third output gate 9 internally isolated from the input gate 8 is coupled through an amplifier 5 to the receiving gate 7 of the vector circuit analyzer 1. At any of the above connections, the 180-degree hybrid member ensures that the signal to its third the output gate 9 corresponds to the difference between the reflection coefficients of the reference impedance 4 and the measured impedance 3. This signal is then measured by the vector circuit analyzer 1 as a transmission coefficient.

Fig. 1 shows one possible circuit of a 180-degree hybrid member 2, in which the output signals at the first output gate 10 and the second output gate 11 when excited to the input gate 8 are in phase and phase of the signal output from the third output gate 9 at excitation to the second output gate 11. it is delayed by 180 degrees relative to the signal output from the third output gate 9 when excited to the first output gate 10.

The method is based on deducting from the reflection coefficient Γ, the measured impedance 3, the value of the reference reflection coefficient T _ref , which corresponds to the reference impedance Z _re f, which corresponds to the reference number 4, due to the properties of the 180-degree hybrid Member 2.

4 may not be known precisely as it will be determined during the calibration process. Subtraction is performed

The difference signal is amplified by the amplifier 5 and is applied to the receiving gate 7 of the vector circuit analyzer I and measured it as the transmission coefficient S21. The measured value of the transmission coefficient is measured. ideally given by b ₂ G with _2l = _ = _ (r _x -r _ref ), (3), a, 2 where ai and b ₂ are incident and output voltage wave, G is the value of amplifier voltage gain 5. If the measured and reference reflection coefficients are approximately the same, the resulting difference signal is very small and can therefore be significantly amplified.

Equation (3) is shown graphically in Fig. 2 in the case of the unit gain G - 1 of the amplifier 5.

The complex plane 12 represents the plane to which the measured value of the transmission coefficient S21 is plotted. To this plane 12 is shown the basic Smith diagram 13 associated with the reference impedance 4 and also the basic Smith diagram 14 associated with the measured impedance 3. The point 15 corresponds to the reference impedance 4 shown in the Smith diagram 13 and the point 16 is centrally symmetrical and represents which is located in the middle of the Smith diagram

14, Point 12 represents the value of the measured impedance 3 shown in the Smith diagram 14. The point is also the point that is measured by the vector circuit analyzer 1 in the complex plane 12.

Thus, the representation according to equation (3) in the complex plane 12 provides an enlarged section of the area around the reference impedance Z _ref 4 from the Smith impedance diagram. This cutout is magnified G / 2 times, making it possible to better utilize the dynamic range of the A / D converters used in the vector circuit analyzer i to increase the sensitivity of the measured transmission coefficient S ₂ i to change the measured impedance 3.

The principle of subtracting the reflection coefficient from the reflection coefficient associated with the measured impedance 3 was experimentally verified on the measurement arrangement of Figure 3. A 180-degree 3 dB hybrid member 2 was a microstrip circular hybrid member 18 with a phase inverter 2L in place the reference impedance 4 and the measured impedance 3 were used with the open end of the SMA connector 20.19. In the experimental verification of this principle, no amplification was applied to the difference signal. The measurement was performed in the 1.5 to 3.0 GHz frequency band with a vector circuit analyzer 1 with a reference impedance of 50 Ω. The frequency dependence of the measured values of the transmission coefficient is shown in Figure 4, where Swp Min corresponds to the initial sweep frequency, Swp Max corresponds to the end sweep frequency, Mag Max is the maximum amplitude value of the transmission coefficient shown on the grid with the largest radius circle transmission between adjacent raster circles. It can be seen that if both the reference impedance 4 and the measured impedance 3 have approximately the same values, the resulting difference signal is very small and can possibly be significantly amplified and utilize the full dynamics of the A / D converters in the vector circuit analyzer L

Industrial applicability

Extreme microwave impedance circuitry and associated new method of measuring circuit impedances in the microwave region of the frequency spectrum are particularly useful for very accurate measurement of very small and very large impedances, such as impedances of carbon nanotube-based microwave elements whose impedance range from tens to hundreds of kQ.

Claims (1)

PATENT CLAIMS 5 1. A circuit for measuring extreme impedances of microwave circuit elements, consisting of a measured impedance, a reference impedance, and a vector circuit analyzer, characterized in that it comprises a 3dB 180-degree hybrid member (2) having first to fourth gates (8, 9, 10) , 11), any of which is the input gate (8) and is connected to the driver gate (6) of the vector circuit analyzer (1), and any remaining ones are the first to third output gate (9, 10, 11), the output gates (10, 11) are internally not isolated from the input gates (8) and one of these output gates (10, 11) is connected to the measured impedance (3) and the other of these output gates (11, 10) is connected to the reference impedance (4) and wherein the third of these output ports (9) is internally isolated from the input port (8) and is connected via an amplifier (5) to the receiving port (7) of the vector circuit analyzer (1), The analyzer (1) is set 15 for measuring the transmission coefficient.