JPS62151766A - Microwave impedance measuring instrument - Google Patents

Abstract

PURPOSE:To obtain a simple, inexpensive measuring instrument by making the frequency of a signal source variable, finding voltage variation between the connection of a load to be measured with a cable end and its short- circuiting, and measuring a maximum value and a minimum value, and frequencies corresponding to them. CONSTITUTION:The high frequency signal source 7 whose frequency is variable is connected through a directional coupler 9 to the input end of a cable 1 for measurement which is long enough as compared with the wavelength of a measurement frequency. Further, the load 6 to be measured is connected to the other end, and a voltmeter 10 which has a detector is connected to the output end of the auxiliary arm of the coupler 9. Then, the frequency of the signal source 7 is varied gradually to read and store the detection output voltage of a reflected wave from the load 6. Then, the terminal of the cable 1 is short- circuited, the frequency of the signal source 7 is varied, and a detection voltage is measured and stored similarly, thereby finding the impedance of the load 6 from the maximum and minimum values of both measured voltages and frequencies corresponding to them. This measurement is taken only by measuring the absolute value of the voltage at one place, so the device is reduced in cost.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention relates to a microwave impedance measuring device, and more specifically, it is a simple and inexpensive device that measures unknown impedance by connecting it to a low-loss transmission line with known characteristic impedance. This article relates to a microwave impedance measuring device.

(Prior Art) When a load having an impedance other than the characteristic impedance Zo of the line is connected to the end of a line and a sine wave is transmitted from the start end, a standing wave is generated on the line. When a load with a characteristic impedance Zo is connected to the terminal end, no reflection occurs from the terminal load, but when a load other than the characteristic impedance Zo is connected, a reflected wave is generated, and the traveling wave and the reflected wave are combined and the magnitude of the voltage and current increases. changes depending on the position on the track. This is called a standing wave. n of a line with known characteristic impedance Zo; unknown impedance zL on ON
The microwave impedance Z of the load can be determined by connecting a load with
L can be measured.

Figure 2 (a) shows the method of microwave impedance measurement. 1 is a cable for measuring characteristic impedance Zo,
2 is the measurement part in the cable, 3 is the high frequency signal source, and 4 is l! ! Part 76, 5 is a voltmeter.

The measuring part 2 of the cable is not actually exposed and the second
As shown in the cross-sectional view of Figure (c), a groove is cut in a part of the outer side of the hollow coaxial cable, so that the short antenna 4a can be moved in the longitudinal direction of the cable. antenna 4
A is integrated with a matching circuit 4b and a detector 4C, and constitutes a sliding part 4. Returning to Figure 2 (a), the detector 4C
The rectified output of is read by a voltmeter 5. When the sliding part 4 is moved with a load zL that is not equal to the characteristic impedance Zo of the line connected, the reading of the voltmeter becomes 2nd.
The waveform changes as shown in waveform a in figure (b). The ratio of the maximum value Vl1lax and the minimum value ■l1in of this waveform is the standing wave ratio ρteal. , o=Vn+ax/Vain From this ρ, load Z
The absolute value of the reflection coefficient ξ due to L is determined.

121-1=(ρ-1)/(ρ+1)...(1)
Next, short-circuiting the ends of the cable creates a standing wave as shown in the figure. The phase angle of the reflection coefficient at this time is πrad. From the distance Δ× of ■1n of this standing wave a, the reflection coefficient is 1
The phase angle of 'L can be calculated, and ZL can be found using the following equation.

ZL = Zo (1+rL)/(1-rL)=
(2) The calculations during this period are omitted.

The measurement system shown in FIG. 2(a) above is called a standing wave indicator.

(Problems to be Solved by the Invention) In the measurement method using the standing wave indicator described above, when the antenna 4a is inserted deeply into the measurement part 2 of the cable 1, the distribution of the standing wave changes, and when it is inserted briefly, the detector Errors occur due to errors due to 4c and mechanical fluctuations due to the moving mechanism. Furthermore, when trying to automate the measurement, it is technically difficult to make the above adjustments or to move the sliding part 4 in the axial direction. There is also a network analyzer that can easily measure impedance, but this is an extremely expensive system that measures the phase of high frequencies internally.

The present invention has been made in view of the above points, and its purpose is to provide a simple and inexpensive microwave impedance measuring instrument.

(Means for Solving the Problems) The present invention solves the above problems by connecting the characteristic impedance of the cable via a directional coupler to the input end of the cable, which has a sufficiently long length compared to the wavelength of the measurement frequency. Connect a high-frequency signal source with a variable frequency having an output impedance not equal to The present invention is characterized in that the impedance of the load is determined by measuring the auxiliary arm output of the directional coupler corresponding to the frequency.

(Operation) Connect the load to be measured to the end of the cable, gradually change the frequency of the signal source, read and store the reading of the detected output voltage of the reflected wave from the load, then short the end of the cable and repeat the same procedure. The frequency of the signal source is gradually changed, the voltage reading of the detection output is memorized, and the impedance of the load is calculated from the maximum and minimum voltage values of the plane measurement values and the frequency value corresponding to those values.

(Example) Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention. 1 is a measurement cable, 6 is a load to be measured, 7 is a high frequency signal source, 8
is the output impedance of the signal source 7, 9 is the directional coupler,
10 is a voltmeter. Δ is a traveling voltage wave, and B is a reflected voltage wave.

In Fig. 1, the traveling voltage wave A at the input end of the characteristic impedance Zo is...
(4) ... (5) It is known that Chi=α+jβ−α+j 2πf /cg (6). where γ is the propagation constant, β is the phase constant, α is the attenuation constant, Cg is the phase velocity in the cable, rs is the reflection coefficient of the signal source, and rL is the reflection coefficient of the load.

If an insulator with relative dielectric constant εr is filled between the cable conductors, then Cg - C/(τgo...(7). (C = speed of light) In general, in high frequency measurements, the signal source is Internal impedance Z
The purpose is to make s equal to the characteristic impedance ZO of the cable as much as possible, but in this case Zs≠Zo, and f changes so that the frequency f of vg gradually increases, so this vector rotates clockwise as shown in Figure 3. Rotate. Then, the magnitude of (1-rs・-2μ "L ε)" becomes larger or smaller. From equation (3), A is the vector (1-・・
-z>1 r 5 rL ε ), so A becomes smaller or larger.

Set the total length l of the cable to be sufficiently larger than the wavelength λ = CQ/f, i.e., a>>co/f"-<B) The change in A with respect to the frequency is measured using the directional coupler 9 and a power meter or detector. Record the output Vl of the voltmeter (as shown in the solid line in Figure 4, where 1 is proportional to 1△1), and record the maximum and minimum values of this Vl as Vm, respectively.
ax and Vmin, and vl near the measurement frequency
Let fl and fl be the two frequencies at which .

Cable length now! is defined as equation (8), so (6)
From the formula... -2i As is clear, the beta 1~le (r5rL ε) rotates once with a slight change in frequency, so the r 5 r
It is possible to ignore the change in
L lε )/-2tlノ(1-l r s rLlε ) ・(9) Therefore, if Vmax /V1n = d, ・(10)
-2etノ1rsrL 16 = (d-1>/(d+1)...
・(11) becomes.

Next, when the cable ends are shorted to calibrate the cable length and characteristics, the reflection coefficient vector becomes ε”(−−1
), but the voltage V when sweeping the frequency in that case is
Record the change in s. (Dotted line in Figure 4) In this case, the voltage, etc. is expressed by adding a dash ('), r
Similarly to the previous case, 2tlJ lrs Iε = (d −-1>/(d −+1)) for l −1.
・(12) From equations (11) and (12), lr(, 1=((d 1)/(d+1>)・((d
'+1)/(d--1)) ...(13) Since it rotates at an angle proportional to the wave number, V and V in Figure 4
The phase angle θ of the rL vector can be calculated from the difference in frequencies at which l- is minimized. Now, the pect of Fig. 5) Lt(-
rsE8-28 and the vector in Figure 3.
-2 (rsrL ε ) start rotating at the same time (in other words, assume that the frequency f in equation (6) is kept exactly the same in FIGS. 3 and 5 and is increased). Then, the vector (-r6ε"e"'> in Figure 5 becomes π-(θS+θL)+θS=π-θL-(14 ), it can be seen from FIGS. 3 and 5 that the rotation is delayed by a certain amount.

Also, in Fig. 4, Vl first becomes minimum at fl, and then Vl
” Until + becomes the minimum at f-2, the vector (-t
・The angle at which εj7ε-26 rotates is 2π(f + 2-r, )/cg...(
15). This value is the angular difference between the two vectors (
14) Since it is the value of formula 2π(f-z-f r ＞/C(1=π-θL...(
16) becomes. - Since the vector (-〆・ε 6ε-′″′) rotates once between −1 and f′2 in Figure 4 of the force cylinder, from equation (6), 2π(f −2−f−1)/ C(7=2π...(17
>, and from equations (16) and (17), θL = π−2π ((f−2ft)/(f ”z
ft )) = (18),
From equation (13) and (18), the reflection coefficient of the measured load is 11
is required.

By incorporating a computer and a cathode ray tube into this measuring device, it is possible to sweep the frequency over a relatively wide range and
By memorizing the maximum and minimum values of l and the frequencies at that time, the reflection coefficients rL and ZL in that frequency range can be measured and displayed on a cathode ray tube. As in the case of a network analyzer, it is also possible to display age in polar coordinates on a Smith chart, or digitally display ZL in complex numbers or polar coordinates.

Similar to equation (3), reflected wave (voltage) at the input end of the cable
Calculating the value of B, ((1/rL) −after S ε−”) ・
(19). The value proportional to the reciprocal of B is the 6th
Since the vector locus is drawn as shown in the figure, the traveling wave 1 person 1 described above
According to the same idea as finding zL from
It is possible to find zL from .

In the end, we used the measurement circuit shown in Figure 1 to find the voltage change when the load under test was connected to the end of the cable and when it was short-circuited by changing the frequency of the signal source, and from the maximum and minimum values and the corresponding frequency, we calculated 1rLl. and θL to find rL,
zL can be found from equation (5).

(Effects of the invention) It is possible to measure and display impedance over a relatively wide range of frequencies in just a few seconds, and unlike network analyzers, there is no phase measurement using a mixer, and only the absolute value of voltage is measured at one location. Because of this, the equipment is inexpensive.

[Brief explanation of drawings]

Fig. 1 is a block diagram of a measuring instrument according to an embodiment of the present invention, Fig. 2 (a) is a block diagram of a conventional measuring instrument, (b) is a waveform of a voltmeter reading, and (c) is a sliding part. Figure 3 is a vector locus diagram for changes in frequency when the load is zL, Figure 4 is a diagram for changes in voltage for changes in frequency when the load is zL and the end of the cable is short-circuited, and Figure 5 is a diagram for the change in voltage with respect to frequency changes when the load zL is short-circuited. FIG. 6 is a vector locus diagram of the change in frequency when the terminal is short-circuited, and FIG. 6 is a vector locus diagram of the reciprocal of the reflected wave. Go...Measuring cable 2...Measuring part of the measuring cable 3...High frequency signal source 4...Sliding part 5...Voltmeter 6...Load to be measured 7...High frequency signal Source 8 - Output impedance of signal source 7 9... Directional coupler 10... Voltmeter Patent applicant Yokogawa Hokushin Electric Co., Ltd. Figure 3 Figure 4 V1'

Claims (1)

[Claims] A variable frequency high frequency signal source having an output impedance that is not equal to the characteristic impedance of the cable is connected to the input end of a cable whose length is sufficiently long compared to the wavelength of the measurement frequency via a directional coupler, and the other end is connected to the input end of the cable. A voltmeter including a detector is connected to the output end of the auxiliary arm of the directional coupler to measure the load, and the output of the auxiliary arm of the directional coupler corresponding to the frequency of the signal source is measured to determine the impedance of the load. A microwave impedance measuring instrument characterized by: