Four-port directional coupler having a main line and two secondary lines, where the two secondary lines are coupled to compensation circuits with attenuation regulator circuits

Abstract

The invention proposes a four-port coupler using micro-strip line in combination with ultra-wide-band compensation circuits. It can be applied for communication systems or information machine systems. The main feature of the invention is the structure and the distribution of the components in the compensation circuit to reduce the size of the coupler. The proposed coupler includes: microstrip directional coupler and compensation circuits, in which the microstrip directional coupler consists of one main line and two secondary transmission lines; the main transmission line has two ports: input and output ports; each secondary is connected to a load and a compensation circuit. The compensation circuit is composed of a low-pass filter and two parallel attenuation regulator circuits.

Claims

1. A four-port coupler with a microstrip line in combination with an ultra-wide-band compensation circuit, including microstrip directional couplers and compensation circuits, of which: the microstrip directional coupler consists of a main and two secondary transmission lines; the main line has an input port and an output port; each secondary line has one port connected to the respective compensation circuit and another port terminated by a resistor; each secondary transmission line is electrically coupled with the main line; each compensation circuit comprises a respectively low-pass filter and corresponding first and second attenuation regulator circuits which are parallel to each other; the corresponding first attenuation regulator circuit is connected between the respective one of the two secondary lines and the respectively low-pass filter, the corresponding second attenuation regulator circuit is placed between the respectively low-pass filter and a coupling port or an isolating port of the respective secondary line.

2. The four-port coupler with a microstrip structure in combination with an ultra-wide-band compensation circuit according to claim 1, in which: the low-pass filter includes resistors and inductors that provides a cut-off frequency equal to a minimum operating frequency of the four-port coupler; wherein as frequency increases up, a loss factor in a frequency stop band is proportional to a coupling factor between the main and the two secondary transmission lines.

3. The four-port coupler with a microstrip structure in combination with an ultra-wide-band compensation circuit according to claim 2 in which: each attenuation regulator circuit is composed of resistors, inductors and capacitors.

4. The four-port coupler with a microstrip structure in combination with an ultra-wide-band compensation circuit according to claim 1 in which: at least one of the attenuation regulator circuits is composed of resistors, inductors and capacitors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the structure of the ultra-wide band coupler;

(2) FIG. 2 shows the measurement results of the isolation factor I, the coupling factor C and the directivity D; and

(3) FIG. 3 shows the measurement results of the attenuation coefficient L and the return loss RL at the input port of the coupler.

DETAILED DESCRIPTION OF THE INVENTION

(4) As shown in FIG. 1, the proposed ultra-wide-band coupler includes the following components: input port 1, output port 2, coupled port 3, isolating port 4; main transmission line 5, secondary transmission line 6 and compensation circuit 7.

(5) The center part of the directional coupler is the main transmission line 5 with the length L, the characteristic impedance of 50Ω, connecting the input port 1 and output port 2. The input signals start at port 1 and come out at port 2 after an extraction.

(6) Two secondary transmission lines 6 with the characteristic impedance of 50Ω and the length L.sub.1 are electrically coupled with the main transmission line 5 at the opposite terminals. Each secondary transmission line is connected to a load and a compensation circuit 7.

(7) Each secondary transmission 6 is connected to a load 8 before going to ground 9 at one end and to a compensation circuit 7 at the other end.

(8) Compensation circuit 7 is composed of a low pass filter 10 and two attenuation regulator circuits 14 that are installed in parallel. The first attenuation regulator circuit is placed between the output port of the secondary transmission line 6 and the low-pass filter 10 while the second regulator circuit is installed between the low-pass filter 10 and the coupled port 3 or the isolating port 4.

(9) The low pass filter 10 including resistor 11 and inductor 12 is designed with the cutoff frequency coinciding with the minimum frequency of the coupler and the loss coefficient in the stop band is frequency proportional to the coupling factor between the main transmission line 5 and secondary transmission line 6.

(10) The attenuation regulator circuit 14 including resistor 15, inductor 16, capacitor 17 and ground terminal 18 compensates the coupling factor between the input port 1 and the coupled port 3.

(11) Without the compensation circuit, the coupling factor goes up as the frequency increases. On the other hand, the attenuation of the low pass filter circuit 10 at the stopband increases as the frequency increases. Since the coupler includes a low pass filter 10, signals from the input port 1 to the coupled port 3 will be reduced due to the attenuation characteristic at the stopband of the low-pass filter 10. The combination of the coupling factor and the filter attenuation will flatten the coupling factor line. To further flatten the coupling factor line, an attenuation regulator circuit is introduced. In general, due to the compensation circuit, the coupling factor line is flat over the operating frequency range of the coupler.

(12) Since the attenuation coefficient in the stopband of the low-pass filter is relatively large, only one low-pass filter is employed. For the attenuation regulator circuit, there are many sub-bands of the operating frequency range of the coupler in which the coupling factor is not flat. Thus the larger number of attenuation regulator circuits will make the coupling factor line flatter. However, a large number of attenuation tuning circuits is not suitable due to the increased size of the coupler. In this invention, the inventors have found that the optimum number of attenuation regulator circuits for a coupler is two.

(13) After entering the port 1, due to electromagnetic induction, the signal appears on the secondary transmission line 6. The ratio between the signal power of the coupled port 3 and that of the input port 1 represents the coupling factor C of the two lines, and is defined as:

(14) $\begin{array}{cc}C=10{\log }_{10}\frac{P_1}{P_3}& \left(b\right)\end{array}$

(15) The compensation circuit 7 is employed to ensure the stable coupling factor over the operating frequency range. Additionally, the compensation circuit 7 also increases the isolation factor between the input port 1 and the isolating port 4.

(16) If the directional coupler in FIG. 1 is an ideally reversed coupler, the signal from the input 1 is induced, appearing at the coupled port 3 and there is no signal at the isolating port 4. Under the such conditions, the directivity D will become infinite.

(17) If the directional coupler does not have a compensating circuit, the directivity of the directional coupler is much poorer. This phenomenon results from the induction between the main transmission line 5 and the secondary transmission line 6 and the value of the directivity D for the microstrip coupler is typically between 7 and 13 dB, depending on the operating frequency. In this invention, the coupler uses a compensating circuit to reduce the induction from the main transmission line 5 to the secondary link 6, thereby improving the D directivity of the entire structure. In addition, the compensation circuit can reduce the effective electrical length of the microstrip line. The results of the invention show that it is possible to shorten the length of the main transmission line from λ0/4 to λ0/30 and the secondary transmission line from λ0/8 to λ0/120 (λ0 is the wavelength of the center frequency of the bandwidth).

(18) Execution Example

(19) This invention proposes the coupler applied for measuring power in an ultra-wide frequency range including HF, VHF and UHF bands. FIGS. 2 and 3 show coupler test results for the frequency range from 20 to 500 MHz. The coupling factor C (FIG. 2) is approximately 56 dB with a deviation of ±1 dB over the frequency range. The attenuation coefficient L (FIG. 3) smaller than 0.1 represents the nearly conserved output power on the main line 5. The required isolation factor I (FIG. 2) is greater than 70 dB, the directional factor D (FIG. 2) also meets the requirement that the directional factor D is higher than 15 dB. The return loss RL (FIG. 3) of the input port 1 and the output port 2 are greater than 20 dB showing that the coupler ports are well impedance matched.

(20) In particular, the invention can also be applied for many other cases such as HF-VHF band, VHF-UHF band or exclusively for each one of the three bands. For higher frequency bands such as L-band, S-band, this invention is also applicable to reduce the size of the coupler.