DIRECTIONAL COUPLER FOR ACCURATE POWER DETECTION

Abstract

A power detection system implemented using a pair of directional couplers and a transmission line (or equivalent) disposed between the directional couplers, wherein the transmission line (or equivalent) provides a 90° phase shift between the directional couplers. Accurate power detection is provided by combining the powers detected at each of the directional couplers, whereby the combined power is independent of load phase. The total power in the forward case is given by Pc1=2*Pf*C, where Pf is the forward power and C is the coupling coefficient the directional couplers. The total power in the reflected case is given by Pc1=2*Pf*C*(ρ2+D2), where Pf is the forward power, C is the coupling coefficient of said directional couplers, ρ is the reflection coefficient, and D is the directivity of the directional couplers.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary embodiment of the present invention's setup having two directional couplers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While this invention is illustrated and described in a preferred embodiment, the invention may be produced in many different configurations. There is depicted in the drawings, and will herein be described in detail, a preferred embodiment of the invention, with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and the associated functional specifications for its construction and is not intended to limit the invention to the embodiment illustrated. Those skilled in the art will envision many other possible variations within the scope of the present invention.

FIG. 1 illustrates an exemplary embodiment of the present invention's setup 100 having two directional couplers 102 and 104. In the preferred embodiment, couplers 102 and 104 are connected in series with a ¼ wavelength transmission line 106 or equivalent. It should be noted that the two isolation ports can either be loaded with 50 ohm loads, as shown in the figure or loaded with external loads. If input1 is connected to the source, then the coupler is set to measure the forward power from the source. If input1 is connected to the load, then the coupler is set to measure the reflected power from the load. Further, the isolation ports can also be used to measure reflected power directly.

In one scenario where the setup is connected for forward power detection, the source is connected to the input 1 and input 2 is connected to a load with an unknown reflection coefficient of ρ and phase of φ. Also, assuming both Coupler 1 and Coupler 2 are identical which have same coupling coefficient of C and directivity of D with negligible loss. The coupled forward power measured from the Coupled1 port P_c1 can be calculated as following:

P _c1 =P _f *C*(1+ρ²*D+2*ρ*D^1/2*cos φ)   (1)

Since ρ is less than land D is much less than 1, ρ²*D<2*ρ*D^1/2<<1 is true for majority of the cases. (1) can be rewritten as following without sacrificing accuracy significantly.

P _c1 =P _f *C*(1+2*ρ*D^1/2*cos φ)   (2)

As shown in (2) for a given reflection coefficient ρ the measured forward power is a function of the load phase angle φ. For a given VSWR the measured forward power will change as the load phase changes. And as D approaches zero the measured forward power P_c1 will approach P_f*C.

In another scenario, if the input1 is connected to the load and input 2 is connected to the source, the coupler is set to measure the reflected power, by going through the similar math, it can be shown that the reflected power P_r measured at coupled 1 is:

P _c1 =P _f *C*(ρ²+D²+2*ρ*D^1/2*cos φ)   (3)

Again, the measured reflected power is a function of load phase and as load phase changes the measured reflected power changes accordingly, although the true reflected power does not change. Again as D approaches zero,

P _c1 =P _f *C*ρ ²   (4)

It should be noted that P_r=P_f*ρ²

Since the load phase φ depends on the characteristics of the load and system implementation, it is impossible to know and, consequently, to compensate the detection error caused by the load phase. The conventional way to solve this problem is to use high directivity couplers if high accuracy is desired. Extensive researches have been going on in the microwave society for many years pursuing high directivity couplers. However, high directivity couplers are still expensive, bulky and very hard, if not impossible, to get integrated into MMIC circuits or modules.

This invention uses a different approach to eliminate the ill effect arising from the finite directivity issue. For power detection purpose the absolute load phase information is not of primary interest as long as the measured power is not a function of the load phase. The invention works as described below.

If another power detector is used to detect the power at Coupled 2, a very similar measured power will be obtained for both forward and reflection cases. But since there is a 90 degree relative phase difference between coupler 1 and coupler 2 with respect to the load, the measured power from Coupled 2 port will be:

P _c1 =P _f *C*[1+2*ρ*D^1/2*cos(φ−180)]  (5)

For forward case and

P _c1 =P _f *C*[ρ ² +D ²+2*ρ*D^1/2*cos(φ−180)]  (6)

For reflected case.

If the detected powers from Coupled1 port and Coupled2 port are combined, the total power will be:

P _c1=2*P_f*C   (7)

for the forward case and

P _c1=2*P_f*C*(ρ²+D²)   (8)

for reflected case.

As shown in (7) and (8), by adding another directional coupler and summing the detected power from both detectors the variation in each individual detected power due to the load phase gets cancelled out. The combined power is not a function of the load phase and the ill effect of the finite directivity of the coupler is nothing more than a constant offset which can be calibrated out in advance, as long as the characteristics of the coupler is well known. Thus the accurate power detection can be achieved without any knowledge of the load phase.

The present invention also provides for a method to detect power comprising the steps of: (a) measuring power at a port associated with a first directional coupler, (b) measuring power at a port associated with a second directional coupler, wherein the first directional coupler is connected in series with the second directional coupler via a transmission line that provides a 90° phase shift between the directional couplers, and (c) detecting total power by combining power detected at the first directional coupler and power detected at the second directional coupler, wherein the combined power is independent of load phase, thereby providing accurate power detection.

CONCLUSION

A system and method has been shown in the above embodiments for the effective implementation of directional couplers for accurate power detection. While various preferred embodiments have been shown and described, it will be understood that there is no intent to limit the invention by such disclosure, but rather, it is intended to cover all modifications falling within the spirit and scope of the invention, as defined in the appended claims. For example, the present invention should not be limited by specific hardware or whether the directional couplers are connected in series or parallel.

Claims

1. A power detection system comprising: a pair of directional couplers;a transmission line disposed between said directional couplers and providing a 90° phase shift between said directional couplers; anda pair of power detectors detecting power at each of said directional couplers,said detected power at each of said directional couplers combined to provide accurate power detection, said combined power independent of load phase, and said combined power in a forward case being given by Pc1=2*Pf*C, where Pf is the forward power, and C is the coupling coefficient the directional couplers.

2. A power detection system, as per claim 1, wherein said transmission line connects said directional couplers in series.

3. (canceled)

4. A power detection system, as per claim 1, wherein said power detection system is integrated into a monolithic microwave integrated circuit or module.

5. (canceled)

6. A power detection system, as per claim 1, wherein said combined power in a reflected case is given by: Pc1=2*Pf*C*(ρ2+D2)where Pf is the forward power, C is the coupling coefficient of said directional couplers, ρ is the reflection coefficient, and D is the directivity of said directional couplers.

7. A system to detect power independent of load phase comprising: a first directional coupler;a second directional coupler;a ¼λ transmission line connecting said first directional coupler to said second directional coupler in series; anda pair of power detectors detecting power at said first and second directional couplers,said detected power at each of said directional couplers combined to provide accurate power detection, said combined power independent of load phase, and said combined power in a forward case being given by Pc1=2*Pf*C, where Pf is the forward power, and C is the coupling coefficient the directional couplers.

8. A system to detect power independent of load phase, as per claim 7, wherein said system is integrated into a monolithic microwave integrated circuit or module.

9. (canceled)

10. A system to detect power independent of load phase, as per claim 1, wherein said combined power in a reflected case is given by: Pc1=2*Pf*C*(ρ2+D2)where Pf is the forward power, C is the coupling coefficient of said first directional coupler and said second directional coupler, ρ is the reflection coefficient, and D is the directivity of said first directional coupler and said second directional coupler.

11. A system to detect power independent of load phase comprising: a first directional coupler;a second directional coupler;a transmission line connecting said first directional coupler to said second directional coupler in parallel, said transmission line providing 90° phase shift between said first and second directional coupler; anda pair of power detectors combining power detected at said first and second directional couplers, said detected power at each of said directional couplers being combined to provide accurate power detection, said combined power independent of load phase, and said combined power in a forward case being given by Pc1=2*Pf*C where Pf is the forward power, and C is the coupling coefficient the directional couplers, and said combined power in a reflected case is given by Pc1=2*Pf*C*(ρ2+D2), where Pf is the forward power, C is the coupling coefficient of said first directional coupler and said second directional coupler, ρ is the reflection coefficient, and D is the directivity of said first directional coupler and said second directional coupler.

12. (canceled)

13. A method to detect power comprising: measuring power at a port associated with a first directional coupler;measuring power at a port associated with a second directional coupler, said first directional coupler connected in series with said second directional coupler via a transmission line, said transmission line providing a 90° phase shift between said first directional coupler to said second directional coupler; anddetecting total power by combining power detected at said first directional coupler and power detected at said second directional coupler, said combined power independent of load phase, thereby providing accurate power detection, and said combined power in a forward case being given by Pc1=2*Pf*C. where Pf is the forward power, and C is the coupling coefficient the directional couplers.

14. (canceled)

15. A method to detect power, as per claim 13, wherein said combined power in a reflected case is given by: Pc1=2*Pf*C*(ρ2+D2)where Pf is the forward power, C is the coupling coefficient of said first directional coupler and said second directional coupler, ρ is the reflection coefficient, and D is the directivity of said first directional coupler and said second directional coupler.

16. A method to detect power, as per claim 13, wherein said first and second directional couplers are implemented in conjunction with a monolithic microwave integrated circuit or module.

17. A method to detect power comprising: measuring power at a port associated with a first directional coupler;measuring power at a port associated with a second directional coupler, said first directional coupler connected in parallel with said second directional coupler via a transmission line, said transmission line providing a 90° phase shift between said first directional coupler to said second directional coupler; anddetecting total power by combining power detected at said first directional coupler and power detected at said second directional coupler, said combined power independent of load phase, thereby providing accurate power detection, wherein said first and second directional couplers are implemented in conjunction with a monolithic microwave integrated circuit or module, and said combined power in a forward case being given by Pc1=2*Pf*C, where Pf is the forward power, and C is the coupling coefficient the directional couplers, and said combined power in a reflected case is given by Pc1=2*Pf*C*(ρ2+D2), where Pf is the forward power, C is the coupling coefficient of said first directional coupler and said second directional coupler, ρ is the reflection coefficient, and D is the directivitv of said first directional coupler and said second directional coupler.

18. (canceled)