Patent Application
20130147576
Standard RF/microwave couplers etched on microstrip have very poor directivity, typically ˜5 dB. Other modified microstrip couplers can achieve 20 dB directivity, but involve narrow etched line widths and spacings that require tight etching tolerances that may not be achievable or repeatable for low cost, high volume production. Also, these modified designs cannot be analyzed for proper function with standard linear simulators. They can only be analyzed with more sophisticated and expensive electromagnetic (EM) simulators. Without an EM simulator, a modified design with improved directivity is not possible in any kind of cost effective or timely manner.
The present invention solves the problem of achieving high directivity (>20 dB) coupling over a reasonable frequency bandwidth on a microstrip transmission line without the need for EM simulation, narrow line widths/spacings, or tight tolerances. The present invention can be implemented in any type of transmission line. It is especially suited to microstrip transmission lines.
An exemplary coupler device includes a combiner, first and second coupling units connected between the combiner and a to-be-measured transmission line. The first and second coupling units comprise first and second coupling devices being in electrical communication with a to-be-measured transmission line, at least one first transmission line coupled between the combiner and the first coupling device and at least one second transmission line coupled between the combiner and the second coupling device. The at least one first and the at least one second transmission line have predefined impedance and phase delay values. The phase delay value of the at least one first transmission line differs from the phase delay value of the at least one second transmission line based on a phase delay value of the to-be-measured transmission line.
In one aspect of the invention, the impedance of the at least one first transmission line is approximately equal to the impedance of the at least one second transmission line.
In another aspect of the invention, the combiner has an isolation value generally greater than 20 dB.
In still another aspect of the invention, each of the first and second coupling units includes a load resistor coupled between a node that is between an end of the first and second transmission lines and the respective coupling device and an electrical ground. The combiner has an isolation value generally less than 20 dB.
In yet another aspect of the invention, the at least one first transmission line comprises first and second sub transmission lines and the at least one second transmission line comprises first and second sub transmission lines. The first sub transmission lines have first ends connected to the coupling device. Each of the first and second coupling units includes a load resistor coupled to second ends of the first sub transmission lines and first ends of the second sub transmission lines. Second ends of the second sub transmission lines are coupled to the coupling devices. Phase delay for at least one of the first or second sub transmission lines is equal.
In still yet another aspect of the invention, the to-be-measured transmission line is located between a transmitter and an antenna.
Preferred and alternative embodiments of the present invention are described in detail below with reference to the following drawings:
In one embodiment, the coupler 20 is used to detect Pf along the microstrip transmission line Z1 located between a transmitter 26 and an antenna 28. The coupler 20 sends a sensed power value to a Power Detector Circuit 30.
The Power Detector Circuit 30 transforms the RF power to a voltage level that is proportional to the RF power level. The voltage is then sent to a field programmable gate array (FPGA) for processing.
The coupler 20 includes a combiner 40 and a first coupler unit 42 and a second coupler unit 44. Each coupler unit 42, 44 includes a coupling device (e.g., resistive, inductive or capacitive device) and a predefined lengths of transmission line Z2, Z3. The lengths depend on the type of combiner (i.e. in phase or quadrature type combiner). For example, resistive coupling is achieved with a chip or thin film resistor, capacitive coupling is achieved with a chip, printed or gap capacitor. The combiner 40 has reasonably high isolation (i.e. Wilkinson, branch line, rat race hybrid, or comparable combiner). Generally greater than 20 dB is considered a high isolation value.
For the case of the combiner being a Wilkinson (in phase type combiner), let impedance for the microstrip transmission lines be as follows Z1=Z2=Z3=50 Ohm , and Zsh1 and Zsh2 have gap capacitance values of 0.029 pF, an approximate 37 dB coupling is achieved. Also let the phase delays for the respective microstrip transmission lines be as follows θ1=90°, θ2=90°, and θ3=0° at a particular frequency fo, fo is the expected frequency of the transmitted signal.
Forward power enters Port 1 and exits at Port 2. A small amount of forward power Pf is coupled off from Zsh1, travels thru Z2 and is incident on the combiner at −90°. Forward power Pf travels thru Z1 and a small amount of Pf is coupled off from Zsh2, travels thru Z3 and is incident on the combiner at −90°. The two coupled signals from forward power Pf are incident on the combiner 40 in phase and thus are added.
The reflected (or reverse) power Pr enters Port 2 and exits at Port 1. A small amount of reflected power Pr is coupled off from Zsh2, travels thru Z3 and is incident on the combiner at 0°. Reflected power travels thru Z1 and a small amount is coupled off from Zsh1, travels thru Z2 and is incident on the combiner at −180°. The two coupled signals from reverse power Pr are incident on the combiner 40 180° out of phase and thus are canceled.
Directivity is defined as forward coupled power minus reflected coupled power, typically expressed in dB. Theoretical analysis indicates directivity to be ≧20 dB for a bandwidth of about 19% for the above values of Z1, Z2, Z3, Zsh1 and Zsh2 when using a Wilkinson combiner.
Different values of phasing for θ1, θ2 and θ3 will be required when using a branch line, rat race or other hybrid as the combiner as one of ordinary skill would be able to determine. Different values for Z1, Z2, Z3, Zsh1 and Zsh2 will result in different coupling, directivity and bandwidths. The values can be different, but typically Z1=Z2=Z3 and Zsh1=Zsh2.
The coupler includes a second set of microstrip transmission lines 104, 108 with respective phase delay 114, 116 that is equal to the transmission lines Z2, Z3 shown in
While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.
