Method and apparatus for measuring true transmitted power using a broadband dual directional coupler

[0002] This application claims the benefit of U.S. Provisional Application No. 60/186,546 filed Mar. 2, 2000, which is herein incorporated by reference.

[0003] This invention relates to radio frequency directional couplers and, more particularly, to a dual directional coupler that generates a true transmitted power measurement.

[0004] The performance of cellular telephony and similar duplex wireless communications systems depend strongly on dynamic control of the power transmitted by each base station and each portable terminal. The Federal Communications Commission (FCC), to keep power levels within a safe level, mandates precise control of the power transmitted from the portable terminal.

[0005] Prior art methods of measuring transmitted power in a portable terminal use a directional coupler to only sample the incident power at the radiating element. A radiating element may not be perfectly matched to the characteristic impedance of the feedline and a radiating element's impedance characteristics change as the portable terminal changes its location and proximity to other objects. Also, if the antenna were to be disconnected, the power to the antenna would be measured as being correct even though no power was transmitted. Therefore, it cannot be assumed that the transmitted power be derived with any degree of accuracy from the measured incident power alone.

[0006] The present invention overcomes the deficiencies found in the prior art and satisfies the need for a device that can provide an accurate measurement of true transmitted power at the radiating element within a small footprint and a low cost mandated by portable terminals.

[0007] The present invention is a dual-directional coupler comprising two detector circuits that generate voltages proportional to the incident power and reflected power. The voltages from the detector circuits are coupled to a differential amplifier. The differential amplifier generates a voltage proportional to the difference between the incident power and reflected power that represents the true transmitted power at the radiating element. The dual-directional coupler is fabricated on a ceramic substrate to facilitate a compact, surface mount implementation. Such a compact coupler can be easily installed in a cellular telephone.

[0008] The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which:

[0009] FIG. 1 depicts a schematic diagram of a prior art directional coupler;

[0010] FIG. 2 shows a schematic diagram of a dual-directional coupler in accordance with the invention;

[0011] FIG. 3 shows a performance graph of the tuned coupler section; and

[0012] FIG. 4 shows a perspective view of a dual-directional coupler contained in a surface mount device in accordance with the invention.

[0013] To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures.

[0014] FIG. 1 shows a schematic diagram of a prior art directional coupler 100 comprising a detector 101, a terminal impedance 102, a tuned length coupling section 103 and a radio frequency (RF) feed-line 104 placed in close proximity to the coupling section 103. When RF energy is directly coupled into the feed-line 104, a small amount of energy is coupled into the tuned coupling section 103. The detector 101 detects RF energy in the coupling section and produces an output voltage proportional to RF energy in the feed-line 104. The prior art directional coupler circuit only samples incident power present at the radiating element and does not detect any probable reflected power loss due to impedance mismatch at a radiating element 106, the reflected power being dissipated in the terminating resistance 102.

[0015] FIG. 2 shows a schematic diagram of dual directional coupler 200 in accordance with the invention. Dual directional coupler 200 comprises a first detector 201 (e.g., an RF diode) having an anode and cathode terminal, a second detector 202 (e.g., an RF diode) having an anode and cathode terminal, a differential amplifier 203 having a first input terminal coupled to the anode of the first detector 201, a second input terminal coupled to the anode of the second detector 203 and an output terminal, a tuned length coupling section 204 having an input terminal coupled to the cathode of the first detector 201 and an output terminal coupled to the cathode of the second detector 202, and an RF feed-line section 205 having an input and output terminal placed in close proximity to the coupling section 204. The coupling section 204 and feedline section 205 are striplines that are mounted on a substrate 208. The output terminal of the differential amplifier 203 may be connected to analog or digital voltage measuring circuits 206 known in the art. These circuits may comprise an analog to digital converter (ADC) for producing a digital word representing the true transmittal RF power. The ADC may or may not be affixed to the coupler substrate 208.

[0016] The dual-directional coupler 200, with directivity D, provides separation between the incident power Pi present at the input terminal of the coupling section 204 and the reflected power Pr present at the output of the coupling section 204. Disregarding device losses, the transmitted power can be represented as:

[0017] P_t=P_i−P_r

[0018] For directivity D<<1, and adding coupling variable C, the transmitted power can be represented as:

[0019] P_t=(1/C)(P′_i−P′_r)

[0020] First and second detector units operate in the square law region such that the output voltages are linearly proportional to P_iand P_r.

[0021] FIG. 3 shows a graph plotting frequency (axis 304) versus power (axis 306) of tuned coupling section 204. Coupling performance shown in the upper trace 301 of the graph indicates the amount of RF power sampled from the RF feedline 205. Return loss performance shown in the middle trace 302 of the graph is a measure of how well the coupler is matched to a 50 ohm system. Isolation performance shown in the bottom trace 303 of the graph indicates the magnitude of the undesired signal present at each coupled port.

[0022] It is desirable to construct a tuned coupling section 204 to have a flat impedance response across the 850 MHz Cellular and 1.9 GHz PCS mobile telephony bands. It is also desirable for the tuned coupling section 204 to have approximately −20 db of coupling throughout the desired frequency range. To achieve the foregoing with a desired coupling section length of 0.22 cm and a desired electrical length of 13.1°, it is determined through mathematical expressions well known in the art that even and odd mode impedances of a 7.7 db coupler are required. In one embodiment, with a groundplane spacing of 1.36 mm, the tuned coupling section 204 has a width of 0.30 mm and a spacing of 0.30 mm from the RF feed-line 205 and a ceramic substrate. The ceramic substrate can be manufactured using a low temperature ceramic circuit (LTCC) technique to achieve a tailored dielectric constant. This implementation produces a dual-directional coupler in a ceramic package that is approximately 100 mils×100 mils. Such a small package enables the coupler to be used in cellular telephones and other small electronic applications.

[0023] In operation, RF power is applied to the input terminal of the RF feed-line 205. A small amount of RF power is induced into the tuned coupling section 204. The first detector 201 detects incident power present and couples a proportional voltage to the first input of the differential amplifier 203. The second detector 202 detects reflected power present and couples a proportional voltage to the second input of the differential amplifier 203. The differential amplifier 203 algebraically combines voltages from the first and second inputs and produces a voltage proportional to the actual power present at the radiating element. In some applications, the analog output from the detectors 201 and 202 may be used without the differential amplifier 203. A further application may couple the sampled RF to circuitry that is not on the coupler's substrate.

[0024] FIG. 4 depicts a perspective view of one embodiment of the invention. Due to the substantial limitations on the use of printed circuit boards (PCB) surface area in such mobile devices as cellular telephones, the invention is implemented using a printed/hybrid implementation. This implementation is selected to enable the device to operate in both the cellular and PCS bands simultaneously while maintaining a low profile of less than 2.54×2.54×3 mm in a surface mount package (module). In the depicted embodiment, the coupler 200 is surface mounted to PCB 400. Circuit traces 402, 404, and 406 carry input RF, output RF an detected signals, respectively. The detectors 201 and 202 and the differential amplifier 203 are mounted to the top of the coupler 200.

[0025] Although various embodiments which incorporate the teachings of the present invention have been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings.