The present disclosure relates to radio frequency sensors and, more particularly, to an orthogonal radio frequency voltage/current sensor.
A radio frequency (RF) current sensor, which is also known as a probe, generates a signal that represents the magnitude of current flow through an RF conductor. The current probe can be combined with a voltage probe to form an RF voltage/current (VI) probe that generates a second signal that represents the RF voltage with respect to a reference potential, such as an RF ground or shield conductor.
RF current and VI probes are used in RF control circuits to provide feedback information. The feedback information may be used to control an RF amplifier that provides the RF power that is being measured. In some applications the RF power is employed to generate plasma for semiconductor manufacturing, metal coating, or micromachining processes.
In various embodiments of the present disclosure, a radio frequency (RF) sensor that measures RF current is disclosed. The RF sensor includes a substrate including a first exterior layer, a second exterior layer, a first interior layer, a second interior layer and an inner perimeter that defines an aperture through the substrate. The RF sensor further includes a first loop having a first plurality of sensor pads coupled to a first plurality of vias by a first plurality of traces, and a second loop having a second plurality of sensor pads coupled to a second plurality of vias by a second plurality of traces. The first and second plurality of sensor pads are arranged on the inner perimeter of the substrate. A center conductor for carrying RF current extends through the aperture and the first and second loops generate an electrical signal that represents RF current flow through the center conductor.
In various embodiments of the present disclosure, a radio frequency (RF) sensor that measures RF current is disclosed. The RF sensor includes a substrate having a first ground plane, a second ground plane and an inner perimeter that defines an aperture through the substrate. The RF sensor further includes a first loop that has a first plurality of sensor pads coupled to a first plurality of vias by a first plurality of traces, and a second loop that has a second plurality of sensor pads coupled to a second plurality of vias by a second plurality of traces. The first and second plurality of sensor pads extend along and are spaced uniformly across the inner perimeter of the substrate. A conductor for carrying RF current extends through the aperture and the first and second loops generate an electrical signal that represents RF current flow through the conductor.
Further areas of applicability of the present disclosure will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the disclosure, are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.
The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:
a)-(c) are plan views of the current sensor traces of
a) is schematic diagram of a current sensor of the prior art;
b)-(c) and 6 are schematic diagrams of a current sensor according to various embodiments of the present disclosure;
a)-(b) are plan views of circular conductive rings of a voltage current sensor according to various embodiments of the present disclosure.
The following description is merely exemplary in nature and is in no way intended to limit the disclosure, its application, or uses. For purposes of clarity, the same reference numbers will be used in the drawings to identify similar elements. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical or. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure.
As used herein, the term module refers to an Application Specific Integrated Circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, and/or other suitable components that provide the described functionality.
Referring now to
A second plurality of vias 22 are electrically insulated from ground planes 14 and connect current loop-back traces that are located on inner layers of PCB 12. A third plurality of vias 24 are also electrically insulated from ground planes 14 and implement part of a voltage sensor H field cancellation structure. Vias 22 and 24 and their associated traces on inner layers of PCB 12 are described below in more detail.
PCB 12 may also include traces and pads, which are generically indicated at 28, for mounting electronic circuit components that may be connected to ground plane 14 and the various traces of the inner layers of PCB 12. Examples of components include: connectors for providing power to the electronic circuits and/or taking electrical signals from them, amplifiers, transformers, and/or filters that buffer and/or condition the signals that represent the RF voltage and/or current of conductor 20, and the like.
Referring now to
First and second inner layers 32, 34 are positioned underneath and insulated from respective ones of ground planes 14. Vias 22 extend between and connect associated traces 44 on the first and second inner layers as is described below. Associated traces 44 are also connected by respective current sensor pads 30 that are formed along walls of aperture 18. Current sensor pads 30 can be plated to the edge of aperture 18 in substrate 36 and cut to shape by laser, mechanical abrasion or other manufacturing technique.
Insulator substrate 36 gives current sensor pads 30 a length L. While RF current flows through conductor 20, a magnetic field rotates around conductor 20. The magnetic field can be defined by Biot-Savart Law, which provides
where radius is the distance between conductor 20 and current sensor pads 30, ACcurrent is the current flowing through conductor 20, and μ0 is the magnetic constant, equal to 4π×10−7 H/m. The magnetic field crosses current sensor pads 30.
From Faraday's law, the induced voltage is a function of the length L of current sensor pads 30, a rate of change of the magnetic field, and a height of a loop formed by current sensor pads 30, traces 44, and vias 22. Increasing the length L, such as by increasing a thickness of substrate 36, increases coupling between the RF coaxial cable (not shown) and VI probe 10.
As L increases, the diameter of vias 22 may also need to be increased in order to reduce the risk of breaking drills during PCB 36 fabrication. Increasing the diameter of vias 22 also increases the size of the sensor and/or reduces the number of current sensor pads 30 or loops (described below) that fit along the perimeter of aperture 18. Increasing the diameter of vias 22 also yields a proportional increase to the capacitive coupling to conductor 20, which allows the electric field (“E field”) produced by conductor 20 to contaminate the desired current signals and reduce the dynamic range of the VI probe 10. In various embodiments, the width of the current sensor pads 30 may be made as narrow as is practicable to mitigate E field contamination and dynamic range issues. Edge plated current sensor pads 30 reduce the size and number of vias 22 required to make VI probe 10, which reduces E field contamination of the current signal. The current signal represents the current flowing through conductor 20 and is taken from traces 44.
Traces 44 are shown in more detail in
By alternating the current sensor pads 30 with spatial uniformity along the inner perimeter of the opening 18, the current loops provide an autocorrecting feature from movement of the conductor 20, such as from assembly/disassembly of VI probe 10 or thermal changes in the VI probe 10 during operation. The autocorrecting feature of the design works by maintaining a constant sum total distance from the conductor 20 to the current sensor pads 30. If, for example, the conductor 20 moves to the right, the right current sensor pad 30 to conductor 20 distance decreases, but the left current sensor pad 30 to center conductor 20 increases by the same amount, keeping the sum total distance and current signal level the same.
Another view of a PCB 12 according to various embodiments of the present disclosure is shown in
A schematic representation of a prior art current sensor is illustrated in
A schematic representation of a current sensor according to various embodiments of the present disclosure is illustrated in
A schematic representation of a current sensor according to various embodiments of the present disclosure is illustrated in
By placing the loops next to each other as depicted in
The transformer 50 generally has a turn ratio (“N”) with respect to the primary and secondary windings. The resistors 55 compensate for the real portion of the output impedance. The imaginary portion of the output impedance may be compensated for by presenting real transmission line sensor sections or matching/filter circuits. In various embodiments, the values of the resistors 55 may be defined by:
where Zoutput is the desired output impedance presented to the analysis unit and N is the transformer turns ratio.
Referring now to
From Ampere's law, with the RF current traveling into the page, a magnetic field will be generated in a clockwise direction around the conductor 20. The resistors 55 may be used to match the sensor impedance to any accompanying processing unit that may be utilized with the VI probe 10, which will result in lower reflected signals, lower noise and maximum sensor signal power transfer to the processing unit. The magnetic field will induce a current on the pickup loops 60 to flow through the transformer 50 and resistors 55. As discussed above, the electric field picked up by loops 60 will not be shorted to ground, as in the case of
As described above, the voltage potential at points A and B are proportional to the electric potential in the conductor 20, due to capacitive coupling. The voltage potential at points A and B can be thought of as the electrical potential of the conductor 20 plus an additional component due to contamination from the current signals generated on loops 60 by the magnetic field.
Utilizing the arrangement of
V
e
=V sin(ωt);
I
m
=I cos(ωt); and
s=jω.
Kirchhoffs current law equations for each node yields:
If we collect like terms:
Solving for VA and VB yields:
Adding VA and VB signals (Vsum=VA+VB) with a center tap of the transformer 50 (or, alternatively, by an analog-to-digital converter/digital signal processing (“ADC/DSP”) system) yields:
The result is the E Field of conductor 20, Ve, and an attenuation term. Subtracting VB from VA (Vdiff=VA−VB) with a transformer 50 (secondary winding) or ADC/DSP system yields:
The result is current flowing through conductor 20, Im, and an attenuation term. From these equations, the voltage (Ve) and current (Im) of conductor 20 may be determined.
A schematic representation of a voltage/current sensor according to various embodiments of the present disclosure is illustrated in
Referring now to
Those skilled in the art can now appreciate from the foregoing description that the broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, the specification, and the following claims.
This application claims the benefit of U.S. Provisional Application No. 61/043,934, filed on Apr. 10, 2008. The disclosure of the above application is incorporated herein by reference.
Number | Date | Country | |
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61043934 | Apr 2008 | US |