Power sensor standard

A power sensor system, assembly and method for use as a power sensor standard in the 50 to 75 GHz frequency range. The power sensing system comprises a housing comprising a dual ridged waveguide impedance transformer, and a resistive component attachable to a back side of the housing. The resistive component comprises a terminating element electrically, but not thermally isolated from a sensing element. The sensing element operates at a constant resistance and is perpendicularly oriented to the terminating element.

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BACKGROUND

Waveguide power transfer standards enable the measurement of radio frequency (RE), microwave, and millimeter power for varying frequency ranges. A waveguide structure guides electromagnetic waves by limiting expansion of the transverse waves perpendicular to the axis of propagation. A sensor is then employed within the waveguide structure to either absorb or detect the guided incident electromagnetic energy and provide a means with which to measure and determine power. Sensors commonly used have typically been bead sensing devices or fm-line type of transforming circuits. Unfortunately these types of sensors result in lower power measurement uncertainty capability. Additionally, the calibration efficiency factor is significantly limited by thermal leakage of the input power to the waveguide housing upon which the sensor is attached.

Low frequency sensors typically use an actual change in temperature or in resistance as the sensed property. However, operating at a constant temperature or resistance is superior as linearity is substantially improved. This will also significantly decrease uncertainty in power measurements and improve calibration efficiency. In addition, standard waveguide configurations normally used in the millimeter frequency range provide fundamentally narrower bandwidth capability as compared to other waveguide topologies such as dual ridged waveguide. An additional issue with AC- or DC-substitution sensors made using thermistor “beads” such as are used in HP486A, 8486B, and Weinschel/TEGAM 1110 through 2510 is that the beads are made on fine (approximately 0.0007″ diameter) wires. While such wires provide high sensitivity, they also result in a step change in wave line width that causes a reflective discontinuity. Existing waveguide thermistor sensors such as Hughes/Militech exhibit this problem in high VSWR performance. The present invention is able to provide a smooth, wideband match with improved reflection over the thermistor design without sacrificing the linearity of substitution designs.

Accordingly, there is a long felt need in the art for a WR-15 standard interface waveguide sensor that measures power in the 50 to 75 GHz frequency range (V-band) or other frequency bands when scaled appropriately. Specifically, a device that can be used as a power standard due to lower power measurement uncertainty capability is needed. Additionally, a device is needed that can separate a termination resistance from a sense resistance while coupling the termination structure to a waveguide so that the resistive structure works thermally while providing high frequency match.

SUMMARY

A power sensing assembly comprising a housing and a resistive component. The housing comprises a standard waveguide interface and a dual ridged waveguide impedance transformer that runs through the housing from a front side through a back side. The dual ridged waveguide impedance transformer topology is configured to concentrate high frequency power as well as transform the waveguide impedance as it travels through the assembly. As such, the dual ridged waveguide impedance transformer may comprise a plurality of specifically proportioned heights and lengths of steps or ridges narrowing an axial separation distance between corresponding ridges within the waveguide from the front side through the back side. Other embodiments of the impedance transformer may comprise a plurality of tapered slopes with specifically designed contours to meet desired frequency responses within the waveguide bandwidth and may be employed equally well in either rectangular or circular waveguides.

The resistive component is attached to the back side of the housing and aligned with both ridges of the waveguide. The resistive component comprises a substrate, a terminating element, and a sensing element. The sensing element is separated from the terminating element by the substrate, and is substantially perpendicularly oriented to the terminating element. The sensing element is thermally, but not electrically coupled to the terminating element.

The power sensing assembly may further comprise a mounting plate, a backside short and a backside short shim. The backside short shim is configured to optimize a distance from a plane of a resistive termination to the backside short inside the waveguide. The mounting plate comprises an opening for accepting the resistive component and a plurality of attachments for connection of the resistive component to external bias circuitry for further processing.

Further, a method of measuring power comprises providing a power sensing assembly comprising a dual ridged waveguide impedance transformer, a terminating element, and a sensing element. The terminating element and the sensing element are aligned with the dual ridged waveguide transformer to achieve good match characteristics. The terminating element is electrically, but not thermally isolated from the sensing element. Tuning capability is also provided if necessary to achieve an optimal matched termination for specific frequency ranges within the fundamental waveguide bandwidth that is independent from the sensing element.

DETAILED DESCRIPTION

Conventional waveguide sensors suffer from poor reflection performance and high power measurement uncertainty. The present invention is useful as a power measurement standard for V band (50-75 GHz) or other frequency bands, such as to at least 110 GHz (WR-10) for example, when scaled appropriately. Additionally, larger embodiments will provide good results at lower frequencies as well. The power sensing assembly provides a lower uncertainty method for measuring microwave power. The power sensing assembly is configured to allow connection to a standard WR-15 waveguide interface. The reflection performance is substantially improved over both existing bead sensing devices or fin-line type transforming circuits resulting in significantly lower power measurement uncertainty capability. The calibration efficiency factor of the present invention is in the range of approximately between 55-60 percent limited by thermal leakage of the input power to the waveguide housing upon which the microwave circuit is attached. However, the variation of the efficiency factor within the frequency band is less than existing approaches.

Reference is now made to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding thereof. It may be evident, however, that the novel embodiments can be practiced without these specific details. In other instances, well known structures and devices are shown in block diagram form in order to facilitate a description thereof. The intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the claimed subject matter. The invention relates generally to a power sensing assembly and method of measuring power.

FIG. 1illustrates a power sensing assembly100. The power sensing assembly100comprises a housing102and a resistive component130. The power sensing assembly100further comprises a mounting plate142, a backside short shim150, and a backside short154.

As illustrated inFIGS. 1-5, the housing102is typically manufactured from a base material such as, but not limited to, copper, beryllium copper, nickel, aluminum, or similar metal or alloy. In addition, thin plated highly conductive coatings such as silver, copper, gold or other similar alloys may be added to enhance the conductivity of the base metal. Composite type coatings consisting of dielectric/metallic plating may also be employed to protect against corrosion. The housing102comprises a front side108, a back side112, and a dual ridged waveguide impedance transformer120. The front side108comprises a standard waveguide interface opening110, and the back side112comprises a dual ridged waveguide output114. The dual ridged waveguide output114comprises a midpoint118and an attachment position116for the resistive component130. The attachment position116is oriented approximately vertically across the dual ridged waveguide output114. The attachment position116may be a pair of ridges (top and bottom) that are substantially identically symmetrical and positioned along the midpoint118as illustrated inFIG. 8. The dual ridged waveguide impedance transformer120extends through the housing102from the standard waveguide interface opening110through the dual ridged waveguide output114.

In one embodiment, the dual ridged waveguide impedance transformer120is substantially rectangular in configuration and may comprise a plurality of steps122or ridges designed to narrow the separation distance between opposing ridges of the dual ridge waveguide impedance transformer120from the standard waveguide interface opening110through the dual ridged waveguide output114as shown inFIGS. 5 and 6. As such, the dual ridged waveguide impedance transformer120is designed to convert the impedance of a transversely propagated high frequency wave within a standard waveguide interface to the impedance of a small resistive termination located at the dual ridge waveguide output114. In addition, as high frequency power enters the power sensing assembly100, the steps122or ridges of the dual ridged waveguide transformer topology concentrate the high frequency power substantially perpendicular to an axis of high frequency wave propagation within the dual ridged waveguide impedance transformer120and convert the field energy to thermal energy via the closely coupled thermal path to the resistive component130where the energy is dissipated. Alternatively, the dual ridged waveguide impedance transformer120may be quadrilateral, circular, or any other shape in configuration that produces a good response. Similarly, dual ridged waveguide impedance transformer120may comprise stepped, or tapered ridges, or a tapered contour that terminate in a planar resistor and RTD assembly to improve the bandwidth of the match to the termination.

The housing102further comprises a plurality of alignment holes104and a plurality of attachment holes106. The power sensing assembly100further comprises a plurality of alignment pins156for engaging the plurality of alignment holes104, and a plurality of fasteners158for engaging the plurality of alignment holes104.

As illustrated inFIGS. 8 and 9, the resistive component130in one embodiment is a microwave thin film circuit132on a single substrate configured to contain both a terminating and a sensing device attached at the end of a dual ridged waveguide impedance transformer120. The resistive component130comprises a substrate134, a terminating element136, and a sensing element138. The resistive component130is aligned with the dual ridged waveguide impedance transformer120and is conductively attached electrically to the housing102at the attachment position116via an attachment element141such as, but not limited to, a pair of epoxy pads, or by other such electrically appropriate conductive means. Alternatively, mechanical fastening may be feasible for larger, lower frequency designs where the necessary space would be available. The drawback would be the fasteners would draw thermal energy away from the sensing element, but insulated screws or spring loaded designs could minimize this effect. Additionally, it could be clamped using clamping arrangements not described, or using a solder system. Once attached, the terminating element136and the sensing element138are essentially suspended at the end of the dual ridged waveguide impedance transformer120. The terminating element136will ideally be located adjacent to the midpoint118of a gap between the attachment positions116of the dual ridged waveguide output114. The substrate134will similarly be centered on the gap.

The resistive component130is essentially a power sensor. The terminating element136provides a matched termination for the rigid waveguide topology while simultaneously providing a source of heat energy, the high frequency power dissipated by the terminating element136, for the sensing element138. The terminating element136is a resistive or other energy absorbing device, such as a matched termination resistor, or any other device that dissipates power from a high frequency source. The sensing element138may be any resistive structure with a positive or negative temperature coefficient such as, but not limited to a resistive temperature detector (RTD), a platinum sensor, a thermistor, a metallic RTD, a semiconductor with bulk resistance and a high temperature coefficient such as Germanium, or any structure with a strong temperature coefficient of resistance. The sensing element138may also be any other energy detecting device, suitable for sensing the energy from the terminating element136.

The substrate134is typically a dielectric material. The terminating element136and the sensing element138are oriented substantially perpendicular or 90 degrees to each other. Additionally, the terminating element136is located on the one side of the substrate134facing the housing102, and the sensing element138is positioned on the opposite side of the substrate134. This electrically isolates the terminating element136from the sensing element138, yet allows thermal energy to propagate. This separation and perpendicular orientation minimize electromagnetic disturbances to the applied signal to be measured. As such, each energy absorbing and energy detecting function can be optimized largely independent of each other, thereby leading to a much lower reflection capability than previously obtained with other approaches which results in lower power measurement uncertainty capability. While not connected electrically, the terminating element136and the sensing element138are still thermally coupled allowing the concentrated power to be transferred to the sensing element138.

The sensing element138ideally operates at a constant resistance in a self-balancing system. The power sensing assembly100operates as a DC-substitution bolometer using a closed-loop balancer to maintain constant resistance. The sensing element138may be integrated with another device (not shown) designed to control power to the sensor to keep the element at a constant resistance. The other device reacts to the change in resistance and uses that to calculate high frequency (RF) power. Therefore, the sensitivity is manifested as the needed change in DC power to keep the sensing element138at constant resistance. This is desirable as now a high frequency (RF) termination may match with an adjacent thermally coupled RTD or thermistor in which the change in resistance is the measurand to be converted to power. This is a distinct improvement over other low-frequency sensors that use the actual change in resistance or temperature as the sensed property. Operating at a constant resistance or temperature is superior as linearity is substantially improved. The present invention separates the termination resistance from the sense resistance and uniquely couples the termination structure to a stepped waveguide that allows the resistive structure to be small enough to work thermally while also providing excellent high frequency (RF) match.

The matched termination of the wave may also be tuned independently from the sensing element138. As illustrated inFIGS. 10-16, the mounting plate142may comprise a printed circuit board (PCB) comprising an opening144and a plurality of attachments146. The power sensing assembly100further comprises a pair of bond wires140. The mounting plate142is positioned so that the opening144is located adjacent to the resistive component130. The mounting plate142is aligned with the housing102via the alignment pins156and is secured via the fasteners158. Each of the pair of bond wires140is long a thin wire that connects the sensing element138to the plurality of attachments146of the mounting plate142. This connection provides high frequency isolation to an external sensing bias circuitry of the PCB. The plurality of attachments146are typically PCB pads, connecting wires or the like on an outward facing side of the mounting plate142. The dimensions of the wires provide a sizable inductive reactance to any high frequency signals that may inadvertently leak onto them in the frequency range of interest, thereby blocking them from ever reaching and possibly corrupting the desired DC bias circuitry used to measure the change in resistance of the sensing element138.

As illustrated inFIGS. 17 and 18, the backside short shim150comprises a channel cutout152. The channel cutout152is positioned adjacent to the opening144of the mounting plate. This allows the pair of bond wires140to project out and align with a projected PCB trace and then turn downward to align with the channel cutout152. The pair of bond wires140typically will remain within the approximately 0.020 inch depth profile, but could be made deeper as permitted by the overall thickness of the backside short shim150to remain inside the channel cutout152in the backside short shim150as illustrated inFIGS. 13 and 14.

The backside short shim150may be varied in thickness which will vary a distance from a plane of a resistive termination at the dual ridged waveguide output114to the backside short154. This variation in thickness will alter the constructive/destructive interference effect between a forward wave and any existing residual reflected waves returning back from a back short ground plane. This allows an overall input reflection response to be tuned as a function of frequency if desired. This also provides a capacity to specifically optimize other frequency bands within the 50 to 75 GHz bandwidth if desired. As with the mounting plate142, the backside short shim150and the backside short may be aligned with the housing102via the alignment pins156and is secured via the fasteners158.

In an additional embodiment, a method for measuring power comprises providing a power sensing assembly100comprising a dual ridged waveguide impedance transformer120, a terminating element136, and a sensing element138. The terminating element136and the sensing element138are aligned substantially perpendicularly to each other and are positioned with respect to the dual ridged waveguide impedance transformer120to achieve good match characteristics for a high frequency wave. Next, the terminating element136is electrically isolated from the sensing element138while remaining thermally coupled by placing a substrate134between them. Tuning capability is also provided if necessary to achieve an optimal matched termination for specific frequency ranges within the waveguide bandwidth that is independent of the sensing element138.

The method allows for attachment of the resistive component130to a housing102of the power sensing assembly100that allows for good match characteristics while simultaneously providing good thermal isolation to the waveguide structure to maximize efficiency. This is achieved by the alignment of the terminating element136and the sensing element138with respect to the dual ridged waveguide impedance transformer120. As the tuning capability is affected by a distance between a backside short154and a plane of resistive termination, a backside short shim150of an appropriate thickness is selected and positioned between the housing102and the backside short154. The thickness of the backside short shim150is selected based on a desired interference effect between a forward wave and a residual wave returning from the backside short154. Additionally, a desired DC bias circuitry used to measure the change in resistance of the sensing element138may be further isolated by providing an inductive reactance to high frequency signals.

The power sensing assembly100uses the inherent broadband nature of the dual ridged waveguide topology and dual ridged waveguide impedance transformer to provide a wideband terminating match, via the terminating element136, to minimize mismatch power sensing error while simultaneously indirectly measuring broadband power via the sensing element138. The power sensing assembly100is scalable in configuration and is applicable to all geometric sizes and shapes of rigid waveguide assemblies. The power sensing assembly100provides an interface to mate with all corresponding standard waveguide configurations. The functionality is derived from the alignment of the sensing element138and the terminating element136to the dual ridged waveguide impedance transformer120. These elements may be modified to accommodate different frequency ranges, primarily by changing the waveguide configuration, or by altering resistor and sensor placement, value, or type. For example, in an additional embodiment, if the desired frequency bandwidth is narrow enough, the impedance transformer may be bypassed in its entirety and the resistive component130may be simply attached to the output of a standard waveguide configuration at midpoint location118. The value of the matched termination is then changed accordingly to match that of the standard waveguide impedance for the intended frequency response.