Device for detecting and indicating power

In accordance with an embodiment a device is usable to measure radio frequency (RF) signals including microwave signals to provide an alarm when a power level at its input exceeds predetermined levels. A user can attach the device to a coax cable on a microwave or wireless tower to determine if certain power levels are present and what levels are exceeded. If high power is indicated by the device, the user will then avoid attaching that coax cable to other measurement equipment which would be damaged by excessive RF power. The device is further usable, for example, to apply power to one coax cable in a cable bundle then identify which cable of the bundle is getting the power by connecting the device on the output end of each coax in the bundle, one by one.

TECHNICAL FIELD

The present invention relates generally to devices for detecting and indicating power at a potential signal source.

BACKGROUND

Signal measurement devices, such as vector network analyzers (VNAs) and scalar network analyzers (SNAs), are commonly used for measuring parameters indicative of performance of a device under test (DUT). A technician can measure the performance of an antenna in a cellular network, for example, by disconnecting the antenna from a transmitter associated with the antenna and connecting the antenna to the signal measurement device. The signal measurement device can transmit test signals and receive reflected signals in response. However, signal measurement devices commonly operate at powers below 100 milliwatt (mW) (i.e., below 20 dBm), much lower than typical transmit powers of DUTs.

In a telecommunication network environment, many opportunities present themselves for inadvertently damaging equipment via accidental exposure to a high power signal source. For example, an antenna for a high power transmitter can be remotely connected to the transmitter via multiple cables. For example, a cable leading to an antenna and a cable leading from a high power transmitter may be connected together in a jumper room. A technician testing the antenna will typically do so in the jumper room by disconnecting the appropriate cable and reconnecting the cable to a signal measurement device. This presents an opportunity for a low power signal measurement device to be accidentally connected with a cable leading to a high power transmitter rather than to the antenna. A signal transmitted at the high powers associated with a transmitter for a cellular site can damage the circuitry of a signal measurement device, which is designed to operate at powers many magnitudes lower.

SUMMARY

Embodiments of the present invention include a device usable to measure radio frequency (RF) signals including microwave signals to provide an alarm when a power level at its input exceeds predetermined levels. In an embodiment, a user can attach the device to a coax cable on a microwave or wireless tower to determine if certain power levels are present and what levels are exceeded. If high power is indicated by the device, the user will then avoid attaching that coax cable to other measurement equipment which would be damaged by excessive RF power. The device is further usable, for example, to apply power to one coax cable in a cable bundle then identify which cable of the bundle is getting the power by connecting the device on the output end of each coax in the bundle, one by one.

In an embodiment, an input for a RF measurement device includes a series distributed capacitor adapted to couple an RF signal from a center pin of a coaxial cable, the series distributed capacitor including a capacitor terminal comprising a multilayer metal horseshoe-shaped trace interconnected with a circuit of the RF measurement device, a shunt lumped capacitor to ground, and a metal washer configured to allow fine adjustment to a series capacitor value of the input.

In an additional embodiment, a device usable to measure radio frequency (RF) signals including microwave signals to provide an alarm when a power level at its input exceeds predetermined levels can include a probe that can indicate the presence of an electric field when brought in proximity to a power source.

DETAILED DESCRIPTION

The following description is of the best modes presently contemplated for practicing various embodiments of the present invention. The description is not to be taken in a limiting sense but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be ascertained with reference to the claims. In the description of the invention that follows, like numerals or reference designators will be used to refer to like parts or elements throughout. In addition, the first digit of a reference number for an embodiment of the invention identifies the sequence in which an individual embodiment is described.

It would be apparent to one of skill in the art that the present invention, as described below, may be implemented in many different embodiments of hardware, software, firmware, and/or the entities illustrated in the figures. Further, the frequencies given for signals generated and/or used, and the values for electronic components (e.g., resistors, capacitors, etc.) in the figures and description are merely exemplary. Any actual software, firmware and/or hardware described herein, as well as any frequencies of signals generated thereby and any values for electronic components, are not limiting of the present invention. Thus, the operation and behavior of the present invention will be described with the understanding that modifications and variations of the embodiments are possible, given the level of detail presented herein.

Devices for detecting and/or measuring signal power have been available in different forms. U.S. Pat. No. 4,032,910 to Hollway, et al. describes a device that senses a radio frequency (RF) field with an antenna. The full strength of the signal is placed on a hot carrier diode, which is connected to an incandescent alarm. However, the device is not capable of interfacing a transmission line. U.S. Pat. Publ. US 2002/0039021 to Wong, et al. describes a root mean square (RMS) power sensor having an 84 dB dynamic range. The device is usable in a power meter application where a 50 ohm load impedance is desired. However, it is not usable when very large powers are present and is therefore not useable as an alarm indicating that preset power levels have been exceeded. Further, the device of Wong is designed for precise power measurement, and as a result the device is costly and not intended for a self-contained and portable, battery operated application.

Devices in accordance with embodiments are usable to determine (and indicate) if a connector is “live” with RF power that could, for example, damage sensitive measuring equipment (i.e., the device is an RF power indicator). In an embodiment, the device is adapted to be mated with a connector to indicate the presence of high-level RF power. In an embodiment, the device is configured to interface a coaxial transmission line (rather than an antenna) and can be temporarily attached to a coax transmission line output to detect RF transmission. For example, a user can attach the device to a coax cable on a microwave or wireless tower to determine if certain power levels are present and what levels are exceeded.

FIG. 1is a circuit diagram of a device100for detecting and indicating RF power in accordance with an embodiment. In the embodiment, the device can include a horseshoe distributed capacitor104as an input device. A test signal with a frequency of 2 MHz is generated at 3.4 V. The horseshoe distributed capacitor can further be seen inFIG. 2extending from a printed circuit board (PCB)102and configured so as to surround the end of a center pin of an RF coaxial adapter110(shown inFIGS. 3 and 4connected with the device100). The horseshoe distributed capacitor104can be further seen inFIG. 6andFIG. 7.

In an embodiment, the device includes a distributed and lumped capacitive voltage divider which can be used to reduce power entering the device to allow very high power inputs without burnout. In an embodiment, the device further uses digital integrated circuit (IC) comparator circuits and dual zero bias Schottky diodes to allow signal detection and comparator voltage reference within the same package, thereby providing for temperature stability. Trigger points can be provided by digital IC comparators which can run on extremely low current, thereby preserving battery life.

If high-level RF power is detected, the device can provide an alarm in the form of a cue to a user that one or more predetermined power levels is exceeded. Alarm trip points can be determined by the reference voltage. In an embodiment, a visual cue is provided in the form of visual indicators that are activated when predetermined thresholds are exceeded. In other embodiments, the device can further include, or alternatively include, an audial cue such as a pre-recorded message or alarm tone, for example.

As shown inFIG. 4, the device includes a yellow LED that is turned on when RF power exceeds 17 dBm (50 mW) and a red LED that is turned on when RF power exceeds 27 dBm (500 mW), although in other embodiments some other threshold limits can be set along with other colors or forms of cues. In the embodiment shown, the device is usable to detect RF power levels of up to 50 dBm (100 W) from a 50Ω source without being damaged. However, operating at such specification, the device has a very high voltage standing wave ratio (VSWR) and should not be used as a 50Ω termination.

The use of LEDs112to provide the alarm can contribute to saving battery life, as LEDs are power efficient. However, in other embodiments other light emitting devices can be used. While the embodiment described above uses a yellow LED and a red LED, any available LED colors can be used. When detected signal exceeds a preset trigger level, the corresponding LEDs will light up. The design is scalable so that in other embodiments more than two trigger circuit comparators/LEDs can be configured.

If high power is indicated by the RF Power Indicator, a notified user can avoid attaching that coax cable to other measurement equipment which would be damaged by excessive RF power. The device is further usable, for example, to apply power to one coax cable in a cable bundle then identify which cable of the bundle is getting the power by connecting the device on the output end of each coax in the bundle, one by one.

In an embodiment, the device is “always on” and ready to be used as needed, having a self-contained battery106that can last for years with normal use and is field-replaceable. In an embodiment, the device further includes a “self-test” button114that, when pressed, causes both indicators (red and yellow) LEDs to light if internal circuits and battery are functioning.

In an embodiment, the device is a handheld battery operated RF power indicator which is self-contained. The user temporarily connects it to their coax cable output. If certain power levels are exceeded, then one or more alarm LEDs will light depending on how much power is applied. If LEDs light up the user would avoid connecting the same powered cable to a test instrument that could be damaged by excessive input power. As shown, inFIGS. 3-7, the device100comprises the RF coax adapter110, washer116, snout118, handle120, rear cap122, battery106, spring, loaded PC board102, and a lanyard108, although in other embodiments the device can comprise fewer or additional components. In other embodiments, the device can be housed in some other fashion, such as sealed with an enclosed rechargeable battery that is not accessible to the user.

Devices in accordance with embodiments comprise a capacitive RF voltage divider used at the power indicator front end. RF voltage is reduced to levels that can be tolerated by the detector diode. The capacitive RF voltage divider dissipates very little of the incident power, eliminating the need for a large heat sink such as would be required for a resistive attenuator. The capacitive RF voltage divider further protects detector components from electrostatic discharge (ESD) on the center conductor, and is itself tolerant to high levels of ESD.

In an embodiment, the capacitive RF voltage divider can comprise an arrangement of components including a series distributed capacitor that couples RF signal from a coax center pin, a shunt lumped capacitor to ground completing the capacitive divider, and a metal washer that can provide fine adjustment to series capacitor value. The series distributed capacitor includes a first capacitor terminal comprised of multilayer metal horseshoe-shaped trace that can be interconnected with vias on the end of a PC board and a second capacitor terminal comprised of the center pin of a coax line.

Devices in accordance with some embodiments further comprise a power indicator that includes a set of components to achieve best bandwidth and response flatness, as provided in the circuit inFIG. 1. The power indicator can provide flatter response with power input alarm trigger points that change little versus frequency over the intended bandwidth. Response controls comprise a number of components. A series resonance acts to increase detector sensitivity at the high frequency end of the measurement band. This resonance is provided by transmission line and parasite inductances in series with detector diode capacitance and stray capacitance. A series resistor damps the series resonance described above. Adjustment of the series resistor changes bandwidth of the resonance, thereby facilitating detector “flatness” adjustment at the high frequency end of the measurement band. Shunt resistance to ground controls low frequency cutoff because the source impedance presented by the capacitive divider increases as frequency decreases. It also provides direct current (DC) ground return for a detector. The self-series resonance of shunt divider caps occurs above the band and decreases sensitivity.

Devices in accordance with some embodiments further comprise a self-check oscillator signal injected into a front end. The oscillator signal can provides the ability for checking that the device is fully functional prior to use. The oscillator signal is injected into the RF detector diode through the same circuitry that provides control of the low frequency band edge.

Devices in accordance with some embodiments further comprise a portable coaxial power indicator that provides a high impedance power detector front end for sensing high power and facilitating multiple trip point power level alarms.

As shown inFIG. 5, the circuit ofFIG. 1can be fabricated on a PCB, using commercially available components. For example, the device can use for signal detection an SMS7630 series component including surface mount detector Schottky diodes available from Skyworks Solutions, Inc. The PCB can also include a model LTC6900-low power, 1 kHz to 20 MHz oscillator available from Linear Technology Corp., and pair of low power comparators each comprising a model LTC1540 low power comparator with reference also available from Linear Technology Corp. As indicated inFIG. 5, a pair components150,152to provide series resonance are shown mounted on the PCB, as well as the IC comparator circuit components156,158, a low power oscillator154and a series resonance component160for the test switch. Depressing the test switch allows a user to check that the device is fully functional.

FIG. 8is a circuit diagram andFIG. 9is a simplified plan view of a device200for detecting and indicating RF power in accordance with a further embodiment. In the embodiment, the device includes a probe204in substitution for the horseshoe distributed capacitor ofFIG. 1as an input device. The device is a relative measuring device, with LEDs that blink faster as the probe gets closer you get to a source of RF. The probe is enclosed, as shown by a plastic tip or dome, and RF is coupled into the circuit as the probe is brought into proximity of an RF source. As such, the probe is not arranged at a consistently fixed distance from the RF source, as is the case with previous embodiments. The fixed distance of the input of the device100ofFIG. 1can allow for calibrated measuring. However, the device200ofFIG. 8can be used without the use of an RF coax adapter. As can be seen, the device includes a plastic tip enclosing the probe, an LED (or other indicator), a test switch, a housing, and a screw cap. In other embodiments the device can include some other indicator, such as an audial cue, and can be housed in some other fashion, such as sealed with an enclosed rechargeable battery that is not accessible to the user.

FIGS. 10-12are partial circuit diagrams and schematics of a front end of a device300for detecting and indicating RF power in accordance with a further embodiment. In the embodiment, the device includes frequency compensation in the form of additional circuit components of a PCB302including a compensation capacitor Ccomp. An example of the difference in amplitude response with change in frequency is shown for the circuit inFIG. 12.