Methods and apparatus for highly reliable signal distribution

Methods and apparatus for providing a passive non-redundant signal distribution system including a passive combiner coupled to the first and second controlled signal sources to receive first and second signals, the combiner having an output, and a passive splitter coupled to the combiner output to provide reference signals to signal sinks, all to provide reliable dissemination of a reference signal despite failures of signal generation or distribution.

BACKGROUND

As is known in the art, conventional high availability radar systems include at least two identical reference signal generation and distribution systems. Each component needing a reference signal receives a copy of the signal from each of the reference distribution systems to allow continued operation despite a failure in generating or distributing the reference signal. Components receiving the reference signal are called sinks herein.

FIG. 1shows a prior art signal distribution system100having first and second identical signal sources112a, b, each driving a respective one of first and second identical active splitters114a, b. A first signal generation and distribution system comprises the first signal source112aand the first active splitter114a, and a second signal generation and distribution system comprises the second signal source112band the second active splitter114b. Signal sinks116a-N receive signals from both the first and second signal generation and distribution systems. As can be seen, the redundant signal distribution systems enable the sinks to receive a reference signal while meeting a required availability despite failures.

There are a number of disadvantages to this traditional approach. One disadvantage is the cost of duplicating distribution and reception systems. A technical disadvantage is that the two distribution systems necessarily differ in manufacturing tolerances and errors, requiring radar calibration for each system, thus increasing the amount of radar timeline occupancy required to calibrate the radar. Radar calibration refers to the continuous process whereby hardware imperfections are compensated for to enable the radar to generate sharp beams with low sidelobes and deep nulls, despite manufacturing variations and subsequent changes due to component aging, temperature variation, power supply voltage variation, and the like.

Another disadvantage is that prior-art signal distribution systems include active components, thus reducing reliability and increasing cost, complexity and sensitivity to temperature variations. Yet another disadvantage is that each and every component requiring a reference signal must be equipped to accept and choose between or combine at least two reference signals, thus at least doubling the expense and complexity of the component's signal reception interface.

There are alternate prior-art approaches where a single distribution system is driven by a pair of sources through a complicated redundant switching arrangement, but it can be difficult to find an adequate approach because the switches themselves are active and/or mechanical, and thus of limited reliability.

SUMMARY

The present invention provides method and apparatus for non-redundantly distributing a reference signal throughout a system, such as a radar system, to coordinate and align processing and signal generation. A precisely aligned and distributed reference signal is required for system performance. Exemplary embodiments of the invention provide efficient and non-redundant signal distribution while achieving relatively low probability of loss of the reference signal to meet stringent availability requirements.

In one aspect of the invention, a passive non-redundant signal distribution system comprises: a passive combiner coupled to the first and second controlled signal sources to receive first and second signals, the combiner having an output, and a passive splitter coupled to the combiner output to provide reference signals to signal sinks in the passive non-redundant signal distribution system.

The system can further include one or more of the following features: the combiner and the splitter are combined, a further splitter, wherein the splitter and the further splitter are merged into a hybrid coupler, a first controlled signal source generating the first signal and a second controlled signal source generating the second signal, the first controlled signal source includes a signal generator and a directional coupler coupled to a threshold detector, the first controlled signal source includes a signal switching module to selectively enable the first signal, the first controlled signal source includes a modulator to combine a sinusoidal signal with a further signal, the reference signals comprise one or more of one sine wave modulating another, a linear frequency modulation chirp, and/or a sinewave modulated with a binary sequence, and/or the first controlled signal source includes a signal adder to combine a sinusoidal signal with a further signal.

In another aspect of the invention a radar system includes: a receiver to receive a first signal and is generate a first output signal and a second output signal having a number of pulses per second, a modulator to combine the receiver first and second output signals and generate a modulator output signal, a directional coupler to provide a sample of the modulator output signal to a threshold detector, a passive combiner to receive the modulator output signal and generate a combiner output signal, and/or a passive first splitter to receive the combiner output signal and generate a number of reference signals for signal sinks for providing a passive non-redundant reference signal distribution system.

The radar system can further include one or more of the following features: a passive second splitter to split at least one of the reference signals from the first splitter, a passive non-redundant first signal distribution for a first part of a system and a passive non-redundant second signal distribution for a second part of the system, multiple antenna faces each having a passive non-redundant signal distribution system, and/or a first one of the signal sinks comprises a digital receiver/exciter.

In a further aspect of the invention, a method comprises: receiving first and second signals from respective first and second controlled sources, combining the first and second signals using a device to generate a third signal, splitting the third signal using a passive device to generate a number of reference signals for providing a passive non-redundant reference signal distribution system.

The method can further include one or more of the following features: receiving a first one of the reference signals at a radar transmit/receive system, receiving a first one of the reference signals at a DREX, enabling only a first one of the first and second signals, enabling and/or summing the first and second signals, and/or employing a further splitter, wherein the splitter and the further splitter are merged into a hybrid coupler.

DETAILED DESCRIPTION

Exemplary embodiments of the invention provide a non-redundant passive signal distribution system that provides a relatively low overall failure rate that is useful in high availability systems. Exemplary signals include references for frequency, phase, time, local-oscillator, digital clock, and the like that can be used to coordinate and align various activities and signal generation processes. It will be appreciated that the degree to which coordination and alignment is achieved is a fundamental limit to radar system performance. In addition, loss of a reference signal can interrupt system operation. Exemplary embodiments of the invention provide a non-redundant signal distribution system that enables reliable operation, superior coordination, and cost effective implementation, despite failures of active devices.

FIG. 2shows an exemplary non-redundant passive signal distribution system200having first and second controlled sources202a,b, each driving a passive combiner204. In one embodiment, the first and second controlled sources202a,bare identical. The combiner204drives a passive splitter206that generates references signals208a-N that can be connected to various signal sinks210a-N, e.g., system components, circuit cards, etc., that use the reference signal. In one embodiment, a single combiner receives the signals from the controlled sources202a,b, and drives a single splitter. It is understood that the combiner and splitter components can be merged into a single component.

As used herein, a passive device refers to a device that has no source of power save the signals it handles. By conservation of energy, the outputs of a passive device cannot in aggregate provide more power than the sum of the power levels of the input signals. In contrast, an amplifier always has an external source of power which is gated or controlled by its input signal or signals to yield a more powerful signal than that which is doing the gating or controlling.

Due to the multiple signal sinks210, and the passive configuration of the combiner204and the splitter206, the reference signal power as received at the signal sinks210will be less than the power emitted by the controlled sources202a,b. But because passive devices can have relatively high, e.g., almost perfect, reliability, the probability of loss of the reference signals208at the signal sinks210can be made quite low, e.g., governed by the probability that all controlled sources202fail simultaneously.

As can be seen, either or both of the first and second controlled sources202a,bcan provide the reference signal. That is, the first and second controlled sources can be selectively enabled so that only one of the controlled sources is enabled at any given time. The resulting signal or signals are individually fed to the combiner204. In an exemplary embodiment, the combiner204comprises a passive device that sums the incoming signals to provide a reference signal that is fed to the splitter206. In an exemplary embodiment, the splitter206is a passive device that divides the incoming signal from the combiner204into as many copies as are needed to supply the various signal sinks210a-N.

It is understood that while the illustrative embodiment shows the use of two controlled sources202a,bfor simplicity, any practical number of controlled sources may be used. It is further understood that the combiner and splitter may be implemented electrically or optically, and that signal transmission may be accomplished by means of electrical signals carried on transmission lines or by optical signals carried over optical fibers, or by some combination of the two. The combiner204may be implemented in various ways, including a passive combiner component, a hybrid coupler, or a combination of these kinds of components. It is also understood that the splitter and combiner, as well as other components, may be implemented as unitary components or as interconnected collections of unitary components.

FIG. 3shows an exemplary implementation of a controlled source300, such as the controlled source202aofFIG. 2. A signal generator302generates a reference signal and feeds it to a signal switching module304, which receives a control input signal, such as from a computer or subsystem, to enable or disable the reference signal output. The signal generator302output is fed to a signal sampling mechanism306, such as a directional coupler306, which extracts a representative sample of the outgoing signal and feeds the remaining signal to a combiner (not shown), such as the combiner204ofFIG. 2. The representative sample is fed to a threshold detector308, which reports status. The status output can indicate whether the representative sample meets a specified signal power level, and/or provide the actual measured signal level.

This allows the external control entity (not shown, but typically a computer or hardware controller or the like) to verify that its commands have in fact been followed, thus allowing detection and remedy of failures in the components of the controlled sources. In general, failures can be remedied by manual or automatic intervention to bypass failed components, followed by manual repair.

It is understood that directional coupler refers to any device that can detect signal energy propagating in a particular direction. In the illustrative embodiment ofFIG. 4, the directional coupler416can detect that the power amplifier414is emitting a reference signal. It is understood that the directional coupler is not fooled by power backfeeding from other sources.

That is, looking atFIG. 4, the directional coupler414distinguishes between energy propagating outward (to the right) from energy propagating inward (to the left, flowing backwards into the output of the power amplifier414).

It will be appreciated that combiners and splitters are relatively simple passive devices, and thus are of almost perfect reliability. By contrast, distribution amplifiers, and the like, are more complex and contain active devices such as RF amplifiers, and so have a significantly higher failure rate. It is further understood that a splitter can be used for combining and a combiner can be used for splitting: the difference is in intended use, not necessarily internal design. These are collectively called “reciprocal devices”. Passive hybrid couplers and directional couplers are also reciprocal devices.

The signal switching module304has a control input that allows an external control entity (not shown) to enable and disable emission of the reference signal by the controlled source300. It is understood that the signal generator302and signal switching module304may be merged into a single directly controlled signal source, if the signal source may be so controlled without undue degradation or variation of the generated signal when the output signal is enabled and disabled.

In this configuration, the external control entity commands one of the controlled sources to provide a reference signal and commands all other controlled sources to silence, and the one enabled controlled source provides the reference signal used by all the signal sinks. The external control entity receives reports from the threshold detectors of the controlled sources, and if the wrong number of controlled sources are reported to be providing a reference signal, the external control entity chooses a different controlled source to provide the reference signal.

It is understood that if the signal generators of the various controlled signal sources are synchronized and aligned such that summing their signals yields a valid and adequate reference signal, one may omit and/or bypass the signal switching module and/or the directional coupler, and allow the resulting reference signals to be summed in the combiner, the sinks utilizing the summed reference signal. In this embodiment, all sources ‘sing’ in chorus, and the song continues even if some sources fall silent, so long as there is at least one surviving source still singing.

FIG. 4shows an exemplary implementation of a controlled source400, such as the controlled source102aofFIG. 2. A receiver402, such as a GPS receiver, receives signals, such as GPS signals, using an antenna404. The receiver402provides first and second output signals406a,bto a clock coding module408. In one embodiment, the first output406acomprises a sinusoidal signal, e.g., a 10 MHz signal, and the second input406bcomprises a signal with one pulse per second to provide a time boundary. In this implementation, every ten millionth cycle of the 10 Wiz sinewave is tripled in amplitude, this larger cycle being referred to as a pip or mark. The one pulse per second signal is used by the modulator412to generate the pip on the sinewave seen at the output of412, and in amplified form, at the output of power amplifier414.

The first output406ais provided to a phase-locked loop (PLL)410, which has an output coupled to an amplitude modulator412. A second input to the modulator412comprises the second output406b(the pip signal) from the receiver402. In one embodiment, the signals are combined for transmission in a single cable to maintain coherence between the signals. The modulator412output is provided to a power amplifier414, which provides a signal that can be sampled using a directional coupler416for analysis by a threshold detector418, as described above. The remaining signal is output as a Composite Timing Signal (CTS) to a combiner (not shown inFIG. 4).

While the illustrative embodiment utilizes amplitude modulation, it is understood that other types of modulation can be used, such as amplitude, phase, frequency, pulse, pulse amplitude, pulse width, pulse position, pulse code, polarization, propagation mode, combinations of modulation types, and so on. One may also simply sum the signals, versus using one signal to modulate another. In general, a reference signal, however composed or constructed, can be disseminated with very high availability, despite failures in active devices.

FIG. 5shows an exemplary passive non-redundant distribution implementation500in accordance with exemplary embodiments of the invention. The illustrative system500comprises a passive signal distribution system for a radar system. A first system502, which can be provided as a cabinet similar to that shown inFIG. 4outputting a Composite Timing Signal (CTS), provides a number of timing signals504a-N. A first one of the timing signals504ais provided to a passive power combiner506. In one embodiment, an attenuator508can adjust signal levels as needed. Similarly, a second system502balso provides a tuning signal510ato the power combiner506.

It is understood that the term cabinet refers to a relatively arbitrary collection of components in close proximity to each other.

The combiner506output is received by a passive first splitter512, shown in the illustrative embodiment as a 1:12 splitter. A first output514of the first splitter512is provided to a passive second splitter516, shown in the illustrative embodiment as a 1:4 splitter. Outputs of the second splitter516are provided to various signal sinks518, such as DREXs (digital receiver/exciters) in the radar system.

The illustrated embodiment is well suited for a radar system having multiple radar faces. The configuration ofFIG. 5can provide references signals to DREXs for a given radar face. Similar configurations can provide reference signals to the other radar. In such an arrangement, the five CTS signals504a-ecan be used to provide signals to the radar faces, each having a number of DREXs.

FIG. 6shows an exemplary passive non-redundant distribution implementation600in accordance with exemplary embodiments of the invention. A first system602, which can comprise a cabinet, provides first and second signals604a, b, which can comprise CTS signals shown inFIG. 4. A second system606provides third and fourth input signals604c,d, which can also comprise CTS signals. In one embodiment, the CTS signals are selectively enabled.

The first input signal604ais provided to a first combiner608aand the second input signal604bis provided to a second combiner608b. Similarly, the third input signal604cis provided to the first combiner608aand the fourth input signal604dis provided to the second combiner608b. The first combiner608aoutput is provided to a 1:12 first splitter610that feeds a 1:12 second splitter612. Outputs from the second splitter612are provided to sinks614in the system, such as system cabinets. With this arrangement, any desired number of sinks, e.g., DREX cabinets, can acquire the reference signal.

The second combiner608bfeeds a 1:12 third splitter616, which feeds a fourth splitter618supplying reference signals to sinks620, such as DREX cabinets in different parts of the radar.

In one embodiment, the first and second combiners608a,bmay be merged into a single hybrid coupler. It is understood that the relevant property of a hybrid coupler is that power flowing into one of input flows out of both outputs, but very little of the input power flows from the other input, so there is little backflow of power from one source into the other. This holds true even if both sources are providing power to the hybrid combiner, so a hybrid coupler is useable for embodiments where only one source sings at a time, and also for those where all sources sing in chorus.

It is understood that any practical type and number of passive splitter can be used to meet the needs of a particular application. It is further understood that the number of reference signals, e.g., CTS signals, received by the combiner can be adjusted to meet the needs of a particular application.

In one exemplary embodiment, a two-way combiner is provided as Mini-Circuits Part No. ZFSC-2-4, a twelve-way splitter is provided as Mini-Circuits Part No, MC-12-1, and a four-way splitter is provided as Mini-Circuits Part No. ZFSC-4-1.

While certain exemplary embodiments of the invention are shown and described in conjunction with radar systems, it is understood that exemplary embodiments of a signal distribution system are applicable to systems in general in which it is desirable to provide high reliability distribution of a signal. In other embodiments, passive optical components are used to provide a non-redundant signal distribution system providing very high availability. Exemplary embodiments of the invention are useful in any system in which a number of signal sinks must be kept aligned by means of a reference signal, with high availability.