Patent Publication Number: US-9411039-B2

Title: Phased-array receiver, radar system and vehicle

Description:
FIELD OF THE INVENTION 
     This invention relates to a phased-array receiver, a radar system and a vehicle. 
     BACKGROUND OF THE INVENTION 
     A phased array or smart antenna is an electronically steerable directional antenna used for example in radar or in wireless communication systems. A phased array receiver contains a group or matrix of antenna elements and associated receive channel circuits in which the relative phases of received signals are varied in such a way that the effective reception pattern of the array is directed in a desired direction and suppressed in undesired directions. Beamforming is used in sensor arrays for directional signal transmission or reception. This spatial selectivity is achieved by using adaptive or fixed receive/transmit beam patterns. 
     A signal may be a time varying physical quantity carrying information, e.g. a varying voltage level, for example occurring at an antenna element when receiving an electromagnetic wave. 
     A phased-array receiver employing beamforming controls the phase and relative amplitude of the incoming signal received by each antenna element and combines the output signals delivered by receive channels associated with each antenna element in such a way that a particular radiation pattern can preferentially be observed. A phased-array receiver uses the phase difference of an incoming signal received at different antenna elements of an antenna matrix to determine the angular position of a target. Phased-array receivers are for example used in radar systems, where the bandwidth is a fraction of the centre frequency, and a time delay introduced by the spatial distance between receiving antenna elements can be mapped to a phase-shift between received signals. The spatial difference translates to a phase difference of the output signals of the receive channels, which may for example be downconverted signals, i.e. may have a frequency below the frequency of the received signal. This principle can be used to steer the antenna beam by introducing a phase-shift at the receive channels. This phase-shift can be realized using digital beamforming (DBF) in the digital domain, i.e. after sampling, or using analog beamforming (ABF) in the analog domain, i.e. prior to sampling. 
     Conversion of the incoming signal into a different frequency range may be achieved by means of a mixing circuit arranged to mix the incoming or received signal with a signal generated by a local oscillator circuit. 
     As shown in  FIG. 1 , a first prior art receive channel  100  of a first phased-array receiver can contain an antenna element  110  which delivers a radio frequency (RF) signal to a phase-shifter circuit  112  arranged to phase-shift the signal in the RF domain and provide it to mixer circuit  114  coupled to a local oscillator (LO) circuit  116 . A generated output signal is delivered for analog or digital post-processing  118 . Post-processing may include summing output signals delivered by multiple receive channel. Alternatively, as shown in  FIG. 2 , a second prior art phased-array receiver  200  has multiple receive channels, each containing an antenna element  210 ,  220 ,  224 , which deliver radio frequency signals (RF 1 , RF 2 , RF 3 ) to corresponding phase-shifter circuits  212 ,  222 ,  226  arranged to phase-shift the signals and provide them to a summation circuit  228  for power combining in the RF domain before provision to a mixer circuit  214  coupled to a local oscillator circuit  216 . A generated output signal is then delivered for analog or digital post-processing  218 . 
     In Koh et. al. “An X- and Ku-Band 8-Element Phased-Array Receiver in 0.18-μm SiGe BiCMOS Technology”, IEEE Journal of Solid-State Circuits, Vol. 43, No. 6, June 2008, pp. 1360-1371, a phased-array receiver implemented using an all-RF architecture is shown, where the phase-shifting and power combining is carried out at the RF-level. Similarly, in Koh et. al. “A Q-Band Four-Element Phased-Array Front-End Receiver with Integrated Wilkinson Power Combiners in 0.18-μm SiGe BiCMOS Technology”, IEEE Transactions on Microwave Theory and Techniques, Vol. 56, No. 9, September 2008, pp. 2046-2053, an all-RF architecture is shown, where phase-shifting is applied in the received signal path. 
     In Yu et al. “A 22-24 GHz 4-Element CMOS Phased Array With On-Chip Coupling Characterization”IEEE Journal of Solid-State Circuits, Vol. 43, No. 9, September 2008, pp. 2134-2143, the incoming signal is split into an in-phase (I) and a phase-shifted quadrature (Q) component. Generation of I and Q signals and summing is done in the RF signal path. 
     As shown in  FIG. 3 , a third prior art receive channel  300  of a third phased-array receiver can contain an antenna element  310  which delivers a radio frequency (RF) signal to mixer circuit  314 . A generated output signal is delivered for analog or digital post-processing  318 . Local oscillator circuit  316  is coupled to mixer circuit  314  through a phase-shifter circuit  312  arranged to phase-shift the local oscillator (LO) signal, i.e. phase-shifting may alternatively be performed not in the received signal path, but in the LO path before applying the LO signal to a mixer device for mixing with the received RF signal. In Jeon et al., “A Scalable 6-to-18 GHz Concurrent Dual-Band Quad-Beam Phased-Array Receiver in CMOS”, IEEE Journal of Solid-State Circuits, Vol. 43, No. 12, December 2008, pp. 2660-2673, an integrated phased-array receiver is shown, wherein phase-shifting is performed in the local oscillator path. 
     Referring to  FIG. 4 , a prior art receive channel  400  is shown, wherein an antenna element  410  delivers a radio frequency (RF) signal to a power splitter circuit  412 . In-phase (I) and quadrature (Q) signals are generated by mixing with a local oscillator  416  signal using directional coupler  418  and I- and Q-mixers  414 ,  424 . I- and Q-signals are separately applied to dedicated analog-to-digital converters (ADC)  420 ,  422  for sampling the I- and Q-signals and subsequent digital processing. In PCT/US2006/046792, a linear FM radar system is presented, wherein two analog-to-digital conversion (ADC) circuits are used for sampling the output signals of an IQ-mixer. 
     As shown in  FIG. 5 , another prior art receive channel  500  of a phased-array receiver may contain a mixer circuit  514  for mixing received radio frequency (RF) signals received at antenna element  510  with local oscillator  516  signal in order to frequency-shift the received signal to a different frequency, for example an intermediate frequency (IF) below RF. The generated IF signal may then be split using an IQ generation module  518  into an in-phase signal  520  and a 90° phase-shifted quadrature signal  522  which may then be applied to a vector modulator or phase-shifter or phase rotator  512  and a weighting amplifier  524 . In other words, the incoming RF signal is first downconverted into an IF signal, and IQ-generation and phase-shifting is applied afterwards in the IF domain. 
     In PCT/GB95/01607, a circuit module for a phased-array radar is shown, wherein superheterodyne, single-sideband receive channel modules with dedicated ADCs for each I- and Q-output are used. In Erkens et al., “A Low-Cost, High Resolution, 360° Phase/Gain Shifter in SiGe BiCMOS”, IEEE 2009, polyphase filters are used for I- and Q-signal generation. 
     SUMMARY OF THE INVENTION 
     The present invention provides a phased-array receiver, a radar system and a vehicle as described in the accompanying claims. 
     Specific embodiments of the invention are set forth in the dependent claims. 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details, aspects and embodiments of the invention will be described, by way of example only, with reference to the drawings. In the drawings, like reference numbers are used to identify like or functionally similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. 
         FIG. 1  schematically shows a block diagram of a first prior art receive channel of a first phased-array receiver. 
         FIG. 2  schematically shows a block diagram of a second prior art phased-array receiver. 
         FIG. 3  schematically shows a block diagram of a third prior art receive channel of a third phased-array receiver 
         FIG. 4  schematically shows a block diagram of a fourth prior art receive channel of a fourth phased-array receiver. 
         FIG. 5  schematically shows a block diagram of a fifth prior art receive channel of a fifth phased-array receiver. 
         FIG. 6  schematically shows a block diagram of an example of a first embodiment of a receive channel of a phased-array receiver circuit. 
         FIG. 7  schematically shows a block diagram of an example of a second embodiment of a receive channel of a phased-array receiver circuit. 
         FIG. 8  schematically shows a diagram of a received signal level over an output signal level, for different degrees of phase-shift. 
         FIG. 9  schematically shows a diagram of an example of a transfer function of a phase rotator and of a receive channel of a phased-array receiver circuit. 
         FIG. 10  schematically shows a block diagram of an example of an embodiment of a vehicle comprising a radar system with a phased-array receiver circuit. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Because the illustrated embodiments of the present invention may for the most part, be implemented using electronic components and circuits known to those skilled in the art, details will not be explained in any greater extent than that considered necessary as illustrated, for the understanding and appreciation of the underlying concepts of the present invention and in order not to obfuscate or distract from the teachings of the present invention. 
     Referring to  FIG. 6 , a block diagram of an example of a first embodiment of a receive channel of a phased-array receiver circuit is schematically shown. A phased-array receiver  600  comprises a plurality of analog beamforming receive channels  602 , each comprising an antenna element  610  arranged to receive a radio frequency (RF) signal, and comprising a channel output  628  arranged to provide an analog channel output signal (IF,n). At least one of the analog beamforming receive channels comprises an in-phase downconversion mixing circuit  614  connected to the antenna element  610  and a local oscillator source  616  and arranged to provide a downconverted in-phase signal to a phase rotation circuit  612 ; and a quadrature downconversion mixing circuit  620  connected to the antenna element  610  and the local oscillator (LO) source  616  and arranged to provide a downconverted quadrature signal to the phase rotation circuit  612 . The phase rotation circuit  612  is arranged to provide to the channel output  628  a phase-shifted analog output signal generated from the downconverted in-phase signal and the downconverted quadrature signal. 
     The shown receive channel may be an analog beamforming receive channel; no sampling of signals may be performed within the signal path between antenna element  610  and channel output  628 . The phase rotation circuit or vector modulation circuit  612  may be arranged to introduce a defined phase-shift desired for beamforming of the reception pattern of the phased-array, to an output signal generated from combining in-phase- and quadrature signals. The phase rotator implementation may determine the precision of the system. Instead of applying phase-shifting in the LO path, the shown analog beamforming receive channel may apply phase-shifting in the IF signal path, i.e. after downconversion and I- and Q-signal generation, which may reduce channel gain dependence on the phase-setting, reduce required die area, power consumption and PVT variation, i.e. variation in power, voltage and temperature of the integrated circuit, and provide a large IF bandwidth, compared to prior art solutions. 
     The shown receive channel  602  may be considered an analog receive channel, even if control signal values, for example applied to the phase rotation circuit  612  for applying a desired phase-shift, may be provided sampled and quantized, which may be considered a digital representation. 
     The LO source  616  shown in  FIG. 6  providing the LO signal may for example be the transmitting counterpart of a phased-array system. Or it may be implemented at the receiver as a single circuit generating one LO signal. In another embodiment, the LO signal applied to each mixer may be generated using separate LO circuits, for example generating LO signals of the same frequency. In yet another embodiment, the LO signal may be generated using an IQ oscillator, arranged to directly generate I- and Q-LO signals. 
     In-phase mixing circuit  614  and quadrature mixing circuit  620  may be downconversion mixing circuits, i.e. by mixing received RF signal with in-phase and 90° phase-shifted LO signal, I- and Q-mixer output signals may be converted into signals having a frequency below those of the received RF signal, which may be referred to as intermediate frequency (IF). During post-processing, this frequency may or may not be further reduced. It should be noted that the term “frequency” may not only refer to a single distinct frequency value, but to a spectrum of frequencies the particular signal may comprise. 
     At least one of the analog beamforming receive channels  602  may comprise a weighting amplifier circuit  624  or variable gain amplifier circuit connected to amplify the phase-shifted analog output signal before provision to the channel output  628 . Weighting factors may be applied to the weighting amplifier circuit  624  through a control input (not shown), for example by a controller circuit. The weighting amplifier circuit  624  may be an amplitude weighting amplifier or variable gain amplifier (VGA) allowing for example suppression of signals from outer channels and putting larger weight on signals received at inner channels. 
     The phased-array receiver  600  may comprise a first summation circuit connected to the channel outputs  628 ,  630 ,  632 ,  634 ,  636  of the plurality of analog beamforming receive channels and arranged to provide an analog summation output signal  638 . Receive channel output signals IF,  1 , IF,  2 , IF, m- 1  and IF, m, where m represents the channel number, may be provided at channel outputs  630 ,  632 ,  634 ,  636 . In  FIG. 6 , only one  602  of the plurality of analog beamforming receive channels of the phased-array circuit  600  is shown in detail. Other receive channels are indicated by their channel outputs  630 ,  632 ,  634 ,  636 . The shown analog beamforming may allow summation of all receive channel output signals prior to processing, which may greatly reduce digital requirements. Only the summation output signal  638  (IF, OUT) may be converted into a digital representation, whereas phase rotation and weighting may be performed in the analog domain. 
     The phased-array receiver  600  may comprise one analog-digital conversion circuit  640  (ADC), the analog-digital conversion circuit  640  connected to receive the analog summation output signal and provide a digital summation output signal. Only one single ADC  640  may be used for a plurality or all of the receive channels, for example saving die area and reducing power consumption, memory, and digital signal processing requirements, compared to digital beamforming solutions. The shown embodiment may for example be suitable when a large number of receive channels is used. The number of ADCs may remain unchanged, one, since all phase-shifted and weighted signals may be summed in the analog domain. The shown embodiment may allow for low-cost ABF phased-array systems consisting of many receive channels (for example  4 ,  8 ,  16 ,  32  etc., each with high signal-to-noise ratio (SNR)), which may allow high angular resolution analog beamforming (ABF) systems, but require large die areas and become very expensive when using digital beamforming. 
     A large number of receive channels, for example  4 ,  8 ,  16 , or  32 , may be required to build a high-performance automotive radar system with precise angular resolution. Digital beamforming (DBF) solutions having one or two ADCs per receive channel are usually too expensive, power-hungry, and, from an integration standpoint, consume large die area. ABF solves this problem by steering the receive antenna beam with adjustable phase-shifts in each channel and summing the outputs of all channels to one output signal that is then fed to one ADC. This approach may for example allow building very cheap 77 GHz radar sensors. 
     Each receive channel may for example be attenuated or amplified using a weighting amplifier to yield a desired beam pattern. 
     At least one of the analog beamforming receive channel  602  of the phased-array receiver circuit  600  may comprise a power splitter circuit  642  connected between the antenna element  610  and the in-phase downconversion mixing circuit  614  and the quadrature downconversion mixing circuit  620 . The power splitter circuit  642 , or power divider circuit, may be arranged to split the received RF signal into a first and similar or identical second RF signal for provision to the in-phase and quadrature downconversion mixing circuits  614 ,  620 . 
     At least one of the plurality of analog beamforming receive channels  602  may comprise one or more high-pass filter circuits  622 ,  626  arranged to high-pass filter the downconverted in-phase (I) signal and the downconverted quadrature (Q) signal before provision to the phase rotation circuit  612 , which may help decoupling the phase rotator circuit  612  from any DC level introduced for example by the downconversion mixer circuits  614 ,  620 . High-pass filtering the downconverted I- and Q-signals may protect the phase rotation circuit from overloading, i.e. from being driven into saturation, which may allow for very precise evaluation of the received signals. In another embodiment, high-pass filtering may be applied to only the in-phase or the quadrature signal. 
     The in-phase downconversion mixing circuit  614  and the quadrature downconversion mixing circuit  620  may be homodyne downconversion mixing circuits. Homodyne mixing circuits receive a LO-signal having a frequency corresponding to the frequency of the received (RF) signal, allowing for direct conversion of the received signal into an output signal, whereas for example a heterodyne mixing circuit may receive a LO signal having an intermediate frequency different from the RF signal frequency, requiring an additional conversion step before provision of an output signal having a target output frequency. And unlike all-pass filters or polyphase filters used in the IF domain, the shown in-phase downconversion mixing circuit  614  and the quadrature downconversion mixing circuit  620  may be arranged to deliver downconverted broadband signals, for example covering a frequency range of several decades. As an example, the radio frequency signal may be a radar signal, e.g. 77 GHz radar signals, for example used in automotive radar systems. The shown analog beamforming receive channel  602  may allow for provision of in-phase and quadrature signals covering several decades, for example 10 kHz up to 20 MHz, enabling broadband phase-shifting in the receive channel  602 . The shown receive channel may allow IQ signal generation in the RF domain using homodyne mixing circuits  614 ,  620  which generate two signals 90° out of phase over a broad IF range, for example 10 kHz-20 MHz, suitable for automotive radar, and feed these signals to the phase rotation circuit  612  in the IF domain. The IF range of automotive radar beat signals may span over more than three decades, for example from 10 kHz up to 20 MHz. 
     Referring now to  FIG. 7 , a diagram of an example of a second embodiment of a receive channel  700  of a phased-array receiver circuit is schematically shown. An incoming RF signal may be amplified by amplifier circuit  710  and, after power splitting, provided to in-phase downconversion mixing circuit  714  and quadrature downconversion mixing circuit  720 . I- and Q-signals may be provided to a phase rotation circuit  712  through high-pass filters  722 ,  724 , which may for example be implemented as capacitive devices allowing for AC coupling. The phase rotation circuit  712  may comprise a plurality of controllable variable gain amplification devices  726 ,  728 , and a second summation circuit  730 . 
     The plurality of controllable variable gain amplification devices  726 ,  728  may be arranged to separately amplify the downconverted in-phase signal and the downconverted quadrature signal depending on parameter values provided by a controller device (not shown). The downconverted 90° out-of-phase I- and Q-signals may be amplified with different amplification factors and subsequently summed by summation circuit  730  in order to introduce an adjustable phase-shift to the output signal  732  of the shown receive channel  700 . 
     Variable gain control may be applied to variable gain amplification devices  726 ,  728  by applying discrete parameter values for introducing a precisely selectable degree of phase-shift. In another embodiment, gain control may be realized using continuously changing parameter values. 
     The same variable-gain amplifier devices  726 ,  728  may be used for high-precision amplitude weighting of the output signal delivered by the channel. 
     The shown phase rotator circuit  712  may for example be provided as an integrated circuit or as part of an integrated circuit comprising the phased-array. 
     The at least one of the analog beamforming receive channels  602  may comprise a directional coupler circuit  718  connected to an output of the local oscillator source  716 , and arranged to provide an in-phase local oscillator signal to the in-phase downconversion mixing circuit  714  and a quadrature local oscillator signal to the quadrature downconversion mixing circuit  720 . A directional coupler may be a passive device, easy to integrate and may allow applying radio frequency LO signals. 
     The directional coupler may for example comprise a branchline coupler as a passive directional coupler, for example suitable when implemented using microstrip technology. The LO signal may be generated in the RF domain which may enable in-phase and quadrature signals over at least 10% of the LO center frequency in case of a passive branchline coupler. 
     In another embodiment of the phased-array receiver, the directional coupler  718  may comprise a frequency divider circuit. A local oscillator signal generated by the local oscillator source  716  may be provided to the frequency divider circuit using a frequency doubler circuit. 
     Referring now to  FIG. 8 , a diagram  800  of an example of a received signal level  810  over an output signal level  812  (in Volts), for different degrees of phase-shift, is shown. In the shown example, phase-shift is applied in 15° steps (0° to 360°) to a phase rotator of an analog beamforming receive channel receiving an input signal at an IF of 1 MHz. Valid points in the diagram are indicated with an “x”. Referring also to  FIG. 9 , a diagram of an example of a transfer function of a phase rotator  910  and of a receive channel of a phased-array receiver circuit  920  is shown. The transfer function of the phase rotator is given by a set of diagram lines spaced 15° from each other, with the absolute phase-shift α (deg) plotted over the frequency IF (Hz) of the downconverted signal. It can be seen that, for each phase-shift shown, the phase difference does not depend on the IF. Diagram  912  shows that the gain G (dB) of a receive channel may only very little depend on the chosen phase-shift setting. 
     Referring now to  FIG. 10 , a block diagram of an example of an embodiment of a vehicle comprising a radar system with a phased-array receiver circuit is schematically shown. 
     A radar system  1010  may comprise a phased-array receiver  1020  as described above. It may further comprise a sender for radiating radar signals. It should be noted that radar frequency range may be considered a subset of the radio frequency range. The radar system  1010  may be any radar system, for example an automotive radar system. The illustrated radar system may be a low-cost, high-performance automotive radar system with smart phase-shifting functionality implementation by combining broadband I- and Q-signal generation in the RF domain and the usage of the described analog phase rotator in the IF domain. In an embodiment, it may further comprise amplitude weighting in the IF domain. 
     A vehicle  1000  may comprise a radar system  1010  or a phased-array receiver  1020  described above. A vehicle may for example be a car, a ship, a plane, a helicopter, or a satellite. In a car, an automotive radar system may for example be used for tracking safety distance between cars or as a parking assistance system. 
     In the foregoing specification, the invention has been described with reference to specific examples of embodiments of the invention. It will, however, be evident that various modifications and changes may be made therein without departing from the broader spirit and scope of the invention as set forth in the appended claims. 
     The connections as discussed herein may be any type of connection suitable to transfer signals from or to the respective nodes, units or devices, for example via intermediate devices. Accordingly, unless implied or stated otherwise, the connections may for example be direct connections or indirect connections. The connections may be illustrated or described in reference to being a single connection, a plurality of connections, unidirectional connections, or bidirectional connections. However, different embodiments may vary the implementation of the connections. For example, separate unidirectional connections may be used rather than bidirectional connections and vice versa. Also, plurality of connections may be replaced with a single connection that transfers multiple signals serially or in a time multiplexed manner. Likewise, single connections carrying multiple signals may be separated out into various different connections carrying subsets of these signals. Therefore, many options exist for transferring signals. 
     Those skilled in the art will recognize that the boundaries between logic blocks are merely illustrative and that alternative embodiments may merge logic blocks or circuit elements or impose an alternate decomposition of functionality upon various logic blocks or circuit elements. Thus, it is to be understood that the architectures depicted herein are merely exemplary, and that in fact many other architectures can be implemented which achieve the same functionality. For example, in-phase mixing circuit  614  and quadrature mixing circuit  620  may be implemented as a combined IQ mixer. 
     Any arrangement of components to achieve the same functionality is effectively “associated” such that the desired functionality is achieved. Hence, any two components herein combined to achieve a particular functionality can be seen as “associated with” each other such that the desired functionality is achieved, irrespective of architectures or intermedial components. Likewise, any two components so associated can also be viewed as being “operably connected,” or “operably coupled,” to each other to achieve the desired functionality. 
     Furthermore, those skilled in the art will recognize that boundaries between the above described operations merely illustrative. The multiple operations may be combined into a single operation, a single operation may be distributed in additional operations and operations may be executed at least partially overlapping in time. Moreover, alternative embodiments may include multiple instances of a particular operation, and the order of operations may be altered in various other embodiments. 
     Also for example, in one embodiment, the illustrated examples may be implemented as circuitry located on a single integrated circuit or within a same device. For example, mixing circuits  614 ,  620  and phase rotation circuit  612  may be implemented as circuitry implemented on a single integrated circuit. Alternatively, the examples may be implemented as any number of separate integrated circuits or separate devices interconnected with each other in a suitable manner. For example, each analog beamforming receive channel  602  of a phased-array receiver may be implemented on a separate integrated circuit. 
     Also for example, the examples, or portions thereof, may be implemented as soft or code representations of physical circuitry or of logical representations convertible into physical circuitry, such as in a hardware description language of any appropriate type. 
     Also, the invention is not limited to physical devices or units implemented in non-programmable hardware but can also be applied in programmable devices or units able to perform the desired device functions by operating in accordance with suitable program code, such as mainframes, minicomputers, servers, workstations, personal computers, notepads, personal digital assistants, electronic games, automotive and other embedded systems, cell phones and various other wireless devices, commonly denoted in this application as ‘computer systems’. 
     However, other modifications, variations and alternatives are also possible. The specifications and drawings are, accordingly, to be regarded in an illustrative rather than in a restrictive sense. 
     In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. The word ‘comprising’ does not exclude the presence of other elements or steps then those listed in a claim. Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements. The mere fact that certain measures are recited in mutually different claims does not indicate that a combination of these measures cannot be used to advantage. 
     While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention.