阅读说明：本技术 一种高精度双多基地雷达光传输同步装置及方法 (High-precision bistatic radar optical transmission synchronization device and method ) 是由 黄海风 赖涛 唐燕群 王青松 王小青 魏玺章 林柏洪 于 2020-12-22 设计创作，主要内容包括：本发明公开一种高精度双多基地雷达光传输同步装置及方法,该装置包括宽带雷达回波信号激光传输子装置、射频参考信号激光传输子装置、第一波分复用器、第二波分复用器和电控高精度光纤延迟线,所述宽带雷达回波信号激光传输子装置分别与第一波分复用器和第二波分复用器连接,所述射频参考信号激光传输子装置分别与第一波分复用器、电控高精度光纤延迟线和第二波分复用器连接,所述电控高精度光线延迟线分别与第一波分复用器和第二波分复用器连接。该方法为应用上述同步装置的工作方法。本发明具有抗散射噪声及色散抖动的优点。本发明作为一种高精度双多基地雷达光传输同步装置及方法,可广泛应用于固体激光放大技术领域。(The invention discloses a high-precision double multi-base radar optical transmission synchronization device and a method, wherein the device comprises a broadband radar echo signal laser transmission sub-device, a radio frequency reference signal laser transmission sub-device, a first wavelength division multiplexer, a second wavelength division multiplexer and an electric control high-precision optical fiber delay line, the broadband radar echo signal laser transmission sub-device is respectively connected with the first wavelength division multiplexer and the second wavelength division multiplexer, the radio frequency reference signal laser transmission sub-device is respectively connected with the first wavelength division multiplexer, the electric control high-precision optical fiber delay line and the second wavelength division multiplexer, and the electric control high-precision optical fiber delay line is respectively connected with the first wavelength division multiplexer and the second wavelength division multiplexer. The method is a working method applying the synchronization device. The invention has the advantages of anti-scattering noise and dispersion jitter. The high-precision bistatic radar optical transmission synchronization device and method can be widely applied to the technical field of solid laser amplification.)

1. The utility model provides a two multistation radar optical transmission synchronizer of high accuracy which characterized in that, includes broadband radar echo signal laser transmission sub-assembly, radio frequency reference signal laser transmission sub-assembly, first wavelength division multiplexer, second wavelength division multiplexer and automatically controlled high accuracy optic fibre delay line, broadband radar echo signal laser transmission sub-assembly is connected with first wavelength division multiplexer and second wavelength division multiplexer respectively, radio frequency reference signal laser transmission sub-assembly is connected with first wavelength division multiplexer, automatically controlled high accuracy optic fibre delay line and second wavelength division multiplexer respectively, automatically controlled high accuracy light delay line is connected with first wavelength division multiplexer and second wavelength division multiplexer respectively.

2. A high-precision bistatic radar optical transmission synchronization device according to claim 1, wherein the broadband radar return signal laser transmission sub-device comprises a first laser, a first intensity modulator, a beam splitter, a first bias control board, a first optical converter and a first bias voltage module, the first laser, the first intensity modulator, the beam splitter and the first wavelength division multiplexer are connected in sequence, the beam splitter is further connected to a first bias board, the first bias board is connected to the first intensity modulator, and the first bias voltage module, the first optical-to-electrical converter and the second wavelength division multiplexer are connected in sequence.

3. A high accuracy bistatic radar optical transmission synchronization apparatus as claimed in claim 2, wherein said beam splitter is a 1:99 optical beam splitter.

4. A high-precision bistatic radar optical transmission synchronization device according to claim 3, wherein the RF reference signal laser transmission sub-device comprises a second laser, a second intensity modulator, a second bias control board, a circulator, a second photoelectric converter, a second bias voltage module, an RF reference source, a phase detection circuit module, a PID circuit module, a delay line driver and a Faraday mirror, the second laser, the second intensity modulator, the circulator and the first wavelength division multiplexer are sequentially connected, the second bias control board is connected with the second intensity modulator, the second intensity modulator is further connected with the RF reference source, the phase detection circuit module, the PID circuit module, the delay line driver and the electrically controlled high-precision optical fiber delay line are sequentially connected, the photoelectric converter is further connected with the second bias voltage module and the phase detection circuit module respectively, the Faraday mirror is connected with the second wavelength division multiplexer.

5. A high-precision bistatic radar optical transmission synchronization method is characterized by comprising the following steps:

the first laser is used for generating an optical carrier with excellent wavelength stability and monochromaticity and generating an optical carrier with stable central wavelength, stable power, narrow line width and high stability;

the first intensity modulator is used for electro-optical modulation of the radar echo signal;

a beam splitter for extracting the control feedback light and the output signal light;

a first bias control board for controlling the operating point of the first intensity modulator;

the first photoelectric converter is used for performing photoelectric conversion on the optical signal received by the receiving end and outputting a recovered radar echo signal;

the second laser is used for generating a narrow-linewidth high-stability optical carrier with stable central wavelength and stable power;

a second intensity modulator for electro-optical modulation of the radio frequency reference signal;

a circulator for routing the optical signal;

a second bias control board for controlling the operating point of the second intensity modulator;

the second photoelectric converter is used for performing photoelectric conversion on the optical signals transmitted back and forth and outputting recovered radio frequency reference signals;

a radio frequency reference source for generating a radio frequency reference signal;

the phase discrimination circuit module is used for carrying out phase comparison on the radio frequency reference signal recovered by the second photoelectric converter and a radio frequency reference signal generated by a radio frequency reference source and outputting a phase discrimination voltage;

the PID circuit module is used for carrying out analog PID control on the output signal of the phase discriminator and providing the output control signal to the delay line driving module;

the delay line driving module is used for generating a driving signal of the electric control high-precision optical fiber delay line according to the control signal provided by the PID circuit module;

the electronic control high-precision optical fiber delay line is used for realizing the phase-stable transmission of the radio frequency reference signal after carrying out feedback compensation according to the change of the transmission path;

the Faraday mirror is used for reflecting the radio frequency reference signal subjected to electro-optical modulation from the receiving end of the device to the transmitting end and rotating the polarization of the optical signal by 90 degrees;

the first wavelength division multiplexer is used for combining the radio frequency reference signal subjected to electro-optical modulation at the sending end with the radar echo signal subjected to electro-optical modulation, transmitting the combined signal to the receiving end through the same optical path, and simultaneously performing wavelength division demultiplexing on the radio frequency reference signal subjected to electro-optical modulation reflected to the sending end by the receiving end;

and the second wavelength division multiplexer is used for carrying out wave decomposition multiplexing on the electro-optically modulated radio frequency reference signal received by the receiving end and the electro-optically modulated radar echo signal, transmitting the electro-optically modulated radar echo signal to the first photoelectric converter for photoelectric conversion, transmitting the electro-optically modulated radio frequency reference signal to the Faraday mirror and reflecting the electro-optically modulated radio frequency reference signal back to the sending end.

6. The method for synchronizing the optical transmission of a high-precision bistatic radar according to claim 5, further comprising:

providing a bias voltage for the first photoelectric converter based on the first bias voltage module;

the second photovoltaic converter is provided with a bias voltage based on a second bias voltage module.

7. The method as claimed in claim 6, wherein the beam splitter is a 1:99 optical beam splitter, and the step of extracting the control feedback light and the output signal light includes:

1% of control feedback light and 99% of signal light are extracted based on the optical beam splitter;

inputting 1% of the control feedback light to the first bias control board;

99% of the signal light is input to the first wavelength division multiplexer.

Technical Field

The invention belongs to the technical field of double multi-base radars, and particularly relates to a high-precision double multi-base radar light transmission synchronization device and method.

Background

The basic principle of the existing home and abroad optical fiber round-trip radio frequency phase-stable transmission technology is that a reference signal is transmitted in two directions in an optical fiber to obtain phase information of a transmission signal, and then phase drift introduced by channel jitter is compensated by a closed-loop compensation method. However, at present, the influence of optical carrier crosstalk, reference signal crosstalk, frequency dispersion, polarization mode dispersion and rayleigh scattering in optical transmission on the fidelity and transmission phase of radar echo signal transmission is not solved, so that the problem of optical transmission synchronization of double multi-base radar weak echo signals with high signal-to-noise ratio cannot be solved.

Disclosure of Invention

In order to solve the above technical problems, an object of the present invention is to provide a high-precision bistatic radar optical transmission synchronization apparatus and method, which have the advantages of anti-scattering noise and anti-dispersion jitter.

The first technical scheme adopted by the invention is as follows: the utility model provides a two multistation radar optical transmission synchronizer of high accuracy, includes broadband radar echo signal laser transmission sub-assembly, radio frequency reference signal laser transmission sub-assembly, first wavelength division multiplexer, second wavelength division multiplexer and automatically controlled high accuracy optic fibre delay line, broadband radar echo signal laser transmission sub-assembly is connected with first wavelength division multiplexer and second wavelength division multiplexer respectively, radio frequency reference signal laser transmission sub-assembly is connected with first wavelength division multiplexer, automatically controlled high accuracy optic fibre delay line and second wavelength division multiplexer respectively, automatically controlled high accuracy light delay line is connected with first wavelength division multiplexer and second wavelength division multiplexer respectively.

Further, broadband radar echo signal laser transmission sub-device includes first laser instrument, first intensity modulator, beam splitter, first bias control panel, first light converter and first bias voltage module, first laser instrument, first intensity modulator, beam splitter and first wavelength division multiplexer connect gradually, the beam splitter still is connected with first bias board, first bias board is connected with first intensity modulator, first bias voltage module, first photoelectric converter and second wavelength division multiplexer connect gradually.

Further, the beam splitter is a 1:99 beam splitter.

Further, the radio frequency reference signal laser transmission sub-device comprises a second laser, a second intensity modulator, a second bias control plate, a circulator, a second photoelectric converter, a second bias voltage module, a radio frequency reference source, a phase discrimination circuit module, a PID circuit module, a delay line driver and a faraday mirror, wherein the second laser, the second intensity modulator, the circulator and the first wavelength division multiplexer are sequentially connected, the second bias control plate is connected with the second intensity modulator, the second intensity modulator is further connected with the radio frequency reference source, the phase discrimination circuit module, the PID circuit module, the delay line driver and an electric control high-precision optical fiber delay line are sequentially connected, the photoelectric converter is further respectively connected with the second bias voltage module and the phase discrimination circuit module, and the faraday mirror is connected with the second wavelength division multiplexer.

The second technical scheme adopted by the invention is as follows: a high-precision bistatic radar optical transmission synchronization method comprises the following steps:

the first laser is used for generating an optical carrier with excellent wavelength stability and monochromaticity and generating an optical carrier with stable central wavelength, stable power, narrow line width and high stability;

the first intensity modulator is used for electro-optical modulation of the radar echo signal;

a beam splitter for extracting the control feedback light and the output signal light;

a first bias control board for controlling the operating point of the first intensity modulator;

the first photoelectric converter is used for performing photoelectric conversion on the optical signal received by the receiving end and outputting a recovered radar echo signal;

the second laser is used for generating a narrow-linewidth high-stability optical carrier with stable central wavelength and stable power;

a second intensity modulator for electro-optical modulation of the radio frequency reference signal;

a circulator for routing the optical signal;

a second bias control board for controlling the operating point of the second intensity modulator;

the second photoelectric converter is used for performing photoelectric conversion on the optical signals transmitted back and forth and outputting recovered radio frequency reference signals;

a radio frequency reference source for generating a radio frequency reference signal;

the phase discrimination circuit module is used for carrying out phase comparison on the radio frequency reference signal recovered by the second photoelectric converter and a radio frequency reference signal generated by a radio frequency reference source and outputting a phase discrimination voltage;

the PID circuit module is used for carrying out analog PID control on the output signal of the phase discriminator and providing the output control signal to the delay line driving module;

the delay line driving module is used for generating a driving signal of the electric control high-precision optical fiber delay line according to the control signal provided by the PID circuit module;

the electronic control high-precision optical fiber delay line is used for realizing the phase-stable transmission of the radio frequency reference signal after carrying out feedback compensation according to the change of the transmission path;

the Faraday mirror is used for reflecting the radio frequency reference signal subjected to electro-optical modulation from the receiving end of the device to the transmitting end and rotating the polarization of the optical signal by 90 degrees;

the first wavelength division multiplexer is used for combining the radio frequency reference signal subjected to electro-optical modulation at the sending end with the radar echo signal subjected to electro-optical modulation, transmitting the combined signal to the receiving end through the same optical path, and simultaneously performing wavelength division demultiplexing on the radio frequency reference signal subjected to electro-optical modulation reflected to the sending end by the receiving end;

and the second wavelength division multiplexer is used for carrying out wave decomposition multiplexing on the electro-optically modulated radio frequency reference signal received by the receiving end and the electro-optically modulated radar echo signal, transmitting the electro-optically modulated radar echo signal to the first photoelectric converter for photoelectric conversion, transmitting the electro-optically modulated radio frequency reference signal to the Faraday mirror and reflecting the electro-optically modulated radio frequency reference signal back to the sending end.

Further, still include:

providing a bias voltage for the first photoelectric converter based on the first bias voltage module;

the second photovoltaic converter is provided with a bias voltage based on a second bias voltage module.

Further, the beam splitter is specifically a 1:99 beam splitter, and the step of extracting the control feedback light and the output signal light specifically includes:

1% of control feedback light and 99% of signal light are extracted based on the optical beam splitter;

inputting 1% of the control feedback light to the first bias control board;

99% of the signal light is input to the first wavelength division multiplexer.

The method and the system have the beneficial effects that: the optical carrier for transmitting the radar echo signal and the optical carrier for the radio frequency reference signal for phase feedback of the transmission path are respectively positioned in two different channels of dense wavelength division multiplexing, so that the interference of Rayleigh scattering noise generated by the back-and-forth transmission of the radio frequency reference signal on the radar echo signal is avoided, in addition, the optical carrier for transmitting the radar echo signal and the optical carrier for transmitting the radio frequency reference signal are both narrow linewidth lasers, so that the adverse effect of frequency dispersion on the transmission phase of the radio frequency reference signal and the transmission phase of the radar echo signal can be effectively prevented, meanwhile, the polarization of the forward transmission optical signal and the reverse transmission optical signal is just orthogonal by the reflection of a Faraday mirror of the radio frequency reference signal, the polarization dispersion of the back-and-forth optical signal is inhibited, the influence of the polarization dispersion on the phase stabilization feedback compensation precision is avoided, and the crosstalk of the radio frequency reference signal on the radar, the signal-to-noise ratio of the double multi-base radar optical transmission synchronization is higher, and the phase-stable transmission performance is more excellent.

Drawings

Fig. 1 is a schematic structural diagram of a high-precision bistatic radar optical transmission synchronization apparatus according to an embodiment of the present invention.

Detailed Description

The invention is described in further detail below with reference to the figures and the specific embodiments. The step numbers in the following embodiments are provided only for convenience of illustration, the order between the steps is not limited at all, and the execution order of each step in the embodiments can be adapted according to the understanding of those skilled in the art.

As shown in fig. 1, the present invention provides a high-precision bistatic radar optical transmission synchronization apparatus, including a broadband radar echo signal laser transmission sub-apparatus, a radio frequency reference signal laser transmission sub-apparatus, a first wavelength division multiplexer, a second wavelength division multiplexer, and an electronic control high-precision optical fiber delay line, where the broadband radar echo signal laser transmission sub-apparatus is connected to the first wavelength division multiplexer and the second wavelength division multiplexer respectively, the radio frequency reference signal laser transmission sub-apparatus is connected to the first wavelength division multiplexer, the electronic control high-precision optical fiber delay line, and the electronic control high-precision optical fiber delay line is connected to the first wavelength division multiplexer and the second wavelength division multiplexer respectively.

Further as a preferred embodiment of the present invention, the broadband radar echo signal laser transmission sub-apparatus includes a first laser, a first intensity modulator, a beam splitter, a first bias control board, a first optical converter, and a first bias voltage module, where the first laser, the first intensity modulator, the beam splitter, and the first wavelength division multiplexer are sequentially connected, the beam splitter is further connected to a first bias board, the first bias board is connected to the first intensity modulator, and the first bias voltage module, the first optical-to-electrical converter, and the second wavelength division multiplexer are sequentially connected.

Further as a preferred embodiment of the present invention, the beam splitter is a 1:99 beam splitter.

Further as a preferred embodiment of the present invention, the rf reference signal laser transmission sub-apparatus includes a second laser, a second intensity modulator, a second bias control board, a circulator, a second photoelectric converter, a second bias voltage module, a rf reference source, a phase detection circuit module, a PID circuit module, a delay line driver, and a faraday mirror, the second laser, the second intensity modulator, the circulator and the first wavelength division multiplexer are connected in sequence, the second bias control board is connected with a second intensity modulator, the second intensity modulator is also connected with a radio frequency reference source, the radio frequency reference source, the phase discrimination circuit module, the PID circuit module, the delay line driver and the electric control high-precision optical fiber delay line are connected in sequence, the photoelectric converter is also respectively connected with a second bias voltage module and a phase discrimination circuit module, and the Faraday mirror is connected with a second wavelength division multiplexer.

Specifically, in the present application, an optical carrier for transmitting a radar echo signal and an optical carrier for transmitting a radio frequency reference signal for phase feedback of a transmission path are respectively located in two different channels of dense wavelength division multiplexing, so that interference of rayleigh scattering noise generated by round-trip transmission of the radio frequency reference signal on the radar echo signal is avoided; the optical carrier used for transmitting the radar echo signal is narrow-linewidth laser, so that the adverse effect of frequency dispersion on the transmission phase of the radar echo signal can be effectively prevented; the optical carrier used for transmitting the radio frequency reference signal is narrow linewidth laser, so that the adverse effect of frequency dispersion on the transmission phase of the radio frequency reference signal can be effectively prevented, and meanwhile, the radio frequency reference signal is reflected by the Faraday mirror to ensure that the polarization of the forward transmission optical signal is just orthogonal to that of the reverse transmission optical signal, so that the polarization dispersion of the back-and-forth optical signal is inhibited, and the influence of the polarization dispersion on the phase stabilization feedback compensation precision is avoided. The radio frequency reference signal used for path phase feedback and the radar echo signal are not crossed in frequency, so that crosstalk of the radio frequency reference signal to the radar echo signal can be effectively prevented. Based on the method and the device for the synchronous light transmission of the double multi-base radar, which are resistant to scattering noise and dispersion jitter, the signal-to-noise ratio of the synchronous light transmission of the double multi-base radar is higher, and the phase-stable transmission performance is more excellent.

In addition, the synchronization device is applied to the laser phase-stabilizing transmission synchronization of radar echo signals among bases in a multi-base radar system:

laser transmission can be understood as: based on the processes of laser carrier electro-optical modulation, transmission and photoelectric conversion.

Phase-stationary transmission can be understood as: and (3) performing stable-phase transmission of the radio frequency signal based on real-time feedback compensation of the optical transmission path.

At a sending end, two paths of signals of broadband radar echo signal laser transmission and radio frequency reference signal laser transmission are combined through a first wavelength division multiplexer, are transmitted to a receiving end through the same light path (optical fiber or free space) and then are separated through a second wavelength division multiplexer, and main light paths of the two transmissions are the same light path, so that the broadband radar echo signal can also be transmitted stably as long as the stable phase transmission of the radio frequency reference signal light path is ensured.

The broadband radar echo signal laser transmission sub-device is used for transmitting broadband radar echo signals.

The radio frequency reference signal laser transmission sub-device is used for real-time feedback compensation of an optical transmission path, so that stable phase transmission of broadband radar echo signals in the optical transmission path is guaranteed.

The first wavelength division multiplexer and the second wavelength division multiplexer are used for wavelength division multiplexing and wavelength division demultiplexing of optical carriers of the broadband radar echo signal laser transmission sub-device and the radio frequency reference signal laser transmission sub-device, specifically, the first wavelength division multiplexer is used for combining a radio frequency reference signal subjected to electro-optical modulation at a sending end and a radar echo signal subjected to electro-optical modulation, transmitting the combined signal to a receiving end through the same optical path (optical fiber or free space), and simultaneously performing wavelength division demultiplexing on the radio frequency reference signal subjected to electro-optical modulation reflected by the receiving end back to the sending end, so that the radio frequency reference signal subjected to electro-optical modulation reflected back to the sending end is transmitted to the second photoelectric converter for photoelectric conversion. And the second wavelength division multiplexer is used for carrying out wavelength division demultiplexing on the radio frequency reference signal after jitter electro-optical modulation and the radar echo signal after electro-optical modulation received by the receiving end, so that the radar echo signal after electro-optical modulation is transmitted to the first photoelectric converter for photoelectric conversion, and meanwhile, the radio frequency reference signal after electro-optical modulation is transmitted to the Faraday mirror and reflected back to the sending end.

A high-precision bistatic radar optical transmission synchronization method comprises the following steps:

the first laser is used for generating an optical carrier with excellent wavelength stability and monochromaticity and generating an optical carrier with stable central wavelength, stable power, narrow line width and high stability;

the first intensity modulator is used for electro-optical modulation of the radar echo signal;

a beam splitter for extracting the control feedback light and the output signal light;

the first bias control board is used for controlling the working point of the first intensity modulator to enable the first intensity modulator to work at a Q point all the time, so that the electro-optic modulation of a low-distortion radar echo signal can be realized;

the first photoelectric converter is used for performing photoelectric conversion on the optical signal received by the receiving end and outputting a recovered radar echo signal;

the second laser is used for generating a narrow-linewidth high-stability optical carrier with stable central wavelength and stable power;

a second intensity modulator for electro-optical modulation of the radio frequency reference signal;

the circulator is used for carrying out path selection on the optical signal, so that the signal output by the second intensity modulator is transmitted to the first wavelength division multiplexer through the circulator, and meanwhile, the light returned by the first wavelength division multiplexer is transmitted to the second photoelectric converter;

the second bias control board is used for controlling the working point of the second intensity modulator to enable the second intensity modulator to work at a Q point all the time, so that the electro-optic modulation of the radio frequency reference signal with low distortion can be realized;

the second photoelectric converter is used for performing photoelectric conversion on the optical signals transmitted back and forth and outputting recovered radio frequency reference signals;

a radio frequency reference source for generating a radio frequency reference signal;

the phase discrimination circuit module is used for carrying out phase comparison on the radio frequency reference signal recovered by the second photoelectric converter and a radio frequency reference signal generated by a radio frequency reference source and outputting a phase discrimination voltage;

the PID circuit module is used for carrying out analog PID control on the output signal of the phase discriminator and providing the output control signal to the delay line driving module;

the delay line driving module is used for generating a driving signal of the electric control high-precision optical fiber delay line according to the control signal provided by the PID circuit module, and has the function of electric control delay of the optical fiber;

the electronic control high-precision optical fiber delay line is used for realizing the phase-stable transmission of the radio frequency reference signal after carrying out feedback compensation according to the change of the transmission path;

the Faraday mirror is used for reflecting the radio frequency reference signal after electro-optical modulation from the receiving end of the device to the sending end, and meanwhile, the Faraday mirror can rotate the polarization of the optical signal by 90 degrees, so that the polarization of the forward transmission optical signal is just orthogonal to that of the reverse transmission optical signal, the polarization dispersion of the back-and-forth optical signal is inhibited, and the influence of the polarization dispersion on the phase stabilization feedback compensation precision is avoided;

the first wavelength division multiplexer is used for combining the radio frequency reference signal subjected to electro-optical modulation at the sending end with the radar echo signal subjected to electro-optical modulation, transmitting the combined signal to the receiving end through the same optical path (optical fiber or free space), and simultaneously performing wavelength division demultiplexing on the radio frequency reference signal subjected to electro-optical modulation and reflected back to the sending end by the receiving end so that the radio frequency reference signal subjected to electro-optical modulation and reflected back to the sending end is transmitted to the second photoelectric converter for photoelectric conversion;

and the second wavelength division multiplexer is used for carrying out wavelength division demultiplexing on the radio frequency reference signal after the receiving end receives the jitter electro-optical modulation and the radar echo signal after the electro-optical modulation, so that the radar echo signal after the electro-optical modulation is transmitted to the first photoelectric converter for photoelectric conversion, and meanwhile, the radio frequency reference signal after the electro-optical modulation is transmitted to the Faraday mirror and is reflected back to the sending end.

Further as a preferred embodiment of the method, the method further comprises the following steps:

providing a bias voltage for the first photoelectric converter based on the first bias voltage module;

the second photovoltaic converter is provided with a bias voltage based on a second bias voltage module.

Further as a preferred embodiment of the method, the beam splitter is specifically a 1:99 optical beam splitter, and the step of extracting the control feedback light and the output signal light specifically includes:

1% of control feedback light and 99% of signal light are extracted based on the optical beam splitter;

inputting 1% of the control feedback light to the first bias control board;

99% of the signal light is input to the first wavelength division multiplexer.

While the preferred embodiments of the present invention have been illustrated and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.