High-frequency signal generator and radar module

A radar module includes a high-frequency signal generator comprising upper and lower parallel conductive plates, at least one dielectric rod held between the parallel conductive plates, a metal diode mount held between the parallel conductive plates, a gunn diode member mounted on a side of the diode mount, and a printed-circuit board mounted on the side of the diode mount in covering relationship to the gunn diode member and having a bias supply circuit on its surface for supplying a bias voltage to the gunn diode member. One terminal of the gunn diode member extends through a through hole defined in the printed-circuit board, is exposed in the vicinity of the surface of the diode mount, and is connected to the bias supply circuit. The printed-circuit board has a rectangular metal pattern dimensionally adjustable for adjusting the oscillation frequency of the gunn diode member, and a varactor diode for modulating the frequency of a signal generated by the gunn diode member so that the high-frequency signal generator can function as an FM signal generator.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a high-frequency signal generator for use 
in a millimeter wave radar device installed on a motor vehicle, and a 
radar module employing such a high-frequency signal generator. 
2. Description of the Prior Art 
Radar devices for use on motor vehicles such as automobiles in combination 
with warning units for preventing collisions are required to have a high 
degree of resolution for detecting objects in close distances of about 
several tens of centimeters. In view of such a high-resolution 
requirement, an FM radar is preferable to a pulse radar for use in the 
vehicle-mounted radar devices. Since the maximum range that may be 
detected up to a target such as a preceding motor vehicle or an upcoming 
motor vehicle is of a relatively short distance of about several hundred 
meters, it is suitable for such a radar to use radiowaves in the 
millimeter range which have a frequency of about 60 GHz and can be 
attenuated greatly upon propagation in order to prevent radiated 
radiowaves from being propagated beyond a necessary range and also from 
interfering with existing microwave communications equipment. Use of 
millimeter waves is also preferable from the viewpoint of reducing the 
size of a radar module including an antenna, FM signal generators in front 
and rear stages, a mixer, and other components. 
Heretofore, FM radar modules in the millimeter range are constructed in the 
form of a microstrip line or a waveguide. Because the microstrip line 
radiates a large amount of power, it suffers a large loss and tends to 
cause interference between a plurality of modules, resulting in a 
reduction in measuring accuracy. The waveguide is disadvantageous in that 
its circuit is large in size and expensive. 
One of the attempts to solve the above problems is a non-radiative 
dielectric (NRD) waveguide as disclosed in an article "Millimeter wave 
integrated circuit using a non-radiative dielectric waveguide" written by 
Yoneyama et al. and published in the Journal of Electronic Information 
Communications Society, Vol. J 73 C-1 No. 3 pp. 87-94, March 1990. The 
disclosed non-radiative dielectric waveguide comprises two confronting 
conductive plates spaced from each other by a distance slightly smaller 
than a half wavelength and a rod-shaped dielectric member inserted between 
the conductive plates for allowing only propagations along the rod-shaped 
dielectric member. The upper and lower surfaces of the non-radiative 
dielectric waveguide are completely shielded by the conductive plates. 
Since the distance between the conductive plates is shorter than the half 
wavelength, radiowaves are fully prevented from leaking laterally out of 
the non-radiative dielectric waveguide. Therefore, any power radiation 
from the non-radiative dielectric waveguide is very small, effectively 
avoiding radiation loss in a module and interference between modules. 
Various components including a directional coupler and an isolator can 
easily be fabricated by positioning non-radiative dielectric waveguides 
closely to each other or adding ferrite. Therefore, modules employing 
non-radiative dielectric waveguides can be made smaller than the 
conventional microstrip arrangement where components are separately 
produced and interconnected by a waveguide. The above article also 
discloses small-size, high-performance transmitter and receiver structures 
for use in the millimeter wave band which employ non-radiative dielectric 
waveguides. 
FIG. 10 of the accompanying drawings shows in cross section the structure 
of a conventional gunn oscillator for use as a high-frequency signal 
generator in a transmitter in the millimeter wave band. The conventional 
gunn oscillator comprises a gunn diode GD threaded in a diode mount DM. 
The distal end of a bias supply line B is fixedly bonded by silver paste 
to an upper conductor of the gunn diode GD which is exposed from a 
dielectric substrate D by cutting off a portion of the dielectric 
substrate D with a knife. 
The high-frequency signal generator with the gunn oscillator shown in FIG. 
10 cannot be fabricated with good reproducibility because the process of 
cutting off the dielectric substrate D with a knife and bonding the end of 
the bias supply line B to the conductor is relatively complex and 
time-consuming. 
The frequency adjustment for the high-frequency signal generator disclosed 
in the above article is cumbersome as the oscillation frequency is 
adjusted by adjusting the dimensions of a metal foil oscillator. 
While the above article shows the high-frequency signal generator using the 
gunn diode, it does not discuss any optimum arrangement for an FM signal 
generator for use in an FM radar module. 
The article also discloses a gunn oscillator as shown in FIG. 11 of the 
accompanying drawings and a non-radiative dielectric waveguide for guiding 
signals which are generated in the millimeter wave band by the gunn 
oscillator to an antenna or the like. As shown in FIG. 11, the gunn 
oscillator and its surrounding circuits comprise upper and lower 
conductive plates 31, 32 serving as a non-radiative dielectric waveguide, 
a diode mount 33 sandwiched between the upper and lower conductive plates 
31, 32, a gunn diode 34 threaded in the diode mount 33, a printed-circuit 
board 35 fixed to a side of the diode mount 33, a dielectric rod 40 for 
guiding a signal generated in the millimeter wave band by the gunn diode 
34 to an antenna or the like (not shown), and a metal foil oscillator 41 
for guiding the signal generated in the millimeter wave band by the gunn 
diode 34 to the dielectric rod 40. 
In FIG. 11, the distance between the upper and lower conductive plates 31, 
32 is set to a value slightly smaller than half the wavelength of the 
signals used in the millimeter wave band. If the signals have a frequency 
of about 60 GHz, for example, then the distance between the upper and 
lower conductive plates 31, 32, and hence the thickness of the diode mount 
33 is of a small value of about 2.5 mm. Commercially available packaged 
gunn diodes are mounted on a heat-radiating stud which is of a diameter 
ranging from 3 to 4 mm. Therefore, it is necessary to machine them to make 
them ready for use in actual applications, as shown in FIGS. 12(A) and 
12(B). First, as shown in FIG. 12(A), a gunn diode 35 is threaded in a 
metal block which is 5 to 6 mm thick. Then, as shown in FIG. 12(B), upper 
and lower portions of the metal block are cut off to reduce the thickness 
thereof to about 2.5 mm, and grooves dimensioned to a 1/4 wavelength are 
defined in the metal block to prevent the signals from leaking out. In 
this manner, the gunn diode 34 is mounted on the diode mount 33. 
Inasmuch as the gunn oscillator requires complex machining as shown in 
FIGS. 12(A) and 12(B) to fabricate the diode mount 33 that is of a small 
thickness, the process of manufacturing the gunn oscillator is 
time-consuming, and the produced gunn oscillator is costly. 
Furthermore, as shown in FIG. 13 of the accompanying drawings, the diode 
mount 33 has a recessed step on which an upper flange of the gunn diode 34 
is placed. Since the depth .delta. of the recessed step is subject to 
variations, the height .epsilon. that a lower conductor of the gunn diode 
34 projects from the surface of the diode mount 33 also suffers 
variations, resulting in varying oscillation characteristics. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a 
high-frequency signal generator which can easily be manufactured with good 
reproducibility, and a radar module employing such a high-frequency signal 
generator. 
Another object of the present invention is to provide a high-frequency 
signal generator which allows an oscillation frequency to be adjusted with 
ease. 
Still another object of the present invention is to provide a 
high-frequency signal generator which can easily be modified into an FM 
signal generator with easy and reliable FM signal frequency control, and a 
small-size FM radar module in the form of a non-radiative dielectric 
waveguide employing such an FM signal generator. 
Yet another object of the present invention is to provide a high-frequency 
signal generator such as an FM signal generator which can be manufactured 
in a relatively short period of time and at a relatively low cost without 
the need for a complex machining process for reducing the thickness of a 
diode mount. 
According to the present invention, a radar module includes a 
high-frequency signal generator comprising upper and lower parallel 
conductive plates, at least one dielectric rod held between the parallel 
conductive plates, a metal diode mount held between the parallel 
conductive plates, a gunn diode member mounted on a side of the diode 
mount, and a printed-circuit board mounted on the side of the diode mount 
in covering relationship to the gunn diode member and having a bias supply 
circuit on a surface thereof for supplying a bias voltage to the gunn 
diode member. One terminal of the gunn diode member extends through a 
through hole defined in the printed-circuit board, is exposed in the 
vicinity of the surface of the diode mount, and is connected to the bias 
supply circuit. The printed-circuit board has a rectangular metal pattern 
dimensionally adjustable for adjusting the oscillation frequency of the 
gunn diode member, and a varactor diode for modulating the frequency of a 
signal generated by the gunn diode member so that the high-frequency 
signal generator can function as an FM signal generator. 
The high-frequency signal generator may further comprise a lower conductor 
mounted on the side of the mount and serving as a terminal of the 
oscillating element with the oscillating element disposed upside down with 
an operating layer thereof facing downwardly, a package mounted on the 
lower conductor in surrounding relationship to the oscillating element, 
and an upper conductor disposed on top of the package and serving as 
another terminal of the oscillating element. 
The lower conductor may have a threaded member threaded in a threaded hole 
defined in the side of the mount. Alternatively, the lower conductor may 
comprise a disk-shaped pad integrally formed with the side of the mount. 
The printed-circuit board may have a rectangular metal pattern 
dimensionally adjustable for adjusting the oscillation frequency of the 
oscillating element. 
The printed-circuit board may have a varactor diode for modulating the 
frequency of a signal generated by the oscillating element, whereby the 
high-frequency signal generator can function as an FM signal generator. 
The high-frequency signal generator may further comprise a metal foil 
resonator mounted on the conductive plate and interposed between the 
dielectric rod and the oscillating element for propagating a 
high-frequency signal generated by the oscillating element to the 
dielectric rod. 
The above and further objects, details and advantages of the present 
invention will become apparent from the following detailed description of 
preferred embodiments thereof, when read in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, a radar module according to a preferred embodiment of 
the present invention includes a high-frequency signal generator 10 and 
its associated components, the radar module being used as an FM radar 
module in the millimeter wave band for use on a motor vehicle such as an 
automobile. 
As shown in FIG. 1, upper and lower parallel conductive plates 1, 2 serving 
as a non-radiative dielectric waveguide hold therebetween a dielectric rod 
21 that constitutes part of the non-radiative dielectric waveguide, a 
diode mount DM of metal kept at a ground potential, a gunn diode member or 
gunn diode GD mounted on a side of the diode mount DM, a printed-circuit 
board PB fixed to the same side of the diode mount DM and having a bias 
supply circuit for supplying a bias voltage to the gunn diode GD, and an 
resonator 11 interposed between the printed-circuit board PB and the 
dielectric rod 21. 
As shown in FIG. 2, the printed-circuit board PB has a central through hole 
H defined therein and first and second bias supply microstrip lines B1, B2 
disposed on a surface of the printed-circuit board PB one on each side of 
the central through hole H, the first and second bias supply microstrip 
lines B1, B2 serving as the bias supply circuit. A rectangular metal 
pattern MP (described later on) is disposed between the first and second 
bias supply microstrip lines B1, B2, specifically between the through hole 
H and the second bias supply microstrip line B2. As shown in FIG. 1, leads 
L1, L2 are connected respectively to the first and second bias supply 
microstrip lines B1, B2. 
As shown in FIG. 5, the diode mount DM has an internally threaded hole 4 
defined in a body 3 thereof and opening at a side thereof. The gunn diode 
GD has an externally threaded lower conductor 6 which is threaded in the 
hole 4. Therefore, the gunn diode GD is fixed to the central region of the 
side of the diode mount DM. The lower conductor 6 is substantially fully 
embedded in the hole 4 in the diode mount DM. 
A package 8 is held on the distal end of the lower conductor 6 of the gunn 
diode GD, and a gunn diode element 5 as an oscillating element is disposed 
upside down in the package 8 with an operating layer facing downwardly. 
The gunn diode element 5 has a heat-generating terminal connected through 
the externally threaded lower conductor 6, which serves as a terminal 
member, to the body 3 of the diode mount DM, and kept at the ground 
potential that is supplied to the diode mount DM. Therefore, the lower 
conductor 6 functions as a heat radiating stud for the gunn diode member 
or gunn diode GD. 
The gunn diode GD has an upper conductor 7 disposed on the top of the 
package 8 which houses the gunn diode element 5. The other terminal of the 
gunn diode element 5 is connected to the upper conductor 7 by a gold 
ribbon 9. The upper conductor 7 and the gold ribbon 9 serve as the other 
terminal member of the gunn diode GD. 
As shown in FIG. 5, the upper conductor 7 on the top of the package 8 is 
inserted through the hole H that is defined centrally in the 
printed-circuit board PB which is fixed to the side of the diode mount DM 
in covering relationship to the gunn diode GD. The upper conductor 7 is 
disposed either to lie flush with the surface of the printed-circuit board 
PB on which the bias supply circuit is formed or to be exposed in the 
vicinity of the surface of the printed-circuit board PB. The upper 
conductor 7 is electrically connected by thermal compression to a gold 
ribbon or metal foil GL that extends between and is joined by thermal 
compression to an end of the first bias supply line B1 and the metal 
pattern MP on the printed-circuit board PB. 
According to this embodiment, a frequency-modulating varactor diode VD of 
the beam lead type extends between and is joined by thermal compression to 
the metal pattern MP and the second bias supply line B2 on the 
printed-circuit board PB, as shown in FIGS. 3 and 5. Therefore, the bias 
voltage on the gunn diode GD is adjusted by a bias voltage applied to the 
first bias supply line B1, and the bias voltage on the varactor diode VD 
is adjusted by the difference between bias voltages applied to the first 
and second bias supply lines B1, B2. The high-frequency signal generator 
10 which is composed of the gunn diode GD and other components can 
function as an FM signal generator when the varactor diode VD is mounted 
as a variable reactance device. 
Each of the first and second bias supply lines B1, B2 serves as a 
five-stage low-pass filter with its larger and smaller line width 
variations repeated at a constant period which is set to 1/4 of the 
wavelength of FM signals that are to be generated in the millimeter wave 
band by the high-frequency signal generator. In this embodiment, the FM 
signals have a frequency of about 60 GHz, and hence the larger and smaller 
line width variations of the low-pass filter are repeated at the period of 
about 1.25 mm. The distance between the conductive plates 1, 2 of the 
non-radiative dielectric waveguide or the height of the side of the diode 
mount DM is set to a value of 2.5 mm or lower which is slightly smaller 
than half the wavelength of the FM signals. 
FIG. 4 shows an equivalent circuit of the high-frequency signal generator 
10, i.e., the FM signal generator 10 and its associated components, shown 
in FIG. 1. The metal foil resonator 11 which comprises a rectangular metal 
foil MS on the thin dielectric substrate 12 is interposed between the FM 
signal generator 10 or the gunn diode GD and the dielectric rod 21. The FM 
signals can be adjusted by adjusting the dimensions of the metal foil MS. 
The FM signals can also be adjusted by the dimensions of the rectangular 
metal pattern MP on the printed-circuit board PB. The metal foil MS may be 
replaced with a metal rod. 
It has been confirmed that the oscillation frequency of the FM signal 
generator 10 can be roughly adjusted by dimensions a, b, shown in FIG. 3, 
of the rectangular metal pattern MP, and can be finely adjusted by the 
metal foil resonator 11 as shown in FIG. 1. Examples of the rough 
adjustment will be described below. 
When the dimension b of the metal pattern MP was fixed to 0.3 mm and the 
other dimension a was varied, the oscillation frequency was varied as 
shown in Table 1 below before and after the metal foil resonator 11 was 
added to the non-radiative dielectric waveguide shown in FIG. 1 for 
propagation therethrough. 
TABLE 1 
______________________________________ 
Dimension a Before added 
After added 
______________________________________ 
1.2 mm 51.4 GHz -- 
0.8 mm 55.6 GHz 59.5 GHz 
0.6 mm 57.4 GHz 61.0 GHz 
______________________________________ 
when the dimension a of the metal pattern MP was fixed to 0.8 mm and the 
other dimension b was varied, the oscillation frequency was varied as 
shown in Table 2 below before and after the metal foil resonator 11 was 
added to the non-radiative dielectric waveguide shown in FIG. 1 for 
propagation therethrough. 
TABLE 2 
______________________________________ 
Dimension b Before added 
After added 
______________________________________ 
0.4 mm 53.2 GHz 57.5 GHz 
0.3 mm 55.6 GHz 59.5 GHz 
0.2 mm 57.7 GHz 61.8 GHz 
______________________________________ 
FIG. 6 shows the radar module incorporating the high-frequency signal 
generator 10 as an FM signal generator. 
The radar module functions as an FM radar module as the high-frequency 
signal generator 10 functions as an FM signal generator. The FM radar 
module has the FM signal generator 10, the metal foil resonator 11, a 
propagating means composed of various dielectric rods and other parts, and 
an antenna means. 
The propagating means comprises the dielectric rod 21 as a first dielectric 
rod held between the upper and lower parallel conductive plates 1, 2, an 
isolator 25, a linear second dielectric rod 22 coupled to the first 
dielectric rod 21 through the isolator 25, a semicircular third dielectric 
rod 23 coupled to the second dielectric rod 22 through a directional 
coupler 26, and a fourth dielectric rod 24 coupled to the second 
dielectric rod 22 through the isolator 25. A single-diode mixer 29 is 
joined to the distal end of the second dielectric rod 22. The third 
dielectric rod 23 has a transmitting antenna 27 and a receiving antenna 28 
respectively on its opposite ends. A resistive terminator is coupled to 
the distal end of the fourth dielectric rod 24. 
In operation, an FM signal generated by the FM signal generator 10 is 
propagated through the first dielectric rod 21 and supplied to an input 
terminal of the isolator 25. The FM signal is then outputted from an 
output terminal of the isolator 25 to the linear second dielectric rod 22. 
The linear second dielectric rod 22 and the semicircular third dielectric 
rod 23, which has its convex portion disposed near the central region of 
the linear second dielectric rod 22, jointly form the directional coupler 
26. Therefore, a portion of the FM signal outputted from the isolator 25 
is transferred to the third dielectric rod 23, and radiated out from the 
transmitting antenna 27 on one of the ends of the third dielectric rod 23. 
The remainder of the FM signal outputted from the isolator 25 is 
propagated through the linear second dielectric rod 22 to its distal end 
where it is supplied as a local signal to the single-diode mixer 29. 
A reflected wave (reflected signal) from an object is received by the 
receiving antenna 28. A portion of the received signal is transferred 
through the directional coupler 26 to the linear second dielectric rod 22, 
from which it is supplied to the single-diode mixer 29. The remainder of 
the received signal is radiated again from the transmitting antenna 27. 
The single-diode mixer 29 is supplied with the local signal from the 
output terminal of the isolator 25 through the second dielectric rod 22 
and also with the reflected wave from the receiving antenna 28 through the 
directional coupler 26. The single-diode mixer 29 mixes the supplied 
signals and generates a beat signal, which is outputted to a coaxial line 
L3 extending from between the upper and lower conductive plates 1, 2. 
The transmitting antenna 27 also receives the reflected wave from the 
object. The received signal is propagated through the semicircular third 
dielectric rod 23 to the receiving antenna 28, which radiates the signal 
again. A portion of the reflected wave received by the transmitting 
antenna 27 and a portion of the reflected signal which is reflected toward 
the FM signal generator 10 by the single-diode mixer 29 are supplied 
through the directional coupler 26 and the second dielectric rod 22 to the 
isolator 25, and then absorbed by the resistive terminator joined to the 
fourth dielectric rod 24. 
Since the transmitting and receiving antennas 27, 28 share the directional 
coupler 26, these antennas radiate waves again and again. Thus, a portion 
of the received reflected wave is delayed from the reflected wave which is 
required by the single-diode mixer 29, and applied as an unnecessary wave 
to the single-diode mixer 29. However, inasmuch as this unnecessary wave 
is produced by an FM signal that has reciprocated twice or more between 
the antenna and the reflecting object, it is of a level sufficiently lower 
than the level of the reflected wave that is required by the single-diode 
mixer 29 because of a relatively low gain of the transmitting and 
receiving antennas 26, 27 and a large space propagation loss, and hence 
any effect of the unnecessary wave on the radar functions can be ignored. 
As described above, the high-frequency signal generator used in the radar 
module does not require the conventional process of cutting off a 
dielectric substrate (printed-circuit board, etc.) with a knife and 
bonding a bias supply line with silver paste. The high-frequency signal 
generator can thus be assembled with a greatly reduced amount of labor, 
and can be manufactured with good reproducibility. 
While the invention has been described with respect to the radar module 
which incorporates the FM signal generator, the process of forming the 
metal pattern on the printed-circuit board for adjusting the oscillation 
frequency of the gunn diode through dimensional adjustment is not limited 
to the FM signal generator but applicable to various high-frequency signal 
generators. 
The high-frequency signal generator used in the radar module according to 
the above embodiment can easily be arranged for use as an FM signal 
generator, and can easily and reliably control the frequency of an FM 
signal when it is arranged as an FM signal generator in the millimeter 
wave band for use in an FM radar module in the form of a non-radiative 
dielectric waveguide. The high-frequency signal generator is also simple 
in structure and small in size. 
As the metal pattern is formed on the printed-circuit board for adjusting 
the oscillation frequency of the gunn diode through dimensional 
adjustment, the oscillation frequency of a high-frequency signal generator 
such as an FM signal generator can easily be adjusted or varied simply by 
replacing the printed-circuit board. 
The gunn diode member or gunn diode GD is separate from the diode mount DM 
as shown in FIG. 5. However, the gunn diode GD and the diode mount DM may 
be integral with each other as described below. 
FIGS. 7 and 8 show a modification in which a gunn diode member or gunn 
diode GD' and a diode mount DM' are integral with each other. The diode 
mount DM' has a disk-shaped pad 6' positioned substantially centrally on a 
side thereof to perform substantially the same function as the lower 
conductor 6 in the above embodiment. A gunn diode element or a gunn diode 
pellet (or chip) 5' is fixedly mounted on the disk-shaped pad 6' by 
thermal compression or soldering in an upside-down configuration with a 
heat-generating active layer facing downwardly. Therefore, one terminal on 
the active layer of the gunn diode element or gunn diode pellet 5' is 
supplied with a ground potential from the diode mount DM' that is kept at 
the ground potential. The gunn diode element 5' is surrounded by a 
cylindrical ceramics member 8' for performing the same function as the 
package 8 in the above embodiment. The cylindrical ceramics member 8' is 
mounted on the diode mount DM' through a metallized layer. A metal lid 
plate 7' which performs the same function as the upper conductor 7 in the 
above embodiment is attached to the distal end of the cylindrical ceramics 
member 8' through a gold ribbon 9' that is connected to the other terminal 
of the gunn diode element 5'. The gunn diode GD' integral with the diode 
mount DM' may be held between the upper and lower parallel conductive 
plates 1, 2, thus providing a high-frequency signal generator or FM signal 
generator 10 as shown in FIG. 9. 
As shown in FIG. 7, each of the upper and lower surfaces of the diode mount 
DM' has a groove defined therein which has a depth equal to 1/4 of the 
wavelength of the oscillated signal for preventing the oscillated signal 
from leaking out. 
As with the above embodiment, the high-frequency signal generator with the 
modified gunn diode integral with the diode mount can easily be arranged 
for use as an FM signal generator in the millimeter wave band for use in 
an FM radar module in the form of a non-radiative dielectric waveguide. 
The high-frequency signal generator is also simple in structure and small 
in size. 
Since the gunn diode and the diode mount are integral with each other, any 
complex maching process, which would otherwise be needed to reduce the 
thickness of a diode mount for using FM signals of relatively high 
frequencies, is no longer necessary, and hence the integral assembly can 
be manufactured less costly. 
The high-frequency signal generator and the radar module have been 
described as employing the non-radiative dielectric waveguide, the 
propagation line is not limited to the non-radiative dielectric waveguide, 
but may be a dielectric waveguide such as an H guide or an insular 
waveguide. 
The high-frequency signal generating element or oscillating element is not 
limited to the gunn diode, but may be any of various other solid-state 
oscillating elements including an IMPATT diode, a TUNNET diode, a BARIT 
diode, a TRAPATT diode, an LSA diode, etc. 
Although there have been described what are at present considered to be the 
preferred embodiments of the invention, it will be understood that the 
invention may be embodied in other specific forms without departing from 
the essential characteristics thereof. The present embodiments are 
therefore to be considered in all respects as illustrative, and not 
restrictive. The scope of the invention is indicated by the appended 
claims rather than by the foregoing description.