Communications: directive radio wave systems and devices (e.g. – Radar transponder system – Radar transponder only
Reexamination Certificate
2001-05-24
2002-10-08
Gregory, Bernarr E. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar transponder system
Radar transponder only
C342S042000, C342S043000, C342S050000, C342S060000, C342S175000, C375S147000, C375S150000, C375S151000, C375S152000, C375S153000
Reexamination Certificate
active
06462698
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to short range communications using surface acoustic wave (SAW) expanders and compressors.
2. Description of the Prior Art
SAW technology is well known for its excellent radio frequency (RF) performance, low cost and small size. SAW is a passive thin film technology that does not require any bias current in order to function. SAW expanders and compressors have been used in RADAR applications for many years.
The basic “building block” of SAW expanders and compressors is the interdigital transducer (IDT) such as shown in FIG.
1
. An IDT
10
is a series of thin metal strips or “fingers”
12
fabricated on a suitable piezoelectric substrate
14
. One set of fingers is connected to an input/output terminal
16
, while the opposite set of fingers is connected to another terminal
18
. In single-ended IDTs, terminal
18
is grounded. For differential input signals however, terminal
18
is a pulse input/output terminal. Spacing “W” between IDT segments is adjusted to conform to the desired chip period of the coded sequence. When excited by a narrow electric pulse at terminal
16
, the IDT generates a coded output SAW signal that propagates in both directions perpendicular to the fingers
12
. If a similarly coded SAW signal impinges on the fingers
12
, then an autocorrelation function is performed and a peak, with associated side lobes, is generated at terminal
16
. These abilities of SAW expanders and compressors are well known in the prior art, having been demonstrated for example in Edmonson, Campbell and Yuen, “Study of SAW Pulse Compression using 5×5 Barker Codes with Quadraphase IDT Geometries”, 1988
Ultrasonics Symposium Proceedings
, Vol. 1, Oct. 2-5, 1988, pp. 219-222.
Thus, the structure shown in
FIG. 1
can operate as both a SAW expander, generating a SAW output from a single pulse input, and a SAW compressor, generating a single pulse or peak output from a SAW input. Terminal
16
, as well as terminal
18
in differential IDTs, is both a pulse input terminal and a pulse output terminal. Conversion of an output SAW into an electrical signal for further processing in conventional communications circuits and subsequent transmission through an antenna is accomplished by adding a transmit IDT
24
, aligned with the IDT
22
, as shown in FIG.
2
. Both IDTs can be fabricated on the same substrate
14
. A SAW output from IDT
22
is converted into an electrical signal by TX IDT
24
. A SAW receiver would have the same structure as in
FIG. 2. A
signal input to a receive IDT from receiver processing circuitry would be converted to a SAW which is input to IDT
22
. Like the IDT
22
, the TX IDT
24
may be a differential IDT, wherein the grounded lower terminal would be a pulse output terminal.
The geometry of adjacent IDT fingers
12
is shown in
FIG. 3
, where Tf is the width of a metallized finger
12
and Ts is the width of the space between the fingers
12
. In typical designs both Tf and Ts are equal to a quarter of a wavelength, &lgr;/4. For example, for a typical SAW system operating in the Industrial, Scientific and Medical (ISM) band at 2.4 GHz the &lgr;/4 dimension could be in the order of 0.425 microns, depending upon the substrate chosen.
Previous SAW-based communications systems use lower frequency SAW expanders and compressors having larger and further spaced fingers in conjunction with a plurality of components such as mixers and local oscillators, as shown in FIG.
4
. In the typical prior art communication system
30
, a lower frequency 266 MHz signal generated by transmit IDT
20
is up-converted in mixer
34
, which receives a 734 MHz signal from local oscillator
36
. The resulting output from mixer
34
is filtered in high pass filter
38
to generate a 1 GHz signal for transmission through antenna
40
. On the receive side, the process is reversed in antenna
42
, mixer
44
, low pass filter
46
and receive compressor IDT
20
′. The TX and RX IDTs
20
and
20
′ have the structure shown in FIG.
2
. Undesirably, the mixers
34
and
44
, oscillator
36
and filters
38
and
46
from the communications system
30
, result in additional cost, power consumption, occupation in space and a much complex system than is desired for low-cost, low power, short range communication systems. Therefore, there remains a need in the art to reduce the number of components in such a communication system. Previously-reported designs of encoded IDT structures have employed split-electrodes of width &lgr;/8 within each chip segment W to suppress spurious IDT finger reflections (See, for example, M. G. Holland and L. T. Claiborne, “Practical Surface Acoustic Wave Devices”,
Proceedings of the IEEE
, Vol. 62, pp. 582-611, May 1974). In contrast, SPUDT-type reflection gratings can be placed judiciously to enhance spurious IDT finger reflections and thereby reduce device insertion loss. These SPUDT techniques have previously been applied to realize low-loss SAW filters, where all of the IDT segments within each section W of the structure have the same polarity (See, for example, 1) C. K. Campbell and C. B. Saw, “Analysis and Design of Low-loss SAW Filters using Single-Phase Unidirectional Transducers”,
IEEE Transactions on Ultrasonics, Ferroelectrics and Frequency Control
, Vol. UFFC-34, pp. 357-367, May 1987; and 2) C. Campbell,
Surface Acoustic Wave Devices and their Signal Processing Applications
, Boston: Academic Press, 1989). These SPUDT-type enhancement techniques have not been previously applied to encoded IDTs.
High-frequency communication techniques involving more conventional non-SAW based circuits and systems also exist. BLUETOOTH™ wireless technology is one such prior art example. BLUETOOTH™ is a de facto standard, as well as a specification for small-form factor, low-cost, short range radio links between mobile PCs, mobile phones and other portable wireless devices. The current BLUETOOTH™ short range communications specification operates in the 2.4 GHz (ISM) band; however, the BLUETOOTH™ standard in its current infancy undesirably involves high cost, substantial power consumption and relatively complex hardware.
Although high frequency SAW expanders and compressors offer significant reductions in cost, power consumption, size and complexity over prior SAW-based and non-SAW based communications systems, conventional SAW expanders and compressors typically have insertion losses greater than 20 dB. This may affect the RF link budget of a communication system, as more gain would have to be designed into the system for satisfactory operation. Therefore, there remains a need for a SAW SPUDT-type expander and compressor that would improve the insertion loss and positively impact the RF link budget and complexity of the system and thereby make RF communications systems in which RF signals are generated and processed directly and solely by SAW expanders and compressors feasible.
SUMMARY OF THE INVENTION
An object of the present invention is overcome at least some of the drawbacks of the prior art.
Advantageously, the use of low loss SPUDT-type SAW devices as described in the present invention offers improved performance compared to conventional SAW devices in respect to generation of coded RF waveforms (expander) and the autocorrelation of coded RF waveforms (compressor) for communication systems.
It is therefore an object of the invention to provide a low cost SPUDT-type SAW-based communication method and system. As an illustrative example of the cost reduction resulting from the present invention, SAW devices used for filtering at near-ISM band frequencies may cost approximately $1.00 each. In contrast, a comparable semiconductor BLUETOOTH™ solution may cost more than $10.00.
It is a further object of the invention to provide SPUDT-type SAW-based transmit and receive units that are easily manufactured. The manufacturing required for the present invention allows for SAW fabrication that utilizes simple, single layer photolithographic techniques.
Another object of the inv
Campbell Colin K.
Edmonson Peter J.
Gregory Bernarr E.
Jones Day Reavis & Pogue
Meyer, Esq. Charles B.
Pathiyal, Esq. Krishna K.
Research in Motion Limited
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