Communications: directive radio wave systems and devices (e.g. – Radar ew
Reexamination Certificate
1977-02-24
2004-01-13
Sotomayor, John B. (Department: 3662)
Communications: directive radio wave systems and devices (e.g.,
Radar ew
C342S192000, C342S193000, C324S076230, C324S076430, C327S041000
Reexamination Certificate
active
06677882
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to R-F receivers and more particularly to multioctave, high-resolution, super-heterodyne receivers utilizing harmonic mixing and mixed-base coding techniques to resolve ambiguities caused by time-coincident signals.
It is often necessary, in the pursuit of electronic intelligence and electronic countermeasure activities, to detect signals which may occur anywhere within a wide range of the R-F spectrum. In the past, a wide variety of approaches has been taken to resolve this problem, the four most common approaches being:
(1) The channelized receiver;
(2) The double-conversion, channelized, supper-heterodyne receiver;
(3) The instantaneous frequency measurement (IFM) receiver; and
(4) The microscan receiver. Each will now be described.
The channelized receiver represents the ideal receiver if performance is the only consideration; however; to cover the radar frequency range of 0.5 to 20 GHz and provide ±5 MHz resolution would require 1,950 filters plus amplifiers, an inordinately large, costly unit.
The second approach, the double-conversion, channelized, super-heteodyne receiver, prefilters the RF band into convenient sub-bands, each of which is then heterodyned into a common first IF amplifier. The IF output is further channelized by a second set of band filters following the first IF. These filtered outputs are again heterodyned into a second IF amplifier and finally channelized by a set of contiguous bandpass filters. The received signal frequency is determined by decoding the video detector outputs of each channel. Several design problems are inherent in this approach, among which are: a) measuring the frequencies of time-coincident signals, b) resolving the ambiguity caused by a signal entering the receiver at the many subband filter crosscovers, and c) the complexity, large size and expense of the equipment.
The third approach is the instantaneous frequency measurement receiver. Normally, to cover the RF bandwidth, a channelized super-heterodyne is used to reduce the bandwidth to an octave or less. This octave bandwidth is then processed by one or more frequency discriminators that cover the band. Each discriminator consists of two delay lines and a phase detector, the output of the phase detector and subband channel detector then being processed to provide the digitally encoded frequency word. In addition to its size, cost, and complexity, the IFM receiver has the same problems as the double-conversion channelized super-heterodyne receiver, as described above.
A fourth approach is the microscan receiver, the heart of which consists of a dispersive delay line that is in the signal path. These lines result in a relatively narrow bandwidth and high insertion loss; therefore, a channelized super-heterodyne receiver, with all its drawbacks, is required prior to the microscan processing. In operation, the signal whose frequency is to be determined is mixed with a VCO that is rapidly tuned over the band of interest in a time less than the minimum radar pulse width whose frequency is to be measured. The heterodyned IF signal is a chirp (linear FM) signal. After amplification, the signal is recompressed by a dispersive delay line. The time spent in the delay line will be a function of the frequency of the signal. By measuring this time, and knowing in what subband the signal was received, the signal frequency can be determined.
Many problems have been found in implementing this type of receiver. Most of them occur in the delay line and include: a) triple travel; b) insertion loss; c) narrow band width; d) temperature sensitivity; e) slow data rate; and f) all of the problems encountered with a channelized super-heterodyne receiver, as detailed above.
In order to provide the basis for a clear understanding of the present invention, it is helpful at this point to briefly discuss the mixed-base code notation and its relationship to more conventional number systems. The discussion will include basic principles of a simple mixed-base code and of a staggered mixed-base code suitable for use in the apparatus according to the invention.
A variety of different number systems have been employed for coding purposes, the most common being those number systems utilizing a common base. Each digital word is written or coded in shorthand notation as a series of digits a
n
, a
n−1
, a
n−2
, . . . a
0
where the a's are coefficients of successive powers (from right to left) of an integer termed the base or radix—thus the positional notation of common-base number systems is merely an arrangement wherein each position or column is weighted according to successive powers of the base, and therefore constitutes an abbreviated version of the more complex expression &Sgr;K
−a
k
−ra
where r is the base (radix) and o≦a
k
≦r−1. In the decimal system for example, r=10 so that the coefficients a
k
(digits forming a word in the positional notation) extend from 0 through 9. Similarly, binary numbers (words) are based on r=z, with a
k
taking on the values 0 and 1; octal words on r=8, o≦a
k
≦7; and so forth.
The mixed-base notation, employed in the present invention, on the other hand, refers to a coding scheme or digital format wherein the digits in each column or position are referenced to a different base, with no positional weighting. The scheme is, in fact, based on the use of different moduli m for each position so that the digit in each column is actually the remainder deriving from the division of a real integer by an integral multiple of the modulo m
k
for that column or position. In a 3-4-5 mixed-base code, for example, the positional notation is based on modulo
3
, modulo
4
, and modulo
5
, so that the word “15” in decimal notation is designated “030” in 3-4-5 mixed-base notation. It will readily be observed that this result is a consequence of the fact that 15 is an integral multiple of 3, viz 15/3=5 remainder 0, 15/4=3 remainder 3, and similarly, 15/5=remainder 0. It will also be apparent that digits in each column or position can take on only the values o≦b
k
≦m
k
−1 where b
k
is the digit in the position k based on modulo m
k
.
It will be apparent, therefore, that a simple mixed-base code is limited to a series of nonredundant digital words over a range of values equal to the lowest common denominator of the moduli. In the case of the 3-4-5 mixed-base notation, 3×4×5=60 (the lowest common denominator of 3, 4, and 5) so that decimal numbers 0 through 59 inclusive, for example, may be expressed without ambiguity. Of course, the non-redundant word capacity of a mixed-base code increases as the number of moduli increases or as the value of the integer representing each modulo increases.
Detailed discussion of the mixed-base notation is available in the prior art (see, e.g., U.S. Pat. No. 3,488,594 to J. Caballero, Jr., and U.S. Pat. No. 3,019,975 to R. E. Williams), so that the presentation here has been relatively brief, confined to very basic principles of the code. It is convenient for purposes of the present invention, to depart slightly from the usual mixed-base notation by allowing the digits b
k
in each position K to take on the values 1≦b
k
≦m
k
. In other words, a digit in the position based on modulo
3
may have a value of either 1, 2, or 3, corresponding to the normal values 0, 1, or 2, respectively, with similar considerations applying to digits in positions based on the remaining moduli of the code. Here again, this is purely a matter of convenience, rather than a limitation of the inventive principles.
The development of wideband microwave receivers may be accomplished in a manner representing a practical application of the mixed-base coding concept. For example, a receiver may be designed with one channel for each column (position) in the mixed-base code, the number of parallel outputs from each channel corresponding to the numerical value of the base (modulo integer) of each column.
Karasek John J.
Mills John Gladstone
Sotomayor John B.
The United States of America as represented by the Secretary of
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