Delay line filters using multiple in-line four-input couplers

Details Delay line filters using multiple in-line four-input couplers Delay line filters using multiple in-line four-input couplers

2002-04-01

2003-12-16

Summons, Barbara (Department: 2817)

Wave transmission lines and networks

Coupling networks

Delay lines including long line elements

C333S161000, C333S164000, C333S02800T, C330S151000

Reexamination Certificate

active

06664869

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed generally to systems and methods for filtering and/or delaying transmission signals, and, more particularly, to systems and methods for providing integrated filters with a flat delay response.
DESCRIPTION OF THE RELATED ART
The popularity of cellular phones and other wireless communication equipment over the last several years has resulted in the need to obtain greater utilization from the existing frequency spectrum while at the same time reducing the cost of technology used to operate at high frequencies. An important component of any cellular system is the high power amplifier that amplifies signals associated with the cellular system. A particular effective high power amplifier is a feed forward design that typically includes one or more delay lines along a main path and one or more delay lines along a secondary feed forward path.
FIG. 1
shows an exemplary feed-forward high power amplifier
1
. In this embodiment, the main path includes a hybrid splitter
2
, a phase and amplitude adjustment
3
, a main amplifier
4
, an output coupler
5
, a delay line
6
, and an error injection coupler
7
, feeding output
13
. The secondary path in the embodiment shown in
FIG. 1
includes a delay line
8
, a hybrid combiner
9
, phase and amplitude adjustment circuits
10
,
11
, and an error amplifier
12
. The high power amplifier shown in
FIG. 1
is suitable for use as a multi-carrier power amplifier.
The delay lines ideally have a uniform, temperature stable, and fixed amount of insertion delay and constant phase over a predetermined frequency range. Conventionally, the delay lines
6
along the main path are implemented using multi-path delay equalization cavity filters. The multi-path delay equalization cavity filters are configured to utilize cross coupling techniques available due to the greater isolation of cavity filters to minimize delay variations across the passband while handling very high levels of power, e.g., in excess of 1,000 watts for most designs, and over 2,000 watts on a custom basis, even for small cavity diameters. Examples of this type of delay line are available from the assignee of the present application as a standard product line. Filters of this type are typically large measuring from around 15 in
2
to more than 40 in
2
.
Other cavity filter delay lines are, for example, shown in U.S. Pat. Nos. 4,622,523 and 3,699,480. Again, these cavity filters are typically used on the main path of the power amplifier shown in FIG.
1
.
On the secondary “feed forward” path, the conventional implementation of the delay lines (e.g., delay line
8
in
FIG. 1
) is a coiled coaxial cable or micro-strip printed on a high dielectric material. Examples of devices falling within this category are shown in U.S. Pat. Nos. 4,409,568, 5,252,934, and 4,218,664 and in conventional components such as Murata LDH33, LDH36, and LDH46 series delay lines. Conventional printed micro-strip delay lines, however, are disadvantageous in that there is coupling between the various turns across the high dielectric material and a high insertion loss. Coaxial cable type delay lines have little cross coupling, but still are characterized by high insertion loss, and require a large amount of area to implement. Accordingly, a suitable secondary delay circuit is not now available conventionally.
Various attempts have been made to achieve delay equalization using active components to shift various delay response curves and add them together. For example, U.S. Pat. No. 3,942,118, issued to Haruo Shiki suggests cascading together a first frequency converter, a first filter, a first series of all-frequency pass filters having successively rising resonant frequencies and successively lowering Q-values (first delay equalizer), a second frequency converter, with a second filter circuit, a local oscillator, and a second series of all-frequency pass filters having successively rising resonant frequencies and successively lowering Q-values (second delay equalizer). This circuit, however, has the disadvantage in that there are large insertion losses, large volume resulting from the large number of components, and temperature instability due to the number of active components. Other attempts at active delay equalization, such as, U.S. Pat. No. 4,491,808, suffer from similar problems.
In 1964, Dr. S. B. Cohn proposed using a four-port coupler or a three-port circulator to achieve equalization of non-linear phase angle or time delay characteristics of other components. See, for example, U.S. Pat. No. 3,277,403, herein incorporated by reference. U.S. Pat. Nos. 4,197,514 and 4,988,962 cite examples of Dr. Cohn's earlier work as prior art in the background portions of the respective specifications. Over the years, there have been several attempts at implementing the structures suggested by S. B. Cohn through the use of bulky, costly, and large devices such as that shown by the above-mentioned U.S. Pat. No. 3,699,480 showing a cavity filter circulator coupled to an impedance circuit. Heretofore, none of these devices has been practical for low cost applications such as the secondary delay line in the multi-carrier power amplifiers discussed in connection with FIG.
1
.
Attempts to configure miniaturized implementations using the same designs employed in delay equalized cavity filters have thus far proved unsuccessful due to the cross coupling between the various lumped components of the filter. Heretofore, it was not thought possible to implement such a filter using discrete components.
The lack of shielding in the lumped elements necessarily influences and has a deleterious effect on the overall operation of delay equalization implementations using lumped elements. Conventionally, it was thought that cross coupling in this circuit would result in a device that is nonfunctional for its intended purpose. In the secondary loop, heretofore, it has been thought to be impossible to construct a flat delay response using lumped components configured in a miniaturized configuration because of the numerous stray inductances and the lack of shielding between resonators.
Because of these problems, the present state of the art is to use a coaxial delay line in the secondary path of the feed forward power amplifier. The coaxial delay line in the secondary path, however, has many of the problems discussed above and thus is not satisfactory. Accordingly, the present invention seeks to take an altogether new approach to equalizing the delay across the passband using lumped discrete components. The present invention further seeks to provide improved high and low power delay lines in the main and secondary paths, respectively.
SUMMARY OF THE INVENTION
One goal of one or more aspects of the present invention is a delay circuit that is suitable for the secondary path of a feed-forward high power amplifier. According to one embodiment of the invention, such a delay circuit includes a first and a second 3 dB quadrature hybrid coupler that are each preferably miniaturized. The first 3 dB quadrature hybrid coupler has a first node that receives an input signal, a second node that is coupled to a first resonator circuit, a third node that is coupled to a second resonator circuit and a fourth node that is an output node. The first and second resonators are each preferably an inductive-capacitive resonator circuit, and are resonant at a first resonant frequency. The delay response of the first 3 dB quadrature hybrid coupler as a function of frequency is substantially linearly decreasing for increasing frequency greater than the first resonant frequency. The second 3 dB quadrature hybrid coupler has a first node coupled to the fourth node of the first 3 dB quadrature hybrid coupler, a second node that is coupled to a third resonator circuit, a third node that is coupled to a fourth resonator circuit, and a fourth node that is an output node. The third and fourth resonators resonator circuit are each preferably an inductive-capacitive resonator circuit, and are resonant at a second

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