Wave transmission lines and networks – Coupling networks – Balanced to unbalanced circuits
Reexamination Certificate
2000-09-15
2003-09-16
Lee, Benny (Department: 2817)
Wave transmission lines and networks
Coupling networks
Balanced to unbalanced circuits
C333S026000, C333S136000
Reexamination Certificate
active
06621370
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to a device for balanced to unbalanced line transformation, that is, a balun, and more particularly to a space-optimized balun that utilizes a combination of lumped and distributed circuit elements.
BACKGROUND
A balun is a device used to convert between balanced and unbalanced lines for input and output in an electrical system. Special considerations apply to the application of a balun to microwave systems that include printed circuit boards. As is commonly known in the art,
FIG. 1
is a diagram illustrating a ring or ratrace design that is used in printed circuit boards. The ring balun
100
is made from microstrip line
102
, including a conductive material such as copper. (
Microwave Circuit Design,
G. D. Vendelin, A. M. Pavio, and U. L. Rohde, John Wiley and Sons, 1990).
For the unbalanced line the ring balun
100
includes a single-ended port
104
and an isolation port
106
. For the balanced line the ring balun
100
includes a first differential port
108
and a second differential port
110
.
The distances along the microstrip
102
between the ports are related to the operational wavelength &lgr;. As shown in
FIG. 1
, in a clockwise direction, the distance (measured circumferentially) between the single-ended port
104
and the first differential port
108
is &lgr;/4, the distance between the first differential port
108
and the isolation port
106
is &lgr;/4, the distance between the isolation port
106
and the second differential port
110
is &lgr;/4, and the distance between the second differential port
110
and the single-ended port
104
is 3&lgr;/4. In typical operation, the single-ended port
104
is driven by a signal at an operational frequency f and a 50 ohm (&OHgr;) resistor is attached to the isolation port
106
. A differential signal is obtained from difference of the outputs at the first differential port
108
and the second differential port
110
. The first differential port
108
and the second differential port
110
together define an open-ended port.
For the ring balun
100
the operational wavelength &lgr; is related to the operational frequency f through the following relation:
λ
=
c
f
ϵ
r
(
1
)
where c is the speed of light and ∈
r
is a substrate dielectric constant associated with the microstrip
102
. Typically the operational frequency f is fixed by the application and the frequency limits design choices for the properties of the microstrip
102
.
For example, for the case where f=5.3 GHz and ∈
r
=3.38 (i.e., the Rogers Corp. substrate material sold under the trademark RO4003®), the circumferential distance between the single-ended port
104
and the first differential port
108
is approximately &lgr;/4=350 mils. In this case, the ring balun
100
has a diameter of approximately 668 mils and covers an area of approximately 0.35 inch
2
. The ring balun
100
can be approximately contained within a square having a side of length 668 mils and having an area of 0.45 inch
2
.
The desirability of reducing the space occupied by elements on circuit boards has led to limited attempts to reduce the space occupied by the ring balun
100
by some modification of the geometry while keeping the essential features of the design. A difficulty with modifying the geometry of the ring balun
100
may arise due to interference (or coupling) between segments of microstrip that are relatively close together. This interference may adversely affect performance of the ring balun
100
.
For example,
FIG. 2
shows a modified ring balun
150
also made from microstrip line
152
and also having a single-ended port
154
, an isolation port
156
, a first differential port
158
and a second differential port
160
. The circumferentially measured distances between the ports
154
,
156
,
158
,
160
for the modified ring balun
150
are prescribed in terms of the wavelength &lgr; as in the ring balun
100
. However, in the modified ring balun
150
the arc between the first differential port
158
and the second differential port
160
is inverted, thereby saving some space on the circuit board while causing minimal interference near the cusps formed at the first differential port
158
and the second differential port
160
. However, the improvement in reduced space is minimal since the approximate area of a square that contains the modified balun
150
is still 0.447 inch
2
.
Thus, the requirements for the space taken by a printed balun on a circuit board are driven in part by the desired operational frequency and the physical properties of the microstrip. Attempts to modify the conventional ring balun design have led to limited improvements in minimizing the required area on a circuit board.
SUMMARY OF THE INVENTION
It is therefore an object of the invention to overcome the above-described problems and challenges.
The present invention fulfills this object and others by providing in a first aspect of the present invention a lumped-distributed balun. The lumped-distributed balun includes a single-ended port, a first differential port, a second differential port, a first phase shifter circuit, and a second phase shifter circuit. The first phase shifter circuit includes a first inductor and a first capacitor. The first inductor is coupled to the single-ended port and the first differential port. The first capacitor is coupled to the first differential port and is adapted to be coupled to ground potential. At least one of the first inductor and the first capacitor is implemented as a transmission line structure. The second phase shifter circuit includes a second inductor and a second capacitor. The second inductor is coupled to the second differential port and is adapted to be coupled to ground potential. The second capacitor is coupled to the single-ended port and the second differential port. At least one of the second inductor and the second capacitor is implemented as a lumped element.
A balun according to a presently preferred embodiment is presented in a second aspect of the present invention. The balun performs unbalanced to balanced line transformation. The balun includes a first shifting unit and a second shifting unit. The first shifting unit shifts an input signal having an input phase value to a first output signal having a first output phase value. The first shifting unit includes at least one distributed circuit element. The second shifting unit shifts the first input signal having the input phase value to a second output signal having a second output phase value. The second shifting unit includes at least one lumped circuit element. The first and second output phase values have a difference of 180 degrees.
A lumped-distributed balun according to a presently preferred embodiment is presented in a third aspect of the present invention. The lumped-distributed balun includes a single-ended port, a first differential port, a second differential port, a first microstrip, a second microstrip, a third microstrip, and a lumped element capacitor. The first microstrip is coupled to the single-ended port and the first differential port. The second microstrip is coupled to the first differential port. The second microstrip is open-circuited. The third microstrip is coupled to the second differential port and is adapted to be coupled to ground potential. The lumped element capacitor is coupled to the single-ended port and the second differential port.
A method of designing a printed lumped-distributed balun according to a presently preferred embodiment is presented in a fourth aspect of the present invention. A first network is configured to satisfy balun performance criteria. The first network includes lumped elements. The criteria include a requirement that signal outputs are approximately 180 degrees out of phase. Impedance values for the first network are determined at an operating frequency. A second network is configured in accordance with the impedance values. The second network is configured by replacing at least one of the lumped elemen
Atheros Communications Inc.
Lee Benny
Pillsbury & Winthrop LLP
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