Wave transmission lines and networks – Plural channel systems – Having branched circuits
Reexamination Certificate
2002-04-02
2004-02-17
Nguyen, Long (Department: 2816)
Wave transmission lines and networks
Plural channel systems
Having branched circuits
C333S025000, C333S112000
Reexamination Certificate
active
06693499
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to a lumped element balun for a ring mixer and, more particularly, to a circuit layout for a lumped element ring balun, where the balun circuit elements are configured on a monolithic substrate in a manner that conserves space and minimizes parasitic inductances.
2. Discussion of the Related Art
Modern communications systems employ transmitter and receiver designs that attempt to maximize the utilization of the assigned frequency bandwidth associated with the various communications channels because signal bandwidth is a costly investment for the system provider. Maximizing the utilization of the assigned bandwidth translates to providing transmitters and receivers that have extremely high performance. However, the transmitters and receivers must also be low cost. The radio frequency (RF) components in a communications system typically are the highest cost items because they are usually custom designed elements and are not mass produced.
One RF component that falls into this category is a frequency mixer. A frequency mixer mixes two RF or intermediated frequency (IF) signals to create a sum and difference frequency for frequency down-conversion or frequency up-conversion purposes. For example, the signal received in the receiver is mixed with a local oscillator (LO) signal to generate an IF signal suitable for subsequent signal processing. Typically, mixers are critical for setting the performance of the RF signal chain. Thus, mixers with lower intermodulation products and high dynamic range that can be implemented as a cell in an RF integrated circuit (IC) are needed.
One known mixer employed in RF communications systems of the type being discussed herein is referred to in the art as a ring mixer. A ring mixer employs four diodes connected in a ring configuration that mix the RF signal and the LO signal to generate the IF signal. The ring mixer employs a hybrid or balun that splits the RF signal and the LO signal into signals that are 180° out of phase with each other. A ring mixer balun is disclosed in Sturdivant, Rick, “Balun Designs for Wireless, . . . Mixers, Amplifiers and Antennas,” Applied Microwave, Summer 1993, pps. 34-44. The split RF signals and the LO signals are applied to the mixer between the diodes at opposite corners of the ring. The diodes are switched on and off in response to the positive and negative portions of the RF signals to provide modulation. The IF signal is generated between the diodes at the other opposite corners of the ring.
FIG. 1
is a schematic diagram of a known lumped element ring balun circuit
10
. The ring balun circuit
10
includes an electrical ring
12
having four sides defining corner nodes
14
,
16
,
18
and
20
. The ring balun circuit
10
includes an electrical configuration of capacitors C
1
-C
6
, inductors L
1
-L
4
and a resistor R
1
. Each side of the ring
12
includes a capacitor and an inductor that combine to provide a high pass filter that forms a lumped element transmission line that causes a delay of a signal propagating therethrough. As is known in the art, current leads voltage on a capacitor, and voltage leads current on an inductor. Therefore, a series capacitor and shunt inductor provide a phase lead of the signal, and a series inductor and a shunt capacitor provide a phase lag of the signal.
An RF input signal is applied to the node
14
, and the filters provide an RF signal at the node
20
that is 90° out of phase with the signal at the node
14
, an RF signal at the node
18
that is 180° out of phase with the signal at the node
14
, and an RF signal at the node
16
that is 270° out of phase with the signal at the node
14
. Output lines
54
and
56
are coupled to the nodes
20
and
16
, respectively, to provide output signals that are 180° out of phase with each other. DC blocking capacitors
24
and
26
are provided in the output lines
54
and
56
to prevent DC signals from the mixer from entering the ring balun circuit
10
.
The ring balun circuit
10
is applicable for a ring mixer, but is limited in use for other types of mixers, such as star mixers and double doubly balanced mixers, because of the complexities in providing a dual balun in the ring design. Therefore, other balun designs are employed in the art for other types of mixers.
FIG. 2
is a schematic diagram of a lumped element dual-balun circuit
30
including a first balun
32
and a second balun
34
that has particular application for use in combination with a star mixer or a monolithic microwave integrated circuit (MMIC) double doubly balanced mixer (DDBM). The dual-balun circuit
30
receives an RF input signal, and the first balun
32
outputs two signals that are 180° out of phase with each other and the second balun
34
outputs two RF signals that are 180° out of phase with each other. A dual-balun structure of this type is disclosed in Chiou, Hwann-Keo, et al., “Miniature MMIC Star Double Balanced Mixer Using Lumped Dual Balun,” Electronics Letters, Vol. 33, No. 6, Mar. 13, 1997, pps. 503-505, and Chiou, Hwann-Keo, et al., “A Miniature MMIC Double Doubly Balanced Mixer Using Lumped Dual Balun for High Dynamic Receiver Application,” IEEE, Microwave and Guided Wave Letters, Vol. 7, No. 8, August 1997, pps. 227-229.
The dual-balun circuit
30
employs inductor and capacitor filter networks in the same manner as the balun circuit
10
discussed above to provide the RF signals that are 180° out of phase with each other. The balun
32
includes a filter made up of inductor L
1
and capacitor C
1
and a filter made up of inductor L
2
and capacitor C
2
. Likewise, the balun
34
includes a filter made up of inductor L
3
and capacitor C
3
and a filter made up of inductor L
4
and capacitor C
4
. In the balun
32
, the inductor L
1
is coupled to the capacitor C
1
at node
36
, the inductor L
1
is coupled to the capacitor C
2
at node
38
, the capacitor C
2
is coupled to the inductor L
2
at node
40
, and the inductor L
2
is coupled to the capacitor C
1
at node
42
. In the balun
34
, the inductor L
3
is coupled to the capacitor C
3
at node
44
, the inductor L
3
is coupled to the capacitor C
4
at node
46
, the capacitor C
4
is coupled to the inductor L
4
at node
48
, and the inductor L
4
is coupled to the inductor C
3
at node
50
. The RF input signal is applied to the nodes
36
and
44
. An RF output signal that is in phase with the RF input signal is provided at the nodes
42
and
50
, and an RF output signal that is 180° out of phase with the RF input signal is provided at the nodes
38
and
46
.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, a circuit layout for a lumped element ring balun is disclosed where the elements of the balun are patterned on a monolithic substrate in a compact design. The balun includes four inductors and a plurality of capacitors electrically coupled together to filter and delay an RF input signal to provide corresponding RF output signals that are out of phase with each other. The inductors are symmetrically disposed in a rectangular area on the substrate. A first pair of the inductors is positioned at one end of the rectangular area where the inductors are adjacent to each other. A second pair of the inductors is positioned at an opposite end of the rectangular area, where the inductors are adjacent to each other. All of the capacitors are formed on the substrate in a central circuit area between the first pair of inductors and the second pair of inductors.
The design employs metallized traces patterned on the substrate to provide electrical coupling between the inductors and the capacitors, where traces are provided at different levels in the substrate. Each inductor includes a winding having an inner end and an outer end that are electrically coupled to circuit elements in the circuit area. The inner end of each winding is coupled to a trace that extends under the winding through an air bridge to be electrically isolated therefrom. The length of the
Goyette William R.
Lam Trung H.
Peterschmidt Karl D.
Miller John A.
Nguyen Long
Northrop Grumman Corporation
Warn, Burgess & Hoffmann, P.C.
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