Multiplex communications – Duplex – Transmit/receive interaction control
Reexamination Certificate
1998-09-30
2002-03-12
Ton, Dang (Department: 2661)
Multiplex communications
Duplex
Transmit/receive interaction control
Reexamination Certificate
active
06356536
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to an improved front-end circuit for a radio transceiver, and more particularly to a front end circuit for a radio transceiver that has a common signal path for both the transmitted and received signals.
BACKGROUND OF THE INVENTION
For clarity, the following specification and claims assume that a wireless communication mobile terminal includes wireless radio transceivers, such as personal communication assistance, pagers, analog and digital cellular telephones and the like, which are configured to operate in wireless communication systems where mobile terminals communicate via terrestrial and satellite base stations to any number of telephony systems.
Time Division Multiple Access (TDMA) is a time-based method for sharing communication resources in a mobile communications system. In a TDMA system, each communication channel is divided into periodic “frames” with each frame subdivided into several equal duration time “slots”. Each mobile station is assigned a slot in the frame during which the mobile station transmits and receives information in short bursts. In a TDMA system, any given time slot may be used for transmissions in both directions—mobile station—to—base station and base station—to—mobile station. Since there are several slots per frame, a plurality of mobile users can simultaneously use each communication channel. The assignments of timeslots is coordinated by a centrally-located “master”, which, in a in traditional wireless system, is a base station. One of the primary benefits of using a Time-Division Multiple Access (TDMA) is to allow multiple users to time-share a limited radio frequency spectrum.
Time-Division Duplex (TDD) is a variation of the TDMA concept. In a TDD system, specific slots in time are designated for only portables to transmit and base stations to receive (and vice versa). With the hardware operating under these constraints, radio hardware designers are often able to share radio circuitry common to both transmitter and receiver, since signals are only present in either the receive or the transmit direction at any one time. This provides benefits of reduction of circuitry, resulting in reduced cost, smaller size and reduced complexity.
Included in the circuitry shared between receiver and transmitter is the antenna and radio frequency (RF) signal filtering. These items perform similar functions in the receive and transmit modes, but at vastly different signal levels. The receiver is designed for detecting and processing extremely small signals (on the order of pico Watts (10
−12
Watts)), and for that, semiconductor devices with great sensitivity are required. The transmitter, however, is producing power levels typically twelve or more orders of magnitude greater than that—typically 1 Watt. The large signals can damage the much more sensitive receiver circuitry, if the receiver is not protected, or isolated in some way. Thus, some sort of a switching function is needed for such a system.
A variety of solutions have been used over the years in which receiver and transmitters have shared antenna and filtering subsystems. Passive RF power combiners, for example, are designed such that the signal can be routed to each path at the same time, while maintaining isolation between transmitter and receiver. However, in general, they are not a good solution for this problem, as half the power goes to each path all the time. Since each circuit in a TDMA or TDD system is only used half the time that the radio is active, this is very inefficient. RF circulators, comprised of ferrite devices which pass signals in one direction with low loss, but in the opposite with high loss are another option. Until only recently such systems were not suitable for use in mobile radio transceivers because their size, weight and cost were prohibitive. Also, RF circulators might not by themselves provide sufficient performance.
Consequently, a transmit/receive (T/R) RF switch is most often present near the antenna of the radio for TDMA and TDD systems. This T/R switch is generally a single-pole-double-throw (SPDT) switch, but depending upon the presence and extent of antenna and/or other diversity devices, more poles and “throws” may be present.
A block schematic of a mobile communications device is shown in FIG.
1
. These systems generally include a transmitter
10
and receiver
12
alternately switched onto a common signal path
14
by an RF T/R switch
16
. The common signal path
14
leads to an antenna
18
and preferably includes a filter
20
, which is generally a bandpass filter (BPF). The transmitter
10
will usually include a power amplifier
22
while the receiver
12
will usually include a low-noise amplifier (LNA)
24
.
The RF switches in the earliest days of radio were electromechanical relay switches, which were slow to settle between states and suffered from early fatigue, since they were generally limited to less than 1 million operations. As semiconductors improved in technology, bandwidth, and cost, they became a much more attractive solution. Currently, with the speed and reliability required for modern systems (PWT/DECT switches at least 200 times per second during a normal phone call), solid-state (semiconductor) switches are necessary.
Several topologies of RF switch designs using “PiN” diodes as the switching elements are commonly used. These diodes exhibit a distinct feature that when forward-biased, they appear as a short circuit to radio frequencies, and when not biased (or reverse-biased) appear as a very high impedance to RF. In RF switches, PiN diodes are alternately biased on and off to produce short and open circuits, respectively, in different arrangements to form SPST, SPDT and DPDT RF switches.
FIG.
2
(
a
) shows one of the more commonly-used topologies for the RF switch
16
, the series-shunt configuration. This configuration generally includes a series PiN diode
26
and a shunt PiN diode
28
. Both diodes are biased on simultaneously during the transmit mode through an inductor
30
by a control signal, such as V
Bias
. The block diagrams in FIG.
2
(
b
) shows the effective functionality of this circuit. Note that although the shunt diode
28
is biased ON, and thus low resistance, its appearance to the antenna in the transmit mode is that of an open circuit, due to the impedance transformation occurring via the quarter-wavelength (&lgr;/4) transmission line
32
. The transmission line length may be actual or effective based on inductance and capacitance affecting or added to the signal path.
Great strides have also been made in recent years to bring the cost of Gallium Arsenide (GaAs) components to a level more competitive with silicon and thus GaAsFET (GaAs field effect transistor) RF switch solutions have become another widely used solution, generally in the form of a Monolithic Microwave Integrated Circuit (MMIC). The MESFET's are also biased on or off to produce either low or very high resistances to radio frequency signals. The most common circuit topology used is a “branched” T-pad configuration shown in FIG.
3
(
a
), where in the “ON” arm of the switch, the series FETs
40
,
42
are ON (low resistance) and the shunt (to-ground) FET
44
is OFF (high-impedance). In contrast, the OFF arm of the switch has the two series FETs
46
,
48
presenting a high series impedance to signal, while also shorting the signal to ground with the shunt FET
50
between the two series devices
46
,
48
. FIG.
3
(
b
) shows the effective operation of the circuit in a transmit mode. The switch positions are reversed during the receiving operation.
Most of the RF switches are designed and specified to operate in a 50-Ohm environment, which has become the most common standard, due to the ease of implementing passive and active circuits at practically any radio frequency at this impedance. The loss of signal through the intended signal paths (antenna to receiver and transmitter to antenna), called insertion loss, is a parameter of importance for these devices. But without a
Coats & Bennett, PLLC.
Ton Dang
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