Pulse or digital communications – Receivers – Angle modulation
Reexamination Certificate
1999-05-04
2002-04-02
Tse, Young T. (Department: 2634)
Pulse or digital communications
Receivers
Angle modulation
C375S133000, C375S222000, C375S281000, C375S345000, C329S304000
Reexamination Certificate
active
06366622
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to wireless communications, and more particularly, to a radio for use in wireless communications and an apparatus and method which uses a radio, modem and controller for implementing wireless communications.
2. Description of the Related Art
Conventional radios used in wireless communications, such as radios used in conventional cellular telephones, typically include several discrete RF circuit components. This results in such radios having a large size and footprint, being expensive and power consuming. In order to illustrate this, it is useful to analyze a conventional implementation of circuitry that could be used, for example, as the receiver portion of a conventional radio. Specifically, a traditional receiver architecture may employ superhetrodyne techniques as shown in FIG.
1
. In a superhetrodyne architecture an incoming signal is frequency translated from its radio frequency (RF) to a lower intermediate frequency (IF). The signal at IF is subsequently translated to baseband where further digital signal processing or demodulation may take place. Receiver designs may have multiple IF stages. The reason for using such a frequency translation scheme is that circuit design at the lower IF frequency is much more manageable for signal processing. It is at these IF frequencies that the selectivity of the receiver is implemented, automatic gain control (AGC ) is introduced, etc.
In addition to more manageable circuit design, high Q (i.e., high “quality factor”) filters are also easier to implement at IF. High Q filters are used to meet the selectivity and spurious rejection requirements dictated by wireless systems. Surface acoustic wave (SAW) and ceramic technology are typically used for the filtering depending on the frequency of operation. Although these respective technologies have improved in terms of size and performance they are still relatively large. Moreover, due to the relatively high frequency of the most IFs, it is not realistic, yet, to implement this filter using integrated circuit (IC) techniques.
As an alternative to the superhetrodyne techniques, a direct conversion receiver architecture may be used. This is shown in FIG.
2
. This scheme translates the incoming RF signal directly to baseband. The direct conversion architecture has several advantages. First, there is no need for the high-Q filters required for traditional superhetrodyne architecture. Generally, all that is needed is a broadband RF filter which is used to reduce the dynamic range requirements of the RF down-converter. Second, there are a limited number of RF circuit blocks. Third, oscillators may be reduced to one. Fourth, it offers the smallest size solution since bulky off-chip filters are no longer required. Finally, because the low-pass channel filters are readily integrated, a fully integrated solution is achievable.
Although the direct conversion receiver architecture has several advantages, there are several practical implementation problems. In general, wireless communications devices use high-frequency signals: 900 MHz to 1900 MHz for cellular phones and higher (up to 6 GHz) for other systems, such as wireless LANs. Radios for the so called “Bluetooth standard” (discussed below) operate in the unlicensed ISM band at 2.4 GHz. Signals at such frequencies are difficult to generate and control. They also have a tendency to interfere with each other, as they are easily coupled by parasitic properties present in all electronic components, including integrated circuits. In ICs, many of the undesirable parasitic effects result from the conductive silicon substrate on which the circuits are fabricated.
Specifically, in the direct conversion receiver of
FIG. 2
, due to limited local oscillator (LO) to RF isolation in the down-converter, and limited reverse isolation in the low noise amplifier (LNA)
10
, an amount of LO signal can appear at the output of the receiver and effectively be transmitted at the antenna. Wireless regulatory authorities limit the amount of spurious signal that can be radiated by the receiver, so limiting the amount of LO radiation is necessary to meet these specifications. In addition, LO leakage causes particular problems for direct conversion receivers. The lack of LO isolation causes self mixing in the direct down converter that manifests as a DC offset at baseband.
Specifically, there are several mechanisms through which LO leakage may occur. For example, there may be conducted paths between components. This occurs because there is limited isolation from the LO port of the mixers
12
to the RF port of the mixers
12
. There is also limited reverse isolation through the low-noise amplifying stages preceding the mixers
12
. A parasitic signal path for signals through the substrate, as well as a lateral signal path through the substrate, can also occur. In addition to the conducted path, there may also be radiated paths via the bond wires used to interconnect the circuit blocks to the outside world. The bond wires act as antennas and couple RF energy, such as that of the LO, to adjacent pins.
The traditional solution for reducing the amount of signal that appears at the antenna port is to have the LO, i.e., the voltage controlled oscillator (VCO)
14
, at a different frequency than the incoming RF signal, as is indicated in FIG.
2
. This utilizes the filtering effects of matching, etc., to reduce the amount of LO leakage. This solution, however, requires the use of dividers or multipliers
16
, as shown in
FIG. 2
, which adds additional circuitry. Furthermore, this solution does not solve all of the problems of LO leakage associated with direct conversion receivers.
The market requirements for today's mobile communication terminals are such that wireless product manufacturers have gone to smaller and smaller form factors with improved performance and lower cost. This has resulted in radio designers, for both circuits and systems, looking for ways of accommodating these requirements. Therefore, it follows that it would be highly desirable to have an improved radio design that is a low cost, low power and small size solution, and that overcomes the disadvantages discussed above. Such an improved radio design would have many uses in wireless communications, including for example, use in cellular telephones, cordless telephones, personal computer (PC) interconnections, etc.
With respect to PC interconnections, at present, standard wire interconnects are used to link together PC based products, such as laptop and notebook computers and personal digital assistants (PDAs). For example, RS232and Universal Serial Bus (USB) are commonly used standards that are offered as connections on many devices. Some wireless interconnects are also being used, such as infrared (IR). IR suffers from the disadvantage of being somewhat directional in its ability to communicate with other IR devices. It has been predicted that in the near future there will be a convergence of traditional wireless and computer technologies, such as cellular phones, and PC based products. One key to a successful implementation of a standard that facilitates this convergence is to make it almost effortless for the user to use.
There are several wireless communications standards either in existence or being proposed, such as for example, Home RF, IEEE 802.11, etc. One wireless communications standard that is currently being proposed is the “Bluetooth” standard. Bluetooth is a global specification for wireless connectivity. It is based on a low-cost, short-range radio link that enables wireless communication of data and voice and facilitates protected ad hoc wireless connections for stationary and mobile communication environments. The proposal of Bluetooth is to offer a solution that yields rugged wireless connectivity. The Bluetooth standard will be discussed herein as an example of a wireless communications standard, but it should be understood that the teachings of the present invention may be applied to any type of wi
Bourk Terrance R.
Brown Stephen Joseph
Chang Glenn
Estrada Andrew Xavier
Grilo Jorge A.
Jaquez & Associates
Jaquez, Esq. Martin J.
Silicon Wave Inc.
Tse Young T.
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