Telecommunications – Receiver or analog modulated signal frequency converter – Local control of receiver operation
Reexamination Certificate
2000-05-09
2003-02-18
Le, Thanh Cong (Department: 2684)
Telecommunications
Receiver or analog modulated signal frequency converter
Local control of receiver operation
C455S255000, C455S258000, C455S264000, C331S176000, C331S17700V
Reexamination Certificate
active
06522871
ABSTRACT:
BACKGROUND OF THE INVENTION
I. Field of the Invention
The present invention relates to electronic circuits. More particularly, the present invention relates to a novel and improved method and apparatus for compensating Local Oscillator (LO) frequency error by controlling the LO operating environment.
II. Description of the Related Art
Accurate frequency sources are vital to the operation of numerous electronic systems and devices. Frequency sources are used as timing sources within electronic devices and are also used as Local Oscillators (LO) to tune electronic devices to desired communication channels.
Many types of accurate frequency sources are available. The specific type of frequency source implemented within a particular application is determined according to the design constraints of the particular application. Atomic clocks exhibit extreme levels of frequency accuracy, however, their size, cost, and absence of tuning range greatly limit their actual application within an electronic system. Similarly, accurate frequency sources can be designed utilizing the piezoelectric effect of quartz crystals. The small size and relative accuracy of quartz crystal based frequency sources make them popular for most consumer based electronic devices.
The application determines the type and frequency accuracy required of a frequency source. A receiver used for Global Positioning System (GPS) applications requires an LO with a high level of frequency accuracy in order to quickly acquire and maintain synchronization with the signals provided on the PS carrier frequencies transmitted from the satellites. An overview of GPS helps to explain the requirement for LO frequency accuracy in a GPS receiver.
GPS is commonly used for position determination. GPS accomplishes position determination using geometric principles. A constellation of GPS satellites orbits the earth. A receiver can determine its exact position by knowing the positions of the satellites and calculating the distance from the receiver to each of a number of satellites.
The GPS receiver calculates the distance from the satellite to the receiver by determining the time it takes for a signal transmitted by the satellite to reach the receiver. Once the receiver determines its distance from the satellite it knows that it resides on a locus of points equidistant from the satellite. The satellite appears as a point source and the locus of points equidistant from a point is a spherical surface. When the receiver determines its distance from a second satellite the receiver knows that its position is located somewhere on a second spherical surface. However, the potential positions are greatly reduced when the distance from two satellites is known. This is because the location of the receiver lies somewhere on the intersection of the two spherical surfaces. The intersection of two spherical surfaces is a circle. Therefore, the receiver knows that its position lies on the circle of intersection. Determining the distance from the receiver to a third satellite creates a third spherical surface. The third spherical surface intersects the first two surfaces and also intersects the circle that defines the intersection of the first and second spherical surfaces. The intersection of the three spherical surfaces results in two distinct points where the receiver may be located. Once the two points generated by the intersection of three spherical surfaces has been determined the receiver can estimate which of the two points is the correct location or the receiver can determine its distance from a fourth satellite.
The receiver can estimate which one of the two points is its correct location once the distances from three satellites have been determined. This can be done because one of the two points is not a likely location. The correct one of the two points will likely be near the surface of the earth whereas the incorrect point likely will be very far above the surface of the earth or deep within the surface of the earth. The exact position of the receiver will be known if the distance from a fourth satellite is determined. The exact position is known using four satellites because the intersection of four spherical surfaces results in only one point.
The main problem in a GPS implementation is the accurate determination of the distance from the satellite to the receiver. Distance from the satellite to the receiver is calculated by measuring the time of arrival of a signal transmitted from the satellite to the receiver. Each satellite transmits two carrier frequencies each modulated with a unique pseudo random code. One of the carrier frequencies operates at 1575.42 MHz and the other carrier frequency operates at 1227.60 MHz. The receiver demodulates the received signal to extract the pseudo random code. A locally generated pseudo random code is synchronized to the demodulated pseudo random code. The delay between the two pseudo random codes represents the time of arrival of the transmitted signal. The distance from the satellite can then be determined by multiplying the time of arrival by the velocity of light.
All of the transmitting satellites are time synchronized. However, the mobile receiver is only weakly synchronized to the satellites. The weak time synchronization of the receiver to the satellites introduces errors into the position determination. As stated above, a distinct time of arrival corresponds to a distinct distance. The locus of points equidistant from a point is a spherical surface with a radius equal to the distance. However, if the time of arrival is only known to lie within a range of times, that is a measured time plus or minus some error, then the distance can only be known to lie within the corresponding range of values. The locus of points equidistant from the source is a spherical shell in the case where the distance is only known to lie within a range of values. The thickness of the spherical shell is equal to the error in the distance measurement. The intersection of three spherical shells, each shell corresponding to a position estimate based on an additional satellite, results in two solids, one of which represents the position of the receiver. Recall that in the case of discrete distances the intersection of the three spherical surfaces results in two points rather than two solids.
The time synchronization problem is partially solved by including the distance measurement from a fourth satellite. First, the time error is assigned an assumed value, even zero. Then the distances from three satellites are determined. As explained earlier, the intersection of the three spherical surfaces defined by these three distance measurements results in two distinct points, one of which is the position of the receiver. The distance from a fourth satellite defines a fourth spherical surface. Ideally, in the case of no timing error, the fourth spherical surface intersects the other three spherical surfaces at only one point. However, the four spherical surfaces do not intersect when a timing error is present. There is no timing error between the satellites. Therefore, the timing error from the receiver to one satellite is the same as the timing error from the receiver to any of the satellites in the constellation. The timing error can be determined by adjusting the value of the assumed timing error. The timing error is determined when the four spherical surfaces intersect in a single point.
Resolution of the timing error is only one of the problems that must be dealt with when position determination using GPS is implemented. A GPS position determination receiver must be implemented in a small physical size at a relatively low cost. The size and cost constraints become increasingly important when the GPS receiver is implemented in a consumer oriented device. New requirements for wireless phones include the ability to determine a caller's location. The specific location of a wireless telephone is important in the case of an emergency call such as a 911 call within the United States. Yet, despite the physical design constraints,
Patrick Christopher
Younis Saed G.
Brown Charles D.
Cheatham Kevin
Cong Le Thanh
Harry Andrew T
Qualcomm Incorporated
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