Telecommunications – Receiver or analog modulated signal frequency converter – Signal selection based on frequency
Reexamination Certificate
2000-01-05
2002-10-08
Trost, William (Department: 2683)
Telecommunications
Receiver or analog modulated signal frequency converter
Signal selection based on frequency
C455S131000, C455S182100, C455S192100, C375S219000, C375S345000, C714S100000
Reexamination Certificate
active
06463266
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatuses and methods for improvement of radio transmitter and receiver frequencies of a local radio communication unit that is communicating digital data with a remote communication unit.
2. Description of Related Art
Communication systems often comprise a plurality of local units such as radiotelephone handsets that communicate digital data by radio transmissions with a remote unit such as a cellular phone base station. The radio frequencies of the communication channels and frequency error tolerances for transmissions on the channels are typically specified by regulatory rules. For example, for the GSM mobile telephony standard [Ref. 1], the frequency tolerance is specified to be 0.05 ppm for the base station and 0.1 ppm for the handset. The frequency tolerances ensure that the level of radio interference between channels is tolerable and that accurate data demodulation is possible at the local unit and the remote unit. In the base station the transmitter and receiver radio frequencies are typically phase locked to very stable reference oscillator signal available in the base stations in order to meet the radio frequency tolerances specified by regulations. However, the cost of such stable reference oscillators are typically prohibitive for the handsets, so that provision for accurate transmitter and receiver frequencies in the local unit at lowest possible cost is important.
In a local communication unit the conventional solution for accurate radio frequencies is the use of a relatively low cost voltage controlled crystal oscillator (VCXO) to serve as a reference oscillator wherein the oscillator frequency is approximately linearly related to the magnitude of a VCXO control voltage. The transmitter and receiver radio frequencies are phase locked to the VCXO oscillator frequency, but the frequency error in parts per million (ppm) of the transmitter and receiver, or equivalently the frequency error of the VCXO, is not sufficiently accurate with a free running VCXO to meet frequency tolerance specifications. Therefore, the VCXO control voltage is adjusted based on estimated radio frequency error of the receiver in accordance with well known feedback control principals such that the radio frequency errors of the receiver and transmitter are sufficiently reduced by feedback control principals. This methodology for radio frequency control in the local unit is a conventional automatic frequency control (AFC) loop. With modern electronics the analog VCXO control voltage is generated by a digital-to-analog converter (DAC), and the digital feedback control signal applied to the DAC is produced by AFC digital control logic.
A drawback associated with the conventional AFC loop is that the achievable accuracy of the normalized VCXO frequency in ppm, or equivalently the normalized transmitter and receiver RF frequencies in ppm derived therefrom, is limited by the digital-to-analog converter (DAC) quantization error. The DAC step size or quantization error associated with a least significant bit change in the DAC control signal represents the frequency control resolution of the VCXO. The associated VCXO frequency quantization error is an irreducible systematic bias error and not a random error reducible by averaging or filtering in the AFC tracking loop.
The magnitude of frequency quantization error is dependent primarily on two parameters, the number of bits N represented in the DAC and the characteristic slope of the VCXO. The characteristic slope S of the VCXO is defined as the ratio of the normalized frequency change in units of parts per million (ppm) to change in the control voltage. Let B equal the voltage control span of an N-bit DAC. Then the VCXO frequency quantization error q in Hz/LSB is given by
q=S*B/
2
N
Generally, a lower cost VCXO requires a larger frequency swing per volt to correct the larger frequency errors; i.e., a lower cost VCXO tends to have a larger value for the characteristic slope S and hence larger VCXO quantization error. Therefore, with the conventional AFC loop, usage of a lower cost VCXO is constrained by the maximum tolerable level of DAC quantization error which is related to radio frequency tolerance specifications. This is an important constraint because the VCXO typically represents a major cost component in a local unit such as a radio telephone.
As a numerical example of VCXO frequency quantization error induced by the VCXO DAC, the NDK5411B [Ref. 2], which is a high quality VCXO suitable for radio telephone usage, has a characteristic slope of S=15 ppm per volt. A typical DAC implemented in modern integrated circuit technology has a voltage span of B=2 volts with N=10 bits of resolution, and the resultant VCXO frequency quantization error is q=0.03 ppm/LSB. The VCXO frequency quantization error represents a substantial fraction of the frequency tolerance of 0.1 ppm allowed for local units under GSM specifications so that there is very little margin for usage of a lower cost VCXO having a larger quantization error. This example teaches how VCXO quantization error is an important factor in the cost of the local unit.
Another drawback of the conventional AFC is susceptibility of the VCXO frequency to DAC switching noise in addition to the DAC quantization step when the ADC digital control word changes. A common method for mitigation of DAC switching noise is the introduction of a filter with a long time constant, for example one millisecond, between the VCXO and its controlling DAC. This has the drawback that such filtering in the VCXO controller DAC response can significantly extend the settling time of the VCXO in recovery to its original reference frequency immediately after termination of sleep mode. Sleep mode is an interval of inactivity between scheduled communication transactions when all inactive components of the local unit, including VCXO, is powered off to conserve battery life for portable local units. Near termination of sleep mode, the VCXO is powered back on with scheduled allowance for oscillator settling time. Thus, a longer settling time implies a reduction in the power down interval with a consequent reduction in the battery savings. Sleep mode is commonly used in portable communication units such as pagers and cellular phones during idle intervals in standby mode.
Another deficiency of the conventional AFC loop is that the local unit processing of data bursts exhibit large timing errors and VCXO frequency errors when the local unit is powered on. AFC tracking loops are typically inefficient in reducing such large initial errors and for this reason, before the AFC tracking loop is closed, a prior acquisition phase is typically employed to reduce the large initial time and frequency errors to sufficiently small values that efficient closed loop AFC tracking can be switched in and accurate data demodulation performed. The reduction of timing error is a synchronization function which is performed by conventional means and not described in this disclosure. The acquisition time, or duration of the acquisition phase, represents a wasted overhead interval before track mode when accurate data communications is possible so that acquisition time must be reduced to the smallest possible value.
In the local unit local oscillator (LO) signals are generated to serve as mixer signals for frequency upconversion and downconversion. Typically, a subsystem of phase locked loop (PLL) frequency synthesizers and voltage control oscillators (VCO) generate the LO signals which are phase locked to the VCXO signal [Ref. 3]. The PLL/VCXO subsystem and the frequency downconversion and upconversion operations are configured so that the local unit transmitter and receiver radio frequencies have zero frequency error on the assigned channels when the VCXO is operating at its specified nominal frequency. At initiation of acquisition mode, the normalized VCXO frequency error in units of ppm, or equivalently the normal
Broadcom Corporation
Kondracki Edward J.
Miles & Stockbridge P.C.
Tran Congvan
Trost William
LandOfFree
Radio frequency control for communications systems does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Radio frequency control for communications systems, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Radio frequency control for communications systems will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2957248