Method and apparatus for frequency synthesis in direct-conversion transmitters
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Solution Overview
Problem
Direct-conversion transmitters face challenges in reconciling narrow loop bandwidth for sideband phase noise suppression with resistance to frequency pulling, and two-step architectures require expensive high-Q filters and additional LO circuitry, increasing cost and size.
Innovation Solution
A transmitter circuit using a first phase-locked loop (PLL) with a second PLL as a translational PLL, along with frequency dividers to generate an intermediate frequency feedback signal, allowing for flexible selection of transmit frequencies and reduced VCO pulling effects, thereby enabling narrow loop bandwidth for low phase noise and wide bandwidth for pulling resistance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the loop bandwidth of the frequency synthesizer is kept narrow to suppress sideband phase noise, then ACP suppression performance is improved, but the synthesizer becomes more susceptible to frequency pulling from high-power transmit signals
Solution Approach 1:
The frequency synthesizer is divided into two separate loops: a narrow-loop synthesizer for generating the main transmit frequency with low phase noise, and a wide-loop translational synthesizer for frequency translation that provides pulling resistance. This segmentation allows each loop to be optimized for its specific function without compromise.
Solution Approach 2:
A wide-loop translational synthesizer acts as an intermediary between the narrow-loop synthesizer and the modulator. It translates the narrow-loop output to the final transmit frequency while providing isolation from pulling effects, thereby protecting the phase noise performance of the narrow-loop synthesizer.
2Reliability
If the loop bandwidth is increased to improve resistance to frequency pulling, then pulling resistance is improved, but sideband phase noise suppression deteriorates
Solution Approach 1:
The system segments the frequency synthesis function into two specialized loops: one optimized for phase noise performance (narrow bandwidth) and one optimized for pulling resistance (wide bandwidth). The narrow-loop synthesizer generates a clean fundamental frequency, while the wide-loop translational synthesizer handles frequency translation with pulling resistance.
Solution Approach 2:
The wide-loop translational synthesizer serves as an intermediary that protects the narrow-loop synthesizer from pulling effects while maintaining low phase noise. It translates frequencies without allowing transmit signal modulations to affect the narrow-loop VCO.
3Reliability
If a two-step frequency conversion architecture is used to reconcile pulling resistance and ACP performance, then both performance metrics are improved, but circuit complexity and cost increase due to additional VCOs and phase locked loop circuitry
Solution Approach 1:
The wide-loop translational synthesizer performs multiple functions: it provides pulling resistance, translates frequencies, and generates the LO signals for the modulator. This multi-functionality reduces the need for separate dedicated circuits for each function.
Solution Approach 2:
The system merges the frequency translation function with the second LO generation function in the wide-loop translational synthesizer. By combining these functions, the patent reduces the number of separate VCOs and PLL circuits needed compared to traditional two-step architectures.
4Reliability
If a translational loop architecture is used to mitigate VCO injection pulling issues, then pulling resistance is improved, but additional VCO and phase locked loop circuitry are necessary, increasing cost and size
Solution Approach 1:
The wide-loop translational synthesizer is designed to perform multiple functions simultaneously: frequency translation, LO signal generation for the modulator, and providing pulling resistance. This universality reduces the total number of separate circuits needed in the system.
Solution Approach 2:
The patent combines the translational synthesizer function with the second LO generation function into a single integrated circuit block. This merging eliminates the need for separate VCOs and PLL circuits that would otherwise be required, thereby reducing system complexity and size.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This configuration achieves low phase noise and improved Adjacent Channel Power (ACP) suppression with reduced circuit complexity and cost, supporting various wireless communication standards like W-CDMA, GSM, and EDGE.
Implementation Method 1
a first phase-locked loop (PLL) configured to generate a first frequency signal that is non-harmonically related to a transmit frequency signal
Implementation Method 2
a first frequency divider configured to divide the first frequency signal to generate a mixing frequency signal
Implementation Method 3
a mixer configured to generate an intermediate frequency feedback signal by downconverting the transmit frequency signal using a mixing frequency signal
Implementation Method 4
a second PLL configured to generate the transmit frequency signal by phase-locking the intermediate frequency feedback signal to the intermediate frequency reference signal
Data Source
Figure 1~2
Figure 3
Figure 4
AI summary
A method and apparatus for direct-conversion transmission generates a first frequency signal that is non-harmonically related to a transmit frequency signal, divides the first frequency signal to obtain a mixing frequency signal, divides the first frequency signal to obtain an intermediate frequency reference signal, generates the transmit frequency signal by using the mixing frequency signal to downconvert the transmit frequency signal into an intermediate frequency feedback signal, and phase-locks the intermediate frequency feedback signal to the intermediate frequency reference signal. The transmit frequency signal may be phase modulated, and may serve as an input to a saturated-mode power amplifier that can be configured for corresponding amplitude modulation. Alternatively, the un-modulated transmit frequency signal serves as the carrier signal input to a quadrature modulator, which imparts I/Q modulations to it, thereby producing a modulated carrier signal for input to a linear power amplifier.