RF Phase Shifter for High-Speed Polar Modulation
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Solution Overview
Problem
Conventional RF transmitters employing superheterodyne and polar modulation schemes face challenges in achieving high-speed phase modulation due to limitations in multi-band processing and loop bandwidth, leading to difficulties in designing power amplifiers that can handle frequency changes during phase shifts.
Innovation Solution
A phase modulator using a polar modulation scheme that includes an RF phase shifter and a baseband phase shifter, along with a phase control loop, to phase-shift and phase-adjust signals based on digital inputs, generating a phase-modulated signal with a phase value corresponding to the digital input, and utilizing a voltage-controlled oscillator to maintain phase-locking without direct frequency control.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a PLL is used to shift the phase of the VCO in a polar modulation scheme, then phase control is achieved, but high speed phase modulation is difficult to achieve because of loop bandwidth limitations
Solution Approach 1:
The patent extracts the phase shifting function from the VCO and implements it separately using an RF phase shifter that operates on the output signal of the VCO. This allows the PLL to maintain its original function while the phase shifting is performed by a dedicated phase shifter circuit, thereby achieving high-speed phase modulation without being constrained by PLL loop bandwidth limitations.
Solution Approach 2:
The patent segments the phase modulation function into two independent parts: the VCO generates the carrier signal while an RF phase shifter independently controls the phase of the output signal. This segmentation allows each component to operate optimally without interfering with the other, enabling high-speed phase changes without requiring the entire PLL loop to respond rapidly.
2Speed
If the phase of the VCO is directly controlled to shift the phase, then phase modulation is achieved, but a momentary frequency change is unavoidable at the instant of the phase shift
Solution Approach 1:
The patent extracts the phase control function from the VCO's input and applies it to the VCO's output instead. By using an RF phase shifter that processes the already-generated carrier signal, the system achieves phase modulation without requiring direct control of the VCO, thereby eliminating momentary frequency changes that would otherwise occur during phase transitions.
3Reliability
If a conventional superheterodyne scheme is used, then image tone removal is achieved using a SAW filter, but the transmitter is not suitable for multi-band processing
Solution Approach 1:
The patent extracts the image tone suppression function from the hardware domain (SAW filter) and implements it in the signal processing domain through polar modulation. By representing the modulated signal in polar coordinates and controlling the in-phase and quadrature components separately, the system can suppress image tones through signal processing rather than requiring a physical filter, thereby enabling multi-band operation.
Solution Approach 2:
The patent changes the modulation scheme from conventional quadrature modulation to polar modulation, which represents the signal in terms of amplitude and phase rather than in-phase and quadrature components. This parameter transformation allows the system to achieve image tone suppression through the mathematical properties of polar representation, making the transmitter adaptable to multiple frequency bands without requiring band-specific filters.
Data Source
AI summary
A high speed passband phase modulation apparatus and method are provided. In the phase modulation apparatus, an RF phase shifter modulates a phase of a local signal that is generated in a VCO according to a digital input. The RF phase shifter is controlled by a phase-controlled loop so that a baseband phase shifter is phased locked to a modulation reference signal from the local signal and a reference clock signal according to the digital input. The phase-controlled loop phase-locks using the modulation reference signal so that the phase-modulated signal generated in the RF phase shifter has a phase value according to the digital input.