Bluetooth Power Amplifier Feed-Forward Spur Cancellation
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Solution Overview
Problem
Conventional power amplifiers in Bluetooth transmitters suffer from PA-pulling, which degrades Error Vector Magnitude (EVM) and communication quality, particularly at frequencies like 2.4 GHz, due to the interaction between the voltage control oscillator (VCO) and power amplifier (PA), and existing solutions either introduce non-ideal spurs or increase complexity and cost.
Innovation Solution
A power amplifying system for Bluetooth devices that uses a voltage control oscillator (VCO) to provide an input signal with a non-integer multiple frequency, coupled with a frequency divider to generate in-phase and quadrature signals, mixed and integrated using an adder amplifier to produce an output signal that neutralizes spurs via a feed-forward technique, eliminating the need for in-phase/quadrature signals or high-order filters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the VCO frequency is selected to be an integer multiple of the PA frequency (e.g., 2.4 GHz, 4.8 GHz), then the system operates at the desired frequency, but PA-pulling occurs which degrades EVM and communication quality
Solution Approach 1:
The patent applies preliminary anti-action by generating an inverted spur signal before it can cause harmful interference. The feed-forward path creates a compensation signal that is 180 degrees out of phase with the expected spur, thereby neutralizing the PA-pulling effect before it degrades EVM and communication quality
Solution Approach 2:
The patent introduces an intermediary compensation signal through the feed-forward path. This intermediary signal acts as a mediator between the VCO output and the PA input, canceling out the harmful PA-pulling effects through destructive interference with the spur signals
2Object-affected harmful factors
If the VCO frequency is set to two thirds of the LO frequency (1.6 GHz) to prevent PA-pulling, then PA-pulling is avoided, but non-ideal spurs appear at 800 MHz which affect PA features
Solution Approach 1:
The patent converts the harmful non-ideal spurs generated by the fractional frequency multiplication into a beneficial compensation opportunity. By predicting and generating inverted versions of these spurs through the feed-forward path, the system transforms what would be harmful interference into a means for self-correction and spur neutralization
3Object-affected harmful factors
If a frequency divider and 3/2 recovery architecture is used to prevent PA-pulling, then PA-pulling is avoided, but the design becomes complicated and costs increase
Solution Approach 1:
The patent extracts and separates the spur cancellation function into an independent feed-forward compensation path. This allows the main frequency multiplication architecture to remain simple while adding a dedicated, modular compensation mechanism that can be implemented with minimal additional complexity
Solution Approach 2:
The patent changes the frequency parameter of the VCO to a non-integer multiple (5/3 times) of the LO frequency, which fundamentally alters the spectral relationship between the VCO output and PA harmonics. This parameter change enables spur cancellation through feed-forward compensation without requiring complex frequency division and multiplication architectures
4Object-generated harmful factors
If an N-path filter architecture is used to filter out non-ideal spurs, then spurs are reduced, but a clock with specific duty cycle is required and power consumption increases
Solution Approach 1:
The patent replaces the mechanical N-path filter architecture with an electronic feed-forward compensation system. Instead of using complex filtering circuits with multiple clock phases and duty cycle requirements, the system uses signal processing through addition and inversion to achieve spur cancellation with lower power consumption
5Object-generated harmful factors
If a band-pass filter is used to filter out non-ideal spurs, then spurs are reduced, but the filter occupies greater area and increases costs
Solution Approach 1:
The patent extracts the spur cancellation function from the physical filtering domain and relocates it to the signal processing domain. By using feed-forward compensation through addition and inversion operations, the system eliminates the need for large-area band-pass filters while achieving the same spur reduction effect
Data Source
AI summary
A power amplifying system includes a voltage control oscillator (VCO), a frequency divider, a mixer and an adding amplifier. The VCO is configured to provide an input signal having a frequency that is a non-integer multiple of a predetermined frequency. The frequency divider is coupled to the VCO, and configured to receive the input signal and perform frequency division upon the input signal to respectively generate an in-phase signal and a quadrature signal corresponding to the input signal. The mixer is coupled to the VCO and the frequency divider, and configured to mix the input signal transmitted from the VCO and the in-phase signal transmitted from the frequency divider to output a mixed signal. The adding amplifier is coupled to the mixer and the frequency divider, and configured to integrate the mixed signal and the quadrature signal to generate an output signal having the predetermined frequency.