PLL Synthesizer Digital Reference for Low Phase Noise
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Solution Overview
Problem
Current frequency synthesizers for RF transceivers face challenges in achieving high reference frequencies with fine frequency resolution while maintaining low in-band phase noise, as fractional-N synthesizers suffer from elevated phase noise due to non-linearities and integer-N synthesizers experience high reference spurs leading to slow settling times.
Innovation Solution
A frequency synthesizer design that incorporates a digital processor to generate a high-reference frequency with fine-tuning capabilities, using a digital-to-analog converter and phase locked loop to produce a filtered analog reference signal, allowing for integer-N phase locked loop operation with fine frequency resolution and low in-band phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If fractional-N synthesizer is used to achieve high reference frequency with fine frequency resolution, then frequency resolution is improved, but in-band phase noise increases due to non-linearities
Solution Approach 1:
The patent introduces a digital processor as an intermediary component that generates the reference signal digitally with high precision, converts it to analog via DAC, and filters it before input to the PLL. This intermediary digital processing stage enables fine frequency resolution without the non-linearities that cause phase noise in traditional fractional-N synthesizers.
Solution Approach 2:
The patent replaces the traditional analog fractional-N frequency synthesis mechanism with a digital processing approach. The digital processor generates the reference signal, and the DAC converts it to analog, eliminating the non-linear mechanical/electrical switching operations that cause phase noise while preserving fine frequency resolution.
2Object-generated harmful factors
If integer-N synthesizer is used to reduce in-band phase noise, then phase noise is improved, but reference spurs increase causing slow settling time
Solution Approach 1:
The patent changes the reference signal parameters by generating it digitally at a high frequency (e.g., 40 MHz or higher) with fine tunability, rather than using a fixed low-frequency crystal reference. This parameter change allows the system to achieve fast settling times while maintaining low phase noise through the integer-N PLL architecture.
3Loss of time
If high reference frequency is used to reduce reference spurs and improve settling time, then settling time is improved, but achieving fine frequency resolution becomes more difficult
Solution Approach 1:
The patent implements a dynamic digital reference signal generation system where the frequency can be precisely adjusted in real-time. The digital processor dynamically generates reference signals with fine frequency resolution, and the DAC converts these dynamic adjustments to analog domain, enabling both fast settling and fine resolution simultaneously.
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
The solution enables the generation of high-reference frequencies with fine frequency resolution and low in-band phase noise, addressing the limitations of both fractional-N and integer-N synthesizers by combining their advantages without introducing their drawbacks.
Implementation Method 1
A Digital-to-Analog Converter (DAC) converts the digital reference signal to an analog reference signal
Implementation Method 2
a low pass filter filters the analog reference signal to produce a filtered analog reference signal
Implementation Method 3
a phase locked loop for up-converting the filtered analog reference signal from an IF signal to an RF signal
Data Source
AI summary
A frequency synthesizer for use in a transceiver generates a relatively high reference frequency with fine frequency resolution and low in-band phase noise by using a digital processor to generate a digital reference signal at a finely-tuned reference frequency. A Digital-to-Analog Converter (DAC) converts the digital reference signal to an analog reference signal, and a low pass filter filters the analog reference signal to produce a filtered analog reference signal. The frequency synthesizer further includes a phase locked loop for up-converting the filtered analog reference signal from an IF signal to an RF signal.