Dual-Oscillator Frequency Synthesis With Low Phase Modulation
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Solution Overview
Problem
Existing frequency synthesizers in wireless transmitters and receivers face challenges in generating precise local oscillator signals due to discrete frequency steps, leading to undesirable phase modulation and spectral components, particularly in high-frequency operations, which affect receiver performance and transmit spectrum quality.
Innovation Solution
A frequency synthesizer system comprising two variable frequency oscillators, a phase reference generator, and a controller that adjusts control signals based on phase difference and additional signal characteristics, such as manufacturing tolerances, temperature, and supply voltage variations, to optimize frequency synthesis and reduce phase modulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If dithering is used to correct frequency errors by hopping between capacitor values, then the desired frequency precision is improved, but oscillator phase modulation is introduced creating unwanted spectral components
Solution Approach 1:
The patent replaces the mechanical switching of capacitor values (digital control method) with a continuous analog voltage adjustment method. Instead of hopping between discrete capacitor values which causes phase modulation, the invention uses a digital-to-analog converter to generate a continuous control voltage that smoothly adjusts the oscillator frequency, eliminating the harmful phase modulation while maintaining frequency precision
Solution Approach 2:
The invention changes the control parameter from discrete capacitor value switching to continuous analog voltage control. By converting the digital frequency correction signal into an analog voltage through a DAC, the system achieves continuous frequency adjustment without the abrupt transitions that cause phase modulation, thus resolving the contradiction between frequency precision and phase stability
2Device complexity
If discrete capacitor values are used to set oscillator frequency, then the circuit complexity is reduced, but the frequency step size increases reducing precision
Solution Approach 1:
The patent replaces the discrete capacitor switching mechanism with a continuous analog voltage control system using a DAC. This substitution maintains circuit simplicity while achieving fine frequency resolution, as the DAC can generate precise analog voltage levels corresponding to small frequency steps without requiring complex capacitor arrays or switching networks
Solution Approach 2:
The digital-to-analog converter acts as an intermediary between the digital frequency control signal and the analog oscillator. This mediator converts discrete digital values into continuous analog voltages, enabling fine frequency control without directly manipulating discrete capacitor values, thus maintaining both circuit simplicity and frequency precision
3Measurement precision
If frequency hopping is used to achieve desired frequency, then the discrete frequency steps are overcome, but spectral components are created deteriorating transmit spectrum
Solution Approach 1:
The patent substitutes the frequency hopping mechanism (discrete frequency switching) with a continuous frequency tuning approach using analog voltage control. The DAC generates smooth voltage transitions that produce continuous frequency changes instead of abrupt hops, eliminating the spectral splatter and unwanted components while achieving the desired frequency accuracy
4Measurement precision
If capacitor hopping is used to correct frequency errors, then the average frequency is accurate, but phase modulation is introduced affecting receiver performance
Solution Approach 1:
The patent replaces the capacitor hopping method with continuous analog voltage control through a DAC. This substitution eliminates the phase modulation caused by discrete capacitor switching while maintaining accurate average frequency, thereby improving receiver performance through smoother frequency control without harmful phase variations
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 approach enhances the precision and adaptability of synthesized frequencies, constraining frequency changes to improve spectrum quality and switching speed, while accounting for manufacturing and environmental variations, thereby improving receiver performance and transmit spectrum quality.
Implementation Method 1
the phase of the divided oscillator signal is compared with the phase of a frequency reference signal by means of a phase comparator
Implementation Method 2
An output signal of the phase comparator is smoothed in a low-pass filter and then applied to the control input of the oscillator
Implementation Method 3
An inductor-capacitor (LC) tank resonator sets the oscillation frequency
Implementation Method 4
It does so by hopping between capacitor values, either side of the wanted value, and hence hopping the frequency. On average this will give the correct 'fractional' capacitance and hence on average generates the wanted frequency
Data Source
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Figure 5~6
AI summary
A frequency synthesiser (100) has a first variable frequency oscillator (10) for generating a first oscillator signal having a frequency responsive to a first control signal, a second variable frequency oscillator (50) for generating a second oscillator signal having a frequency responsive to a second control signal, and a phase reference generator (40) for generating a phase reference signal. There is a phase difference generator (30) for generating a phase difference signal indicative of the phase difference between the sum of the phases of the first and second oscillator signals and the phase of the phase reference signal. A controller (60) responsive to the phase difference signal generates the first and second control signals. At least one of the first and second control signal are determined dependent on a value of the phase difference signal, and at least one of them are determined dependent on a further characteristic of a signal, the further characteristic being supplementary to the effect of any dithering introduced into the frequency synthesiser (100).