PLL Signal Generator Architecture for Low Phase Noise Tuning
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Solution Overview
Problem
Conventional high-frequency signal generators suffer from poor signal-to-noise ratio and sub-optimal phase noise due to the mixing of noise into intermediate frequencies, limiting their sensitivity and accuracy.
Innovation Solution
A high-frequency signal generator design featuring two phase-locked loops with a frequency splitter outside the loop to preserve phase noise, using passive doubling units and filters to suppress harmonics, and bridgeable mixers to select mixing products, ensuring low phase noise and rapid frequency changes through pre-tuning based on oscillator characteristics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a mixer operates with a multiplier diode to generate short pulses from reference frequency, then the oscillator can be synchronized to clear frequencies in the GHz range, but the signal-to-noise ratio becomes poor due to noise being mixed into the intermediate frequency
Solution Approach 1:
The frequency synthesis is divided into multiple stages with different mixers handling different frequency ranges. Each mixer processes a specific segment of the frequency spectrum, allowing optimized filtering and noise reduction for each segment while maintaining accurate frequency synchronization.
Solution Approach 2:
A frequency splitter is introduced as an intermediary component between the reference frequency generator and the mixers. This splitter separates the reference frequency into multiple divided frequencies that are then used by different mixers, enabling selective mixing and improved signal-to-noise ratio while maintaining synchronization accuracy.
2Manufacturing precision
If a series circuit of several mixers is used for fine frequency adjustment, then the output frequency can be precisely tuned, but the phase noise remains sub-optimal
Solution Approach 1:
The frequency tuning process is segmented into coarse adjustment and fine adjustment stages. The coarse adjustment uses a frequency splitter with integer division for rapid frequency changes, while the fine adjustment uses fractional division. This segmentation allows precise frequency tuning while maintaining low phase noise by avoiding cascaded mixer configurations.
Solution Approach 2:
The frequency splitter performs preliminary frequency division before the signal enters the mixer chain. By pre-dividing the reference frequency into multiple paths with different division ratios, the system can achieve fine frequency adjustment without requiring multiple cascaded mixers, thereby reducing phase noise accumulation.
3Object-affected harmful factors
If the frequency range of the reference signal is restricted to approximately 10% for small discrete steps, then very good voltage-controlled oscillators can be constructed with low phase noise, but the overall frequency range is limited
Solution Approach 1:
The frequency splitter serves multiple functions: it provides both integer division for coarse frequency adjustment and fractional division for fine tuning. It can operate with different reference frequencies and generate multiple output frequencies simultaneously, enabling a single VCO to cover a wide frequency range while maintaining low phase noise characteristics.
Solution Approach 2:
The system adds a temporal dimension to frequency synthesis by using phase modulation of the divided frequencies. The fractional frequency splitter uses phase modulation to achieve fine frequency adjustments beyond the 10% range, effectively extending the frequency coverage while maintaining the low phase noise performance of the VCO operating in its optimal range.
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 achieves improved secondary-line spacing with low phase noise and rapid frequency adjustments, enhancing the sensitivity and accuracy of the signal generator.
Implementation Method 1
two oscillators locked by means of phase-locked loops
Implementation Method 2
The resulting intermediate frequency is synchronized with a digital phase detector
Implementation Method 3
the signal of the oscillator is mixed down
Implementation Method 4
The first mixer, the second mixer and the switches are connected in series. The mixers are connected into the phase-locked loop individually in a selective manner
Implementation Method 5
a frequency splitter outside the phase-locked loop
Implementation Method 6
Passive doubling units with subsequent filtering are used in order to realize an extremely low-noise operation
Implementation Method 7
As a result of the advantageous filters between the doubling units, undesirable harmonics are suppressed
Data Source
AI summary
A high-frequency oscillator comprises a reference-frequency generator and a high-frequency generator. The reference-frequency generator generates a variable reference frequency and supplies it to the high-frequency generator. The high-frequency generator comprises a phase-locked loop and generates a high-frequency signal from the variable reference frequency. The phase-locked loop comprises at least one first mixer, a second mixer and several switches. The first mixer, the second mixer and the switches are connected in series. The mixers are connected into the phase-locked loop individually in a selective manner by means of the switches.