PLL High-Frequency Generator Layout for Low Phase Noise
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Solution Overview
Problem
Conventional high-frequency signal generators suffer from poor signal-to-noise ratio and suboptimal phase noise due to the conversion of harmonics and limited frequency range, which restricts sensitivity and accuracy.
Innovation Solution
A high-frequency signal generator with two phase-locked loops, a frequency divider outside the loop, and a chain of bridged mixers with low-pass filters to minimize phase noise, allowing for precise frequency adjustments and low-noise operation, along with a digital phase detector for synchronization.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a sampling mixer with multiplier diode is used to synchronize the oscillator, then the oscillator can be synchronized to a clean frequency, but the signal-to-noise ratio becomes poor due to noise mixing from all harmonics
Solution Approach 1:
The frequency synthesis is divided into multiple stages: a first phase-locked loop generates a high-quality reference frequency, which is then processed through passive doubler stages with filtering, and finally mixed with the oscillator signal. This segmentation allows each stage to be optimized independently, achieving both good synchronization and low noise.
Solution Approach 2:
A clean reference frequency generated by a dedicated phase-locked loop serves as an intermediary between the original reference signal and the oscillator. This intermediary reference signal has superior noise characteristics because it is generated through a controlled feedback process rather than direct harmonic mixing.
2Adaptability or versatility
If a frequency divider inside the phase-locked loop is used, then frequency division can be achieved, but the phase noise of the reference signal deteriorates
Solution Approach 1:
The frequency divider is extracted from inside the phase-locked loop and placed outside. This allows the phase-locked loop to maintain its reference signal with minimal phase noise, while the frequency division function is performed separately on the already-stabilized output, preventing noise degradation.
Solution Approach 2:
The phase-locked loop first establishes a stable, low-noise reference frequency before any frequency division or mixing operations are performed. This preliminary stabilization ensures that subsequent frequency manipulations start from a clean signal foundation.
3Reliability
If a single phase-locked loop with limited frequency range is used, then voltage-controlled oscillators can be constructed with very good performance, but the tuning range is restricted to approx. 10% of the frequency
Solution Approach 1:
Multiple phase-locked loops operating at different frequency ranges are connected in parallel, extending the overall tuning range from a single dimension (approx. 10%) to multiple dimensions (covering several GHz ranges). Each loop maintains its high performance characteristics while the combination provides extensive frequency coverage.
Solution Approach 2:
The system uses a universal frequency synthesis architecture where multiple phase-locked loops can be selectively activated depending on the desired frequency range. This multi-functional design allows the same basic loop structure to serve multiple frequency bands, achieving both performance and versatility.
4Adaptability or versatility
If mixers are used to expand the frequency range, then more frequency coverage is achieved, but unwanted harmonics and secondary lines are generated
Solution Approach 1:
The unwanted harmonics and mixing products generated by the mixers are converted into a benefit by using them as additional reference signals for other mixers in the chain. This systematic utilization of what would normally be noise or interference allows comprehensive frequency coverage while maintaining signal purity through controlled mixing relationships.
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 a high signal-to-noise ratio and low phase noise, enabling precise frequency control and increased tuning range with minimal secondary lines, thereby enhancing the sensitivity and accuracy of the signal generator.
Implementation Method 1
The oscillator according to the invention preferably has two oscillators controlled by means of phase-locked loops. A first phase-locked loop generates a high-quality reference frequency
Implementation Method 2
The output signal of the oscillator can be mixed with the different reference signals depending on the position of the bypass switch
Implementation Method 3
The preferred use of a frequency divider outside the phase-locked loop means that the phase noise of an original frequency-fixed reference signal is largely retained
Implementation Method 4
The phase noise can be further reduced by using a low-pass filter after each mixer
Data Source
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AI summary
A high-frequency oscillator has a reference frequency generator and a high-frequency generator (2). The reference frequency generator generates a variable reference frequency and feeds same to the high-frequency generator (2). The high-frequency generator (2) has a phase-locked loop (60) and generates a high-frequency signal from the variable reference frequency. The phase-locked loop (60) has at least one first mixer (40), a second mixer (39), and multiple switches (46, 47, 48, 49, 50, 51, 52, 53). The first mixer (40), the second mixer (39), and the switches (46, 47, 48, 49, 50, 51, 52, 53) are connected in series. The mixers (39, 40) are connected into the phase-locked loop (60) by means of the switches (46, 47, 48, 49, 50, 51, 52, 53) in an individually selective manner.