Differential LC Tank VCO Current Feedback for Low Phase Noise
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Solution Overview
Problem
Conventional differential LC tank VCOs face challenges in reducing phase noise due to up-conversion of 1/f noise and thermal noise, which affects the frequency modulation range and noise characteristics.
Innovation Solution
A differential current negative feedback circuit is integrated into the conventional differential capacitive-degeneration LC tank VCO to suppress low- and high-frequency noise, utilizing emitter-degeneration resistors and capacitors to sample emitter currents and input them into negative feedback transistors, thereby offsetting noise components and reducing phase noise.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a conventional differential LC tank VCO is used, then the circuit is simple and easy to manufacture, but the phase noise is high due to up-conversion of 1/f noise and thermal noise
Solution Approach 1:
A current negative feedback circuit is introduced into the differential LC tank VCO. The feedback network samples the emitter currents of the cross-coupled pair and feeds them back through negative feedback transistors to the bases, creating a feedback loop that suppresses low-frequency noise (1/f noise) and thermal noise, thereby reducing phase noise by approximately 25 dB without significantly increasing circuit complexity
Solution Approach 2:
Negative feedback transistors are introduced as intermediary elements between the cross-coupled pair and the LC tank. These transistors act as mediators that transfer and suppress noise components from the current source and cross-coupled pair, preventing their up-conversion to phase noise while maintaining the oscillation function
2Reliability
If emitter-degeneration resistors and capacitors are added to reduce noise, then the noise characteristic improves, but the device complexity increases
Solution Approach 1:
The emitter-degeneration resistors and capacitors serve multiple functions simultaneously: they provide noise suppression by sampling emitter currents, enable frequency tuning through the capacitive degeneration effect, and maintain differential operation. This multi-functionality reduces the need for additional dedicated noise-filtering components
3Reliability
If the Q factor of the inductor is increased to reduce phase noise, then the phase noise decreases, but the manufacturing precision requirements increase
Solution Approach 1:
The current negative feedback circuit actively suppresses noise without requiring high-Q inductors. By feeding back sampled emitter currents through negative feedback transistors, the circuit reduces the impact of thermal noise and 1/f noise, achieving 25 dB phase noise reduction while using standard inductors with typical Q factors, thereby avoiding stringent manufacturing precision requirements
Data Source
AI summary
A differential voltage controlled oscillator (VCO) employed in a frequency synthesizer used as a local oscillator of a wireless communication on-chip transmitter/receiver is provided. More particularly, a differential current negative feedback VCO equipped with a current-current negative feedback circuit that suppresses low- and high-frequency noise is provided.A differential current negative feedback VCO includes a resonator determining oscillation frequency, and an oscillator generating negative resistance. In the oscillator of the differential current negative feedback VCO, transistors Q1 and Q2 form a cross-coupled pair, and negative resistance is generated by positive feedback of the cross-coupled pair. And, transistors Q1 and Q3 together with an emitter resistor and a capacitor form a current negative feedback part, and transistors Q2 and Q4 together with an emitter resistor and a capacitor form another current negative feedback part which is disposed opposite to a resonator. Thus, the VCO operates differentially.In the oscillator of the differential current negative feedback VCO, emitter noise currents generated by base noise voltages of Q1 and Q2 induced by low- and high-frequency noise sources in the bases of Q1 and Q2 are sampled by emitter resistors, amplified through bases of Q3 and Q4, and thus return to the bases of the Q1 and Q2 and suppress the base noise voltages. Measurement of the phase noise of the differential current negative feedback VCO reveals a phase noise reduction of approximately 25 dB compared to a conventional differential VCO.