Complementary LO Buffer for Common-Mode Rejection and Phase Balance
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
High-performance radio transceivers face challenges in maintaining signal balance and rejecting common-mode noise due to lengthy LO distribution lines, which introduce phase errors and unwanted spurs, and existing LO buffers lack sufficient common-mode rejection capability.
Innovation Solution
A local oscillator (LO) buffer circuit utilizing a complementary common-source and source-follower stage configuration with matched transconductances, implemented in CMOS technology, to enhance common-mode rejection and phase error correction, thereby improving image-rejection and harmonic-rejection capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If a lengthy LO distribution line is used to connect the frequency synthesizer to the RX/TX blocks, then the LO signal can reach all necessary components, but the signal amplitude weakens and phase errors increase
Solution Approach 1:
The LO distribution chain is segmented into distinct functional blocks: frequency synthesizer, LO buffer, and frequency divider. The LO buffer acts as an intermediate stage to regenerate and strengthen the signal, breaking the lengthy distribution line into manageable segments that maintain signal integrity.
Solution Approach 2:
The LO buffer serves as an intermediary component between the frequency synthesizer and the frequency divider. It receives the weakened LO signal, amplifies it with proper biasing, and provides a strengthened signal to subsequent stages, thereby compensating for the effects of the lengthy distribution line.
2Device complexity
If a simple common-source amplifier is used as LO buffer, then the circuit is simple, but the common-mode rejection capability is insufficient
Solution Approach 1:
The circuit merges a common-source amplifier stage with a source-follower stage into a single integrated LO buffer. This combination allows the circuit to provide both voltage amplification (from the common-source stage) and high input impedance with good common-mode rejection (from the source-follower stage), achieving superior common-mode rejection without excessive complexity.
Solution Approach 2:
The LO buffer uses a composite transistor configuration combining n-channel and p-channel MOS transistors in a complementary arrangement. This composite structure enables differential operation with enhanced common-mode rejection ratio while maintaining low phase noise and providing the necessary signal strengthening capability.
3Power
If the LO buffer amplifies the differential LO signal strongly, then the signal strength increases, but the phase noise may increase
Solution Approach 1:
The circuit changes the operating parameters of the transistors by applying specific bias voltages to the gate terminals. The biasing scheme adjusts the transconductance and operating point of the common-source and source-follower stages to optimize the trade-off between signal amplification and phase noise generation, ensuring strong output signal with minimal phase noise contribution.
Data Source
Figure 1
Figure 2
Figure 3
AI summary
A local oscillator buffer circuit (404) comprises a complementary common-source stage (406) comprising a first p-channel transistor (MCSP) and a first n-channel transistor (MCSN), arranged such that their respective gate terminals are connected together at a first input node (407), and their respective drain terminals of each of is connected together at a buffer output node (409). A complementary source-follower stage (408) comprises a second p-channel transistor (MSFP) and a second n-channel transistor (MSFN), arranged such that their respective gate terminals are connected together at a second input node (411), and their respective source terminals are connected together at the buffer output node (409).