Observation Receiver LOFT Cancellation in RF Transmitters
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Solution Overview
Problem
Existing methods for canceling local oscillator feedthrough (LOFT) in RF mixers are inefficient, especially when the transmitter is active, as they require extra analog circuitry and do not accurately distinguish LOFT from receiver DC offset, which is critical in 5G mmWave systems.
Innovation Solution
A method and apparatus using an observation receiver to measure and cancel LOFT by determining a DC offset through LOFT correction parameter estimation, allowing for accurate distinction and correction of LOFT even in the presence of DC offset, employing a feedback signal tapped from the transmitter and mixed with a shifted LO signal in the observation receiver.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If power detection with auxiliary ADC and power meter is used to minimize power for LOFT correction, then LOFT cancellation accuracy is improved, but device complexity increases due to extra analog circuitry
Solution Approach 1:
The patent extracts the LOFT measurement function from the main receiver and implements it in a dedicated observation receiver. This separation allows the observation receiver to continuously monitor the transmit signal and measure LOFT without interfering with the main receiver's primary function, eliminating the need for auxiliary ADC and power meter circuitry while maintaining measurement accuracy.
Solution Approach 2:
The observation receiver is designed to perform multiple functions: it serves as both the main receiver for normal operation and as an observation receiver for LOFT measurement. By making the receiver multi-functional, the patent eliminates the need for separate measurement circuitry, thereby reducing device complexity while maintaining measurement capabilities.
2Productivity
If main receiver is used for observation when transmitter is active, then LOFT measurement is enabled, but measurement precision deteriorates due to inability to distinguish LOFT from receiver DC offset
Solution Approach 1:
The patent segments the receiver signal path into two distinct components: the desired signal path and the LOFT path. By introducing a variable gain amplifier that can independently control the amplitude of the LO signal fed back to the mixer, the system can isolate and measure the LOFT component separately from the DC offset, enabling accurate measurement during active transmission.
Solution Approach 2:
The patent changes the amplitude parameter of the LO signal by using a variable gain amplifier to adjust the LO signal level. By varying the LO signal amplitude and observing the corresponding changes in the receiver output, the system can distinguish the LOFT component from the DC offset, thereby improving measurement precision during active transmission.
3Device complexity
If observation receiver LO is at the same frequency as transmitter LO, then system simplicity is maintained, but measurement precision worsens due to DC offset overlap with LOFT
Solution Approach 1:
The patent introduces a variable gain amplifier as an intermediary component between the LO signal source and the mixer. This intermediary allows precise control of the LO signal amplitude, enabling the system to create a test condition where the LOFT component can be isolated and measured independently of the DC offset, even when the observation receiver LO is at the same frequency as the transmitter LO.
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
Enables effective LOFT cancellation without additional analog circuitry, even when the transmitter is active, by accurately determining and applying a DC offset to the transmit signal, improving system performance in both zero intermediate frequency and high intermediate frequency architectures.
Implementation Method 1
A radio frequency (RF) mixer is a non-linear electrical device that generates new frequencies from two input signals
Implementation Method 2
the receiver including a second mixer configured to mix the feedback signal with a second local oscillator signal
Data Source
AI summary
A method and apparatus for cancelling local oscillator feedthrough (LOFT). A transmitter includes a first mixer configured to mix a transmit signal with a first local oscillator signal. An observation receiver receives a fraction of a power of the transmit signal as a feedback signal and processes the feedback signal. The observation receiver includes a second mixer configured to mix the feedback signal with a second local oscillator signal. A LOFT correction estimation circuitry is configured to determine a DC offset to cancel LOFT at the first mixer in the transmitter based on measurements on outputs of the second mixer. An LOFT correction circuitry is configured to add the DC offset to the transmit signal. The LOFT correction estimation circuitry may determine the DC offset based on several measurements obtained by varying the DC offset and a phase shift in the second local oscillator signal in the observation receiver.