Transmitter Circuit DC Offset Correction for Carrier Leak Reduction
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Solution Overview
Problem
Conventional transmitter circuits, particularly in mobile communication systems, face challenges in suppressing carrier leaks due to non-constant envelope modulation, especially with 8-value PSK and QPSK modulations, where offset PLL systems fail to correct DC offsets caused by transistor variations and temperature changes, leading to significant spurious signals.
Innovation Solution
A transmitter circuit with double balanced mixers that detect and correct DC offsets by using a feedback loop to adjust bias voltages, incorporating a limiter amplifier and control unit to stabilize the amplitude of signals, thereby minimizing carrier leaks.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If offset PLL is used for non-constant envelope modulation, then carrier radiation of interfering waves is reduced, but carrier leak suppression is insufficient due to DC offset from transistor variations
Solution Approach 1:
The patent implements a feedback mechanism where the DC offset detected at the mixer output is fed back to adjust the bias voltage of the mixer transistors. This closed-loop control continuously compensates for DC offset variations caused by transistor parameter changes and temperature, thereby suppressing carrier leak while maintaining stable modulation performance.
Solution Approach 2:
The patent dynamically adjusts the bias voltage parameter of the mixer transistors based on detected DC offset levels. By changing the bias voltage in response to measured carrier leak, the system adapts transistor operating conditions to maintain optimal performance and suppress harmful carrier radiation.
2Adaptability or versatility
If double balanced mixer is used for 8-value PSK modulation, then modulation capability is improved, but carrier leak increases due to transistor characteristic variations
Solution Approach 1:
The patent adds a feedback path that detects DC offset at the mixer output and uses this information to adjust the bias voltage of the mixer transistors. This enables the double balanced mixer to compensate for transistor characteristic variations while maintaining its capability for complex 8-value PSK modulation.
Solution Approach 2:
The system performs self-adjustment by automatically detecting its own DC offset and correcting it through bias voltage modification. This self-service mechanism eliminates the need for external manual calibration and maintains optimal mixer performance under varying operating conditions.
3Object-generated harmful factors
If DC offset correction is implemented, then carrier leak is suppressed, but device complexity increases due to additional correction circuits
Solution Approach 1:
The patent combines the DC offset correction function with the existing mixer circuit by integrating the correction circuit directly into the mixer structure. This merging approach allows the correction mechanism to share components and control signals with the main modulation path, thereby reducing overall device complexity while maintaining effective carrier leak suppression.
Data Source
AI summary
A transmitter circuit has two mixers that modulate a carrier wave according to an input signal, outputs a signal having information in a phase and an amplitude, detects a DC offset in each of the mixers, and adds a DC voltage that corrects the detected DC offset to the input signal of the mixers. The mixer is a double balanced mixer having two load resistors, and the transmitter circuit has a resistor that is connected between a node of two load resistors and a power supply, a limiter amplifier that amplifies a signal, and a control unit that changes first and second potentials using a signal that is outputted by the limiter amplifier. The first and second potentials become a potential of the DC voltage that corrects the DC offset.