Transmitter DC Offset Suppression via Dynamic Bias Control
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Solution Overview
Problem
In direct conversion architecture of wireless communication equipment, the local oscillator (LO) carrier rejection exceeds the required threshold when the maximum attenuation is reached, failing to maintain LO carrier rejection below the specified value over the entire gain range, especially in GPRS or EDGE networks.
Innovation Solution
The transmitter device incorporates a low-pass filter and differential transconductor with multiple signal copy cells to control attenuations, ensuring the LO carrier rejection remains below a chosen maximum value across the entire gain range by adjusting the number of copy cells used for the second attenuation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the transconductor generates maximum second attenuation to decrease DC offset, then the amplitude of baseband signals decreases, but the LO carrier rejection becomes greater than the threshold
Solution Approach 1:
The patent implements dynamic control of the transconductor's attenuation by varying the bias current based on the amplitude of the baseband signal. When the signal amplitude is large, the bias current is reduced to decrease second attenuation and maintain LO carrier rejection. When the signal amplitude is small, the bias current is increased to increase second attenuation and suppress DC offset. This dynamic adjustment resolves the contradiction between DC offset suppression and LO carrier rejection.
Solution Approach 2:
The patent changes the operating parameters of the transconductor by adjusting the bias current level according to the baseband signal amplitude. This parameter change allows the transconductor to adapt its attenuation characteristic, enabling it to maintain both adequate DC offset suppression and acceptable LO carrier rejection across different signal conditions, rather than operating at fixed maximum attenuation.
2Adaptability or versatility
If the low-pass filter applies high first attenuation to decrease baseband signal amplitudes, then the gain range increases, but the DC offset between differential signals remains constant while signal amplitude decreases
Solution Approach 1:
The patent employs feedback by monitoring the amplitude of the baseband signal and using this information to adjust the bias current of the transconductor. This feedback mechanism ensures that the second attenuation is dynamically adapted to maintain an appropriate DC offset to signal amplitude ratio, preventing the ratio from becoming excessively large when high first attenuation is applied for extended gain range.
3Reliability
If the transmitter uses state-of-the-art architecture with fixed attenuation, then the LO carrier rejection meets threshold at certain gain points, but it cannot satisfy the requirement over the whole gain range
Solution Approach 1:
The patent transforms the fixed attenuation architecture into a dynamic one by continuously adjusting the transconductor's bias current based on the baseband signal amplitude. This dynamic adjustment enables the transmitter to maintain LO carrier rejection below the threshold across the entire gain range, rather than only at specific fixed gain points, thereby achieving both reliability and adaptability.
Data Source
AI summary
A transmitter device for wireless communication equipment, comprises at least one path (P1) comprising i) a low-pass filter (LPF 1+, LPF1−) for filtering differential signals and applying a chosen first attenuation to each of them to decrease their amplitudes, ii) a differential transconductor (TC1+, TC1−) arranged for applying a chosen second attenuation to each differential signal coming from the low-pass filter (LPF1+, LPF1−) to decrease its continuous component, and iii) a mixer (M1) for mixing separately the differential signals delivered by the transconductor with local oscillator carriers at a chosen radio frequency to deliver output RF signals to be transmitted. The transconductor (TC1+, TC1−) comprises two original cells (OC) for defining two original signals from the differential signals having the first attenuation, and N first and N second signal copy cells (CC1-CC6) each arranged for generating a copy of one of the original signals. The number of signal copy cells used defines the second attenuation and the first and second attenuations to be applied are chosen so that the value (defined by the offset between the continuous component of the differential signals delivered by the transconductor divided by the amplitude of one of these signals) stays below a chosen maximum value over the whole gain range of the path.