CMOS Switch Programmable Attenuator for Wide-Range Linear Gain Control
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Solution Overview
Problem
Implementing a direct-conversion tuner for satellite broadcasting is challenging due to wide input signal bandwidths and low received power levels, along with numerous unwanted channel interferers, which requires RF and baseband attenuators to exhibit good linearity over a wide tuning range to achieve high channel-selectivity.
Innovation Solution
A programmable attenuator using a linear-in-dB gain control with a resistor ladder and parallel switch bank configuration, providing a 30 dB gain range with fine resolution, is integrated into the direct-conversion receiver to maintain a high signal-to-noise ratio and suppress out-of-channel interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a direct-conversion tuner is implemented for satellite broadcasting, then on-chip integration is improved, but linearity over wide tuning range deteriorates due to wide input signal bandwidths and low received power levels
Solution Approach 1:
The attenuator is divided into multiple independently controllable attenuation stages, each with its own switch bank. This segmentation allows each stage to operate within a limited attenuation range, maintaining linearity performance while the overall system achieves wide tuning range through coordinated control of multiple stages.
Solution Approach 2:
The attenuator uses dynamically controllable switches that can be programmed to provide variable attenuation levels. The switch banks are controlled by digital signals that adjust the attenuation characteristics in real-time, enabling the system to maintain optimal linearity across different operating conditions and frequency ranges.
2Adaptability or versatility
If attenuation range is increased to suppress out-of-channel interferers, then channel-selectivity is improved, but signal-to-noise ratio deteriorates due to excessive attenuation of desired signals
Solution Approach 1:
The system incorporates automatic gain control (AGC) feedback that continuously monitors the signal level and adjusts the attenuator settings accordingly. This feedback mechanism ensures that the desired signal is attenuated only to the extent necessary for channel selection, while maintaining the signal-to-noise ratio above acceptable thresholds.
Solution Approach 2:
The attenuator provides programmable control over attenuation parameters, allowing the system to dynamically adjust attenuation levels based on the specific channel being received. This enables fine-tuned control where different attenuation levels are applied to suppress interferers versus preserving the desired signal, optimizing both channel-selectivity and signal-to-noise ratio.
Data Source
AI summary
A programmable attenuator includes a resistor ladder having a plurality of taps to provide a coarse gain control. Coupled to each tap is a plurality of switches. Control logic activates or deactivates individual switches in the plurality of switches to provide a fine gain control. More specifically, a set of activated switches provides fine gain control by determining an overall attenuation level interpolated between an adjacent pair of taps.