Logarithmic RMS Detector Servo Loop for Linear-in-dB Gain
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Solution Overview
Problem
Existing methods for measuring signal power, particularly in wireless communication, face inaccuracies when dealing with variable signals that are modulated with message information or fit within specific frequency bands, due to limitations in maintaining linear gain across a wide dynamic range and increased attenuation at high frequencies.
Innovation Solution
A log-linear variable gain amplifier (VGA) is coupled in a feedback configuration with a difference-of-squares detector and integrator, featuring a sliding current generator that selects and boosts amplifier cells, allowing for adjustable transition circuit parameters and capacitive loading to maintain linear-in-dB gain across a wider dynamic range.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a set of attenuated amplifiers is used to create a log-linear VGA, then the dynamic range is extended, but the gain linearity deteriorates at the first stage for input signals near maximum gain
Solution Approach 1:
The VGA is divided into multiple amplifier stages, each contributing a portion of the total gain. By segmenting the gain function across multiple stages with progressively lower gain slopes, the patent achieves both extended dynamic range and improved gain linearity at each stage, resolving the contradiction between these two parameters.
2Power
If the first amplifier stage has maximum current, then the gain is maximized, but the gain slope becomes non-linear for input signals near maximum gain
Solution Approach 1:
Each amplifier stage is designed with a specific gain slope tailored to its position in the signal chain. The first stage has the highest gain slope, and each subsequent stage has a progressively lower gain slope. This local differentiation of gain characteristics ensures that each stage operates within its optimal linearity range, resolving the contradiction between maximum gain and gain linearity.
3Speed
If capacitive loading is present on output nodes at high frequencies, then attenuation increases at the last amplifier stage, but this causes non-linear gain for input signals near minimum gain
Solution Approach 1:
The patent anticipates the capacitive loading effect at high frequencies by designing the amplifier stages with progressively lower gain slopes from first to last. This preliminary design adjustment compensates for the expected attenuation, ensuring that even when capacitive loading occurs at the last stage, the overall gain remains linear-in-dB across the full dynamic range.
Data Source
AI summary
Measurement of signal power for variable or time varying signals. A log-linear VGA coupled in a feedback configuration to a difference detector and an integrator, includes a set of amplifier cells selectable by a sliding current generator, producing a sum of outputs. Outputs of the sliding current generator include a first control current provided using a sum of amplified currents, a sequence of intermediate control currents, and a final control current provided using a sum of amplified currents. Control currents to be summed can be differentially amplified or attenuated; attenuators include capacitors to compensate for capacitive loading. Selectable amplifier cells are differentially amplified or attenuated. Isolating switches and canceling stages reduce the effects of leakage between adjacent amplifier cells. The sliding current generator can have boosted current to first and last amplifier cells, providing a more linear-in-dB gain near a relative maximum or minimum.