AM Transmitter Gain Control for Low-Distortion Modulation
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Solution Overview
Problem
Conventional AM transmitters face challenges with modulation distortion due to nonlinear power amplifiers, which complicate the design and hinder miniaturization, especially in small-sized transmitters, as they require large peak power to maintain linearity at high modulation levels.
Innovation Solution
The AM transmitter incorporates a PRE DRIVE amplifier, a DRIVE AMP, a power amplifier, LPFs, an antenna switch, a power detector, and an APC amplifier, where the APC amplifier generates a gain controlling signal to manage the power amplifier's gain, reducing modulation distortion by linearizing the modulation degree up to 100% and eliminating the need for a separate modulator.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a power amplifier providing large peak power is used to improve modulation distortion, then modulation distortion is improved, but device size increases and miniaturization becomes difficult
Solution Approach 1:
The patent divides the power amplifier into multiple stages (first power amplifier and second power amplifier) with different functions. The first power amplifier operates in a linear region for accurate amplification, while the second power amplifier provides additional power without requiring the same linear performance. This segmentation allows achieving good modulation distortion performance without requiring a single oversized power amplifier, thus enabling miniaturization.
2Adaptability or versatility
If modulation is performed by a separate modulator and power amplification is performed by a power amplifier, then modulation function is achieved, but device complexity increases
Solution Approach 1:
The patent combines the modulation function and power amplification function into a single power amplifier unit. The first power amplifier performs both amplitude modulation and power amplification by operating in its linear region, while the second power amplifier provides additional power gain. This merging eliminates the need for a separate modulator, reducing device complexity while maintaining full modulation capability.
Data Source
Figure 1
Figure 2A~2B
Figure 3
AI summary
An AM (Amplitude-Modulated) transmitter (1) capable of improving modulation distortion is provided. An adder (20) adds a modulation signal to a pre-set power reference signal. An APC (Automatic Power Control) amplifier (21) compares a level of a detecting signal outputted by the LPF (Low Pass Filter) (14) with a level of an adder signal. The APC amplifier (21), based on the comparison result, generates a gain controlling signal that makes a difference between a level of the detecting signal and level of the signal approach zero and feeds the gain controlling signal to a power amplifier (13). The power amplifier (13) modulates the signal outputted from the amplifier by the gain controlling signal and power-amplifies the modulated signal according to a level of the gain controlling signal. Since the APC amplifier (21) feeds the gain controlling signal to the power amplifier (13), a modulation degree does not depend on a nonlinear distortion characteristic of the power amplifier.