Digital AM Transmitter Pulse Width Modulation RF Drive
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Solution Overview
Problem
Existing AM radio broadcasting RF power amplifier systems require a large number of identical power amplifiers to achieve high resolution, leading to significant costs and non-linearities due to variations in group delay, power gain, and RF drive phase alignment when using binary weighted amplifiers.
Innovation Solution
A digital amplitude modulation transmitter using a plurality of identical RF amplifiers, where the number and pulse width of amplifiers turned on vary based on the audio signal, eliminating the need for big and little step amplification, and maintaining coherent phase alignment for linear output.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Quantity of substance
If binary weighted power amplifiers are used to reduce the number of amplifiers, then the quantity of amplifiers is reduced, but non-linearities increase due to variations in group delay, power gain, and RF drive phase alignment
Solution Approach 1:
The patent changes the control parameter from amplifier selection (binary weighted) to pulse width modulation of a single amplifier. By varying the duty cycle of the RF drive signal, the system achieves continuous amplitude control without requiring multiple amplifiers with different power levels, thereby maintaining linearity while reducing component count.
Solution Approach 2:
A single power amplifier is made to perform multiple functions by using pulse width modulation to control its output amplitude. The same amplifier handles both the carrier signal generation and the amplitude modulation function that previously required multiple amplifiers with different power levels, simplifying the system architecture.
2Measurement precision
If many identical power amplifiers are used to achieve high resolution, then the manufacturing precision and reliability improve, but the device complexity and cost increase significantly
Solution Approach 1:
The patent replaces the mechanical approach of using multiple physical amplifiers with different power levels (binary weighted system) with an electronic control approach using pulse width modulation. This substitution allows high-resolution amplitude control through digital duty cycle variation, reducing hardware complexity while maintaining precision.
3Manufacturing precision
If equal step power amplifiers are used to provide consistent output, then the manufacturing precision improves, but the quantity of amplifiers required increases to achieve good resolution
Solution Approach 1:
The patent employs periodic switching of the power amplifier at high frequency with varying duty cycles. By modulating the ON/OFF timing of the amplifier in a periodic manner, the system achieves continuous amplitude control equivalent to having multiple amplifiers, but with only one amplifier required, thus reducing quantity while maintaining precision.
Data Source
AI summary
A digital amplitude modulation (AM) transmitter with pulse width modulating RF drive is presented. A plurality of RF amplifiers, each, when turned on, amplifies an applied RF drive signal by the same amount to provide a unit step RF output. An encoder supplies turn-on signals to turn on a number of the RF amplifiers wherein the number varies as a function of the value of an applied audio signal. The width of the unit step RF output is varied as a function of the number of the RF amplifiers that are turned on.