Digital FM Transmitter Frequency Stability

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Solution Overview

Problem

Analog FM transmitters used in aftermarket car kits are prone to frequency drift due to temperature variations, requiring frequent retuning and necessitating analog signal conversion from digital sources, making them unsuitable for reliable in-car digital voice and music broadcasting.

Innovation Solution

A digital FM transmitter that operates entirely in the digital domain, utilizing a digitally controlled oscillator with fine frequency resolution and temperature stability, coupled with a digital signal processor and bandpass filter, allowing for precise modulation and RF power adjustment, eliminating the need for analog conversion and reducing retuning requirements.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Ease of manufacture

If analog FM transmitter with LC oscillator is used, then the transmitter can be implemented with off-the-shelf components, but the carrier frequency drifts by up to 10% due to temperature variations requiring frequent retuning

Engineering Contradiction:

Improveease of manufactureVSAvoidfrequency stability

Core Design Contradiction:

Ease of manufactureVSReliability

Solution Approach 1:

The patent replaces the analog LC oscillator mechanism with a digital controlled oscillator that uses a crystal oscillator reference. This substitution eliminates the mechanical/electrical variability of analog components and provides frequency stability of 0.0001% to 0.002% temperature coefficient, resolving the frequency drift issue while maintaining manufacturability through digital implementation

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The patent changes the fundamental operating parameter from analog capacitance adjustment to digital frequency control. The carrier frequency is generated digitally using a crystal oscillator with fixed, stable frequency, eliminating the need for retuning while maintaining ease of manufacture through software-based frequency adjustment capabilities

Inventive Principle:

Principle #35Parameter changes

2Reliability

If digital signal processing is used throughout, then frequency stability and precision are improved, but the device complexity increases

Engineering Contradiction:

Improvefrequency stabilityVSAvoiddevice complexity

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent extracts the frequency generation and modulation functions from the analog domain and places them entirely in the digital domain. By taking out the analog oscillator and replacing it with a digital controlled oscillator, the system achieves frequency stability while managing complexity through modular digital architecture

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The digital signal processor performs multiple functions including frequency generation, modulation, and filtering within a single integrated component. This multi-functionality reduces the need for separate analog circuits, thereby managing device complexity while achieving high frequency stability and precision

Inventive Principle:

Principle #6Universality (Multi-functionality)

3Adaptability or versatility

If analog domain conversion is used for voice/music signals, then compatibility with existing analog systems is maintained, but additional D/A converters are required increasing device complexity

Engineering Contradiction:

ImprovecompatibilityVSAvoiddevice complexity

Core Design Contradiction:

Adaptability or versatilityVSDevice complexity

Solution Approach 1:

The patent substitutes the analog domain with digital domain throughout the signal processing chain. By replacing D/A converters with digital modulation techniques, the system maintains compatibility with FM radio receivers while eliminating the need for additional analog conversion hardware, thus reducing device complexity

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Applied Scientific Principles

This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.

Function Achieved in This Case

The digital FM transmitter provides stable and precise frequency modulation with minimal temperature sensitivity, enabling reliable digital broadcasting of voice and music directly to FM radio receivers without the need for frequent retuning, enhancing usability in hands-free car kits and other applications.

Implementation Method 1

a digital controlled oscillator for generating a modulated RF carrier

Methodology Applied

Scientific EffectDigital Oscillation:

Implementation Method 2

a bandpass filter for filtering frequency components of said modulated carrier outside a predetermined frequency band

Methodology Applied

Scientific EffectFrequency Filtering: Filter (electronic)

Data Source

PatentUS8090316B2Digital FM radio transmitter

Publication Date: 2012.01.03 IP GEM GRP LLC

AI summary

A digital FM transmitter has a digital controlled oscillator for generating a modulated RF carrier. A digital signal processor receives digital input samples and generates a modulating signal for input to the digital controlled oscillator. A bandpass filter for filters frequency components of the modulated carrier outside a predetermined frequency band and supplies the filtered modulated RF carrier to an antenna.