Digital FM Demodulator Using Zero Crossing Detection

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Solution Overview

Problem

Existing video decoders face challenges in decoding SECAM composite video signals due to the susceptibility of phase locked loops (PLLs) to electrical noise and increased complexity and cost, particularly in handling frequency-modulated color information.

Innovation Solution

A digital time-based FM demodulator circuit that uses a zero crossing detector, counter, intercept calculator, and sinusoidal correction filter to refine the period determination of FM waveforms, eliminating the need for PLLs and enhancing accuracy.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If phase locked loops (PLLs) are used for FM demodulation, then the video decoder can handle SECAM video signals, but the circuit becomes susceptible to electrical noise and increases in cost and complexity

Engineering Contradiction:
Improvecapability to decode SECAM videoVSAvoidcircuit complexity
Core Design Contradiction:
Adaptability or versatilityVSDevice complexity

Solution Approach 1:

The patent replaces the analog PLL-based FM demodulation system with a digital time-based measurement system. Instead of using analog phase-locked loop circuits, the invention uses digital signal sampling, zero-crossing detection, and period measurement techniques to achieve FM demodulation, thereby reducing circuit complexity and eliminating the associated electrical noise susceptibility

Inventive Principle:
Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The invention changes the fundamental parameter being measured from phase (in PLL) to time period between zero crossings. By measuring the time interval between consecutive zero crossings of the FM signal and using this to determine frequency, the system achieves demodulation without requiring the complex phase-tracking mechanisms of traditional PLL circuits

Inventive Principle:
Principle #35Parameter changes

2Adaptability or versatility

If phase locked loops (PLLs) are used for FM demodulation, then the video decoder can handle SECAM video signals, but the circuit becomes susceptible to electrical noise

Engineering Contradiction:
Improvecapability to decode SECAM videoVSAvoidsusceptibility to electrical noise
Core Design Contradiction:
Adaptability or versatilityVSReliability

Solution Approach 1:

The patent replaces the analog PLL-based FM demodulation system with a digital time-based measurement system. Instead of using analog phase-locked loop circuits, the invention uses digital signal sampling, zero-crossing detection, and period measurement techniques to achieve FM demodulation, thereby reducing circuit complexity and eliminating the associated electrical noise susceptibility

Inventive Principle:
Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The invention creates a digital copy of the FM signal through sampling, then performs all demodulation operations on this digital representation rather than the original analog signal. This digital copying approach isolates the measurement process from analog noise sources while preserving the essential signal characteristics needed for demodulation

Inventive Principle:
Principle #26Copying

3Measurement precision

If linear interpolation is used to localize zero crossings, then the period determination resolution is refined, but error is introduced due to the sinusoidal nature of the waveform

Engineering Contradiction:
Improvezero crossing localization accuracyVSAvoidperiod determination accuracy
Core Design Contradiction:
Measurement precisionVSManufacturing precision

Solution Approach 1:

The patent introduces a correction mechanism that acts as an intermediary between the linear interpolation result and the final period measurement. By applying a correction term that accounts for the sinusoidal waveform characteristics, the system eliminates the error introduced by linear interpolation while maintaining the benefits of refined zero-crossing localization

Inventive Principle:
Principle #24Intermediary (Mediator)

Solution Approach 2:

The invention employs a feedback correction approach where the measured period is used to calculate and apply a correction to the zero-crossing localization. This feedback loop continuously refines the measurement by compensating for the inherent error in linear interpolation of sinusoidal waveforms

Inventive Principle:
Principle #23Feedback

Data Source

PatentUS7298423B1Time based digital FM demodulator
Publication Date: 2007.11.20 CIRRUS LOGIC INC
  • US7298423B1 patent drawing
  • US7298423B1 patent drawing
  • US7298423B1 patent drawing

AI summary

A time-based, digital FM demodulator circuit receives a stream of digital samples corresponding to an analog FM waveform. The samples are provided to a zero crossing detector, which allows a counter to determine a number of clock cycles between zero crossings. The resolution of this coarse period determination is further refined by an intercept calculation, which further localizes the zero crossing of the FM waveform based on interpolation between samples on either side of the zero crossing. Accuracy of the period determination may be further enhanced by use of a sinusoidal correction filter, which minimizes error caused by the linear interpolation performed on the sinusoidal waveform. Although the FM demodulator circuit is particularly suitable for demodulation of the chroma component of a SECAM video signal, it may advantageously be applied in a wide variety of FM demodulation applications.