Direct-Conversion Receiver With FM Local Oscillator DC Offset Control
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Solution Overview
Problem
Direct conversion receivers face signal distortion due to local oscillator energy leakage, which creates a DC offset that overloads baseband amplifiers and results in significant loss of low-frequency information, particularly in narrow band frequency modulation schemes, leading to increased distortion.
Innovation Solution
Implementing a frequency modulated local oscillator that spreads low-frequency components beyond the high pass filter cutoff frequency, reducing effective attenuation and improving distortion performance by using a modulated local oscillator signal with adjustable frequency deviation and modulation frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If DC blocking components (high pass filters) are used to remove DC offset, then DC offset is attenuated, but low frequency baseband information is lost
Solution Approach 1:
The patent applies parameter changes by modulating the local oscillator frequency around the carrier frequency fc with a small frequency deviation. This frequency modulation transforms the spectral characteristics of the leaked LO signal, moving it away from DC and into a range where it can be effectively filtered without blocking low frequency baseband information. The frequency deviation parameter is carefully controlled to achieve separation between the modulated LO leakage and the baseband signal spectrum.
2Device complexity
If a fixed frequency local oscillator is used, then mixing is simple, but LO leakage creates DC offset that overloads baseband amplifiers
Solution Approach 1:
The patent introduces dynamics by modulating the local oscillator frequency instead of using a fixed frequency. The local oscillator frequency is varied sinusoidally around the carrier frequency fc with a controlled frequency deviation. This dynamic frequency variation causes the leaked LO signal to be spectrally spread, preventing it from concentrating at DC and thereby reducing the DC offset problem while maintaining mixing functionality.
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
Significantly reduces distortion elements and enhances the signal noise and distortion (SINAD) ratio, achieving improved signal quality by minimizing harmonic distortion and maintaining high-frequency information integrity.
Implementation Method 1
The direct-conversion receiver feeds the radio signal into a frequency mixer, just as in a superheterodyne receiver, where it is mixed with a local oscillator signal
Implementation Method 2
any DC blocker utilized in components 118 and 120 will have a cutoff frequency, above which it will substantially pass AC signals, and below which it begins to substantially attenuate them
Implementation Method 3
a frequency modulator coupled to the outputs of the local oscillator
Data Source
AI summary
A direct conversion receiver including an input stage, a frequency modulated local oscillator, a mixer stage, an output stage, and a DC blocker stage coupled between the mixer stage and the output stage. The input stage is receptive to a frequency modulated transmission having a carrier frequency fc and the frequency modulated local oscillator provides a frequency modulated local oscillator signal. In an embodiment, the frequency modulated local oscillator includes a local oscillator having an output with a frequency approximately equal to fc, a modulation source having a modulation signal output, and a frequency modulator coupled to the outputs of the local oscillator.