Local Oscillator Offset for Image Rejection in Direct Downconversion
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Solution Overview
Problem
Direct downconversion receivers face challenges in achieving sufficient image rejection ratios, particularly in integrated circuit-based architectures, leading to interference between signals within a radio channel, such as NTSC television channels, which can result in video picture artifacts due to insufficient image rejection.
Innovation Solution
Offsetting the Local Oscillator (LO) frequency by −0.25 MHz from the normal channel center during baseband conversion to ensure that the image of the luma carrier does not interfere with the chroma carrier, while adjusting the frequency later to maintain the original baseband frequency, thereby enhancing image rejection without requiring changes in filtering or user knowledge of the offset.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If direct downconversion is used in integrated circuit-based receivers, then device complexity is reduced, but image rejection ratio deteriorates leading to intra-channel interference
Solution Approach 1:
The patent applies asymmetry by intentionally offsetting the local oscillator frequency from the channel center frequency. Instead of using symmetric downconversion at the channel center, the LO is tuned to a frequency that is offset by a predetermined amount (e.g., 0.25-2.0 MHz). This asymmetric frequency placement ensures that the image frequency of one signal does not coincide with another signal within the same channel, thereby achieving adequate image rejection without requiring complex external filtering components.
2Reliability
If image rejection ratio is increased to prevent video picture artifacts, then signal quality improves, but device complexity increases requiring additional external components
Solution Approach 1:
The patent changes the frequency parameter of the local oscillator from the conventional channel center frequency to an offset frequency. By adjusting the LO frequency parameter to be offset by a predetermined amount from the channel center, the system achieves adequate image rejection (e.g., 68-70 dB) without requiring additional external filtering components. This parameter change fundamentally alters the frequency relationships within the receiver to prevent intra-channel interference.
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
This approach effectively increases the image rejection ratio above the desired threshold, reducing interference and maintaining signal quality without degrading the video picture, even with frequency drifts up to ±50 ppm, and does not necessitate changes in filtering or user intervention.
Implementation Method 1
Offsetting the Local Oscillator (LO) frequency by −0.25 MHz from the normal channel center during baseband conversion to ensure that the image of the luma carrier does not interfere with the chroma carrier
Implementation Method 2
Many radio receivers convert a radio signal to a baseband signal using signal mixing with a local oscillator frequency
Data Source
AI summary
The present invention provides a method for determining a local oscillator frequency for a baseband converter, comprising obtaining characteristics of a channel comprising a plurality of radio signals at corresponding radio frequencies; using the channel characteristics to select a local oscillator frequency to use in baseband conversion of the plurality of radio signals of the channel to form a first baseband converted signal and a second baseband converted signal; and programming a local oscillator to operate the selected local oscillator frequency; wherein the local oscillator frequency is selected such that an image rejection ratio of the baseband converted second signal to an image of the baseband converted first signal is above a predetermined threshold.