Orthogonal Local Oscillator and Dual-Filter Image Rejection
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Solution Overview
Problem
Superheterodyne receiving apparatuses face interference issues due to incomplete elimination of image signals caused by phase shift and gain deviation, particularly in low-IF systems where the frequency difference between desired and image signals is small, leading to deteriorated communication quality.
Innovation Solution
A receiving apparatus with a local oscillator generating orthogonal signals, a mixer producing intermediate frequency signals, and a dual-filter system that allows desired signals to pass while eliminating image signals, along with a comparator to adjust local oscillator frequencies based on signal level comparisons, ensuring effective separation of desired and image signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a superheterodyne receiving apparatus uses a low-IF frequency to reduce the intermediate frequency, then the frequency difference between desired and image signals becomes small, but image signals cannot be effectively eliminated due to phase shift and gain deviation
Solution Approach 1:
The receiving apparatus segments the filtering function into two separate filters: a first filter that eliminates image signal components and a second filter that eliminates desired signal components. This segmentation allows each filter to be optimized for its specific function, with the first filter designed to pass the desired signal frequency while attenuating the image signal frequency, thereby solving the image signal elimination problem in low-IF systems
Solution Approach 2:
The invention introduces dynamic control mechanisms including a comparator that compares signal levels and a controller that adjusts the local oscillator frequency based on comparison results. This dynamic adjustment allows the system to adapt to phase shift and gain deviation variations, maintaining effective image signal elimination even when operating conditions change
2Stability of the object's composition
If phase shift and gain deviation occur in mixed signals due to part variability and temperature change, then orthogonal phase relationship and amplitude equality cannot be maintained, but image signals remain without being completely eliminated
Solution Approach 1:
The invention implements a feedback control system where a comparator continuously monitors the signal levels from the first and second filters and provides feedback to a controller. The controller adjusts the local oscillator frequency based on this feedback to maintain optimal operating conditions, compensating for phase shift and gain deviation caused by part variability and temperature changes
Solution Approach 2:
The system dynamically changes the local oscillator frequency parameter based on comparison results between the first and second filter outputs. By adjusting this critical parameter, the system maintains the effectiveness of image signal elimination despite variations in phase relationship and amplitude equality caused by environmental and component factors
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 solution effectively eliminates image signals, preventing interference and improving communication quality by dynamically adjusting local oscillator frequencies to match the frequency difference or sum of the desired and image signals, even in low-IF scenarios.
Implementation Method 1
a local oscillator configured to output first and second local oscillator signals whose phases are orthogonal to each other
Implementation Method 2
a mixer configured to mix a received signal with the first local oscillator signal and output a first intermediate frequency signal, and to mix a received signal with the second local oscillator signal and output a second intermediate frequency signal
Implementation Method 3
a first filter configured to allow a component from a desired signal to be received to pass therethrough, and eliminate a component from an image signal having a frequency symmetrical with a frequency of the desired signal with respect to a frequency of the first and second local oscillator signals
Implementation Method 4
a comparator configured to compare levels between output signals of the first and second filters
Data Source
AI summary
A receiving apparatus includes: a local oscillator to output first- and second-local-oscillator signals whose phases are orthogonal to each other; a mixer to output first- and second-intermediate-frequency signals; a first filter to allow a component from a desired signal to pass therethrough, and eliminate a component from an image signal having a frequency symmetrical with that of the desired signal, in the first- and second-intermediate-frequency signals; a second filter to allow a component from the image signal to pass therethrough, and eliminate a component from the desired signal, in the first- and second-intermediate-frequency signals; a comparator to compare levels between output signals of the first and second filters; and a control unit to switch a frequency of the first- and second-local-oscillator signals to a difference frequency between a frequency of the desired signal and the intermediate frequency or a sum frequency thereof, according to a comparison result of the comparator.