Double Conversion Tuner LO Frequency Offset for Interference Rejection
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Solution Overview
Problem
Devices such as set top boxes, cable modems, and embedded multimedia terminal adapters suffer from performance degradation due to radiated interferers like cordless telephones and wireless routers, leading to increased bit error rates or complete signal loss, as the local oscillator harmonics often fall within the bandwidth of these interferers, and existing solutions like mechanical shielding and manipulating LO and IF frequencies do not adequately address this issue.
Innovation Solution
A method for selecting local oscillator frequencies and bandwidths in double conversion tuners to ensure that LO harmonics do not fall within problematic frequencies, involving the calculation of nominal 1st and 2nd LO frequencies and adjusting the 1st IF passband to avoid interference, using specific equations and criteria to determine optimal LO frequencies that are either lower or higher than initially calculated to prevent harmonic overlap with interfering frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the local oscillator frequency is set to a nominal value for broadband tuning, then the tuner can cover a wide frequency range, but the LO harmonics fall within the bandwidth of radiated interferers causing performance degradation
Solution Approach 1:
The patent changes the LO frequency parameter from a fixed nominal value to a modified value calculated by offset equations. The first LO frequency is adjusted by adding or subtracting an offset from the nominal frequency, and the second LO frequency is similarly adjusted. This parameter modification ensures that LO harmonics are positioned outside interferer bandwidths while maintaining broadband tuning capability across all channels.
2Object-affected harmful factors
If mechanical shielding is applied to reduce interferer impact, then some protection is achieved, but the solution is limited and operationally undesirable
Solution Approach 1:
The patent replaces mechanical shielding approaches with an electrical/frequency-domain solution. Instead of physically blocking interferers with shields and enclosures, the invention modifies the LO frequency parameters through calculated offsets to position harmonics outside interferer bandwidths. This substitution eliminates the need for complex mechanical shielding structures while providing more effective interference rejection.
3Object-affected harmful factors
If the 1st IF passband is narrowed to exclude interferer frequencies, then interference is reduced, but legitimate signal channels may be excluded
Solution Approach 1:
The patent performs preliminary frequency positioning by calculating appropriate LO offsets before signal reception. By pre-positioning the LO harmonics outside interferer bandwidths through offset equations, the system ensures that the full 1st IF passband can be used to receive all legitimate channels without needing to narrow the bandwidth for interference rejection. The preliminary frequency arrangement eliminates the trade-off between passband width and interferer rejection.
Data Source
AI summary
A double conversion tuner system is disclosed that avoids the problem of degraded performance as a result of radiated interferers. Several selection criteria are described for specifying the parameters of the tuner, including the nominal first or high intermediate frequency, a first local oscillator signal for each desired channel, a nominal second local oscillator signal, and a bandwidth of the first intermediate frequency.