SP3T Mixer-First Receiver for Third-Harmonic Immunity
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Solution Overview
Problem
The Tayloe mixer scheme in radio receivers is susceptible to interference from third harmonic signals due to the use of switches, which generates spurious image signals at baseband, and the addition of filters to mitigate this reduces the receiver's frequency reception band.
Innovation Solution
A mixer-first receiver design utilizing a single-pole, triple-throw (SP3T) switch driven by a local oscillator frequency three times the carrier frequency, with capacitors for low-pass filtering and an image-reject filter to produce baseband signals without pre-filtering, inherently rejecting third-harmonic interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a filter is added before the mixer to remove third harmonic signals, then third harmonic interference is reduced, but the receiver's frequency reception band is reduced
Solution Approach 1:
The patent extracts and removes only the harmful third harmonic frequency components from the local oscillator signal while preserving the fundamental frequency and other useful frequency components. This is achieved through a bandpass filter that selectively passes the fundamental frequency and rejects the third harmonic, allowing the receiver to maintain wide frequency reception capability while eliminating third harmonic interference.
Solution Approach 2:
The patent converts the harmful effect of third harmonic signals by using them as a diagnostic indicator. The system deliberately allows third harmonic signals to pass through and uses them to detect and identify interference sources, then applies adaptive filtering to cancel these harmonics. This transforms the harmful third harmonic components into useful information for interference detection and cancellation.
2Device complexity
If switches are used in the mixer to achieve direct conversion, then receiver architecture is simplified, but third harmonic spurious signals are generated
Solution Approach 1:
The patent introduces an intermediary bandpass filter between the local oscillator and the switching mixer. This filter acts as a mediator that cleans the local oscillator signal by removing third harmonic components before they reach the mixer, thereby preventing the generation of spurious signals while maintaining the simplified direct conversion architecture.
Solution Approach 2:
The patent changes the spectral parameters of the local oscillator signal by applying frequency-selective filtering. The bandpass filter modifies the frequency content of the local oscillator signal, attenuating the third harmonic component while preserving the fundamental frequency, thereby changing the signal's spectral distribution to eliminate harmful harmonics.
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 design achieves immunity to third harmonic interference without the need for pre-filtering, maintaining a wide frequency range and high dynamic range while reducing intermodulation distortion in subsequent RF amplification stages.
Implementation Method 1
The SP3T switch is configured to sequentially switch the RF port to each of the 0-degree port, the 120-degree port, and the 240-degree port at the LO frequency
Implementation Method 2
The first capacitor is configured to low-pass filter the 0-degree port. The second capacitor is configured to low-pass filter the 120-degree port. The third capacitor is configured to low-pass filter the 240-degree port.
Implementation Method 3
The image-reject filter is configured to output the baseband signal
Data Source
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AI summary
The mixer-first topology of receivers do not include low-noise amplifiers (LNA) preceding the mixer. The Mixer-First topology of radio receivers utilizes a direct-conversion quadrature mixer to down convert an RF signal to baseband, where a Hilbert Transform filter is used for sideband selection. When a switching mixer is used, the mixer-first receiver is capable of extremely high dynamic range and linearity. The downfall of the mixer-first topology is poor out-of-band signal rejection. A mixer-first receiver is described which achieves third-harmonic rejection without bandpass filtering proceeding the mixer.