RLC Low Pass Filter for RF Harmonic Coupling in Wireless Devices
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Solution Overview
Problem
In mobile wireless communications devices, RF coupling through metallic 'cans' creates interference due to voltage standing waves at harmonic frequencies, bypassing filters and affecting signal transmission.
Innovation Solution
An RLC low pass filter is implemented within the transmission line between the power amplifier and antenna switch, using a capacitor and resistor in series and an inductor and second resistor in parallel, to reduce RF coupling of voltage standing waves at harmonic frequencies while maintaining signal transmission at the fundamental frequency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an RF metal shield is used to isolate RF circuitry, then RF filtering between power amplifier and antenna switch is improved, but voltage standing waves at harmonic frequencies couple through the shield bypassing filters
Solution Approach 1:
A low pass filter is introduced as an intermediary component between the power amplifier and antenna switch within the RF metal shield compartment. This filter specifically targets and attenuates harmonic frequencies, preventing them from coupling through the shield to the antenna switch while allowing fundamental frequency signals to pass through normally.
Solution Approach 2:
The low pass filter modifies the frequency parameter of the RF signals by selectively attenuating frequencies above a certain cutoff point. This changes the spectral composition of signals in the transmission line, removing harmful harmonics while preserving the desired fundamental frequency communication signals.
2Volume of moving object
If circuit boards and electronic components are reduced in size and placed closer together, then device size is reduced, but components pick up conducted energy and create interference
Solution Approach 1:
The device is segmented into isolated RF compartments using metal shields. Each compartment (power amplifier compartment, antenna switch compartment, transceiver compartment) is physically separated from others, preventing conducted energy from jumping between components even when they are placed close together on the circuit board.
Solution Approach 2:
Low pass filters are placed as intermediary components in the transmission lines between RF components. These filters act as mediators that allow desired signals to pass while blocking harmful harmonic frequencies and conducted energy from interfering with adjacent components.
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 RLC low pass filter significantly reduces voltage standing wave ratio (VSWR) at harmonic frequencies, preventing RF harmonics from coupling through the RF shield and ensuring reliable signal transmission.
Implementation Method 1
voltage standing waves of single or multiple harmonics
Implementation Method 2
low pass filter formed within the transmission line has an input connected to the power amplifier and an output connected to the antenna switch for reducing any RF coupling
Data Source
AI summary
A mobile wireless communications device includes a housing, antenna, and circuit board carried by the housing and having radio frequency (RF) circuitry operative with the antenna for receiving and transmitting RF signals through the antenna. A power amplifier is connected within a transmission line for amplifying RF signals to be transmitted over the transmission line to the antenna. An antenna switch is connected to the antenna and RF circuitry. An RF shield surrounds the power amplifier and antenna switch and isolates the power amplifier and antenna switch from the antenna and RF circuitry. A low pass filter is connected to the power amplifier and antenna switch for reducing any RF coupling of voltage standing waves of upper harmonic frequencies from the power amplifier into the antenna switch through the RF shield while maintaining transmission of signals through the transmission line at a desired fundamental frequency.