A study was conducted on whether the reception sensitivity for voice communication was reduced when supplying power wirelessly.
As a result, it was found that the reception sensitivity was reduced in the 800 MHz band when power was supplied wirelessly. Also, a reduction in reception sensitivity occurred uniformly in all frequencies.
Noise Generation/Noise Propagation Mechanism
As a result of our noise identification study, we proposed a mechanism by which noise is likely generated in the receiver module.
There are two main sources of noise.
The first source is the charger.
The second source is the receiver IC, which is located in the receiver module.
This noise is thought to be emitted directly from the receiver module board or to flow through the power line or ground line and be emitted from the smartphone board or wires, and this noise results in a reduction in reception sensitivity when it flows to the antenna.
Simple Equivalent Circuit of Wireless Receiver Module
||(1) Charger (Flows to the receiver coil)
(2) Receiver IC
||(1) Emitted from receiver module board
(2) Flow through power/ground line → Emitted from smartphone board or wires
There is noise emitted from the charger and noise occurring within the receiver module.
The standard specifies the capacitors be installed in series with the transmitter coil of the charger and the capacitors be installed in series and in parallel with the receiver coil of the receiver module.
The capacitors after the receiver IC are rectification capacitors.
The wires in propagation mode (2) refer to the wires connecting the receiver module and smartphone and the wires in the smartphone.
Noise Suppression Measures for Wireless Receiver Modules
Noise Issue Mechanism
Noise Suppression Measures
Next, we will explain the simple equivalent circuit (noise suppression circuit) of the wireless receiver module.
Noise Suppression Circuit
Ferrite beads are installed at the base of the receiver coil (2 locations) as shown in the figure.
As a result, noise that is emitted from the charger and flows to the receiver coil is prevented from propagating within the receiver module.
Ferrite beads are installed in the power line and ground line connected to the smartphone.
As a result, noise that is generated from the receiver IC is prevented from flowing to the smartphone.
Use of the BLM15PD800SN1 ferrite beads featuring a compact size and support for large currents is recommended.
Simple Equivalent Circuit of Wireless Receiver Module (Noise Suppression Measures)
The following noise suppression measures enable improved reception sensitivity for voice communication.
- (1) Ferrite beads at the base of receiver the coil (Recommended: BLM15PD800SN1)
- (2) A ferrite bead on the power line connected to the smartphone (Recommended: BLM15PD800SN1)
- (3) A ferrite bead on the ground line connected to the smartphone (Recommended: BLM15PD800SN1)
Measure (1) is effective for noise that is emitted from the charger and that flows to the receiver coil.
Measures (2) and (3) are effective for noise generated in the receiver IC.
If the flux leakage from the charger is large, noise suppression measures for the receiver side only may not improve the reception sensitivity.
Noise suppression measures must also be fully implemented at the charger.
Also, if the flux leakage from the charger is large, the noise that is emitted from the transmitter coil will flow directly to the antenna, and so measures that use ferrite beads will not be effective.
In this case, the electromagnetic wave absorbing sheet can be enlarged beyond the receiving coil (as if affixing to the entire case) for preventing the noise from flowing to the antenna and improving the reception sensitivity.
Effect on Resonance Frequency of Circuit
We studied whether the noise suppression measures in the above figure have an adverse effect on charger operation.
This is an issue because, if the impedance characteristics formed by the circuit (receiver coil + series capacitor, receiver coil + series capacitor + parallel capacitor) are changed when ferrite beads are installed, the power transmission and reception also change.
In the Qi standard,
it is specified that
Resonance frequency for the receiver coil + series capacitor = 100 kHz+5%-10%
Resonance frequency for the receiver coil + series capacitor + parallel capacitor = 1000 kHz±10%
Then, we calculated how the impedance characteristics of the circuit (receiver coil + series capacitor in this example) change when ferrite beads were installed. This was calculated assuming that the inductance value and capacitance value were two different types.
Condition (1): Transmitter coil＝25uH、Series C＝100nF
Condition (2): Transmitter coil＝10uH、Series C＝250nF
Condition (1) is the actual-measured value that was set to the system provided for evaluation, and Condition (2) was set so that the inductance drops to less than half of the above value. (This is because the effect of ferrite beads becomes more pronounced as the inductance of the transmitter coil becomes smaller.)
Installing the ferrite beads has no effect on the impedance characteristics of the circuit.
As a result, installing the ferrite beads had no effect on the impedance characteristics of the circuit. And so, we think that, even if ferrite beads are installed, their effect on charger operation is small.
Compared to the inductance value (about 10 to 20 uH) of the receiver coil, the L value of the BLM15PD800SN1 is a small value of only 210 nH (actual value measured by LCR meter), and so the resonance frequency can be said to remain unchanged.
Effect of Noise Suppression Measures
We evaluated the reception sensitivity when power is supplied wirelessly while the noise suppression measures described in the previous section were implemented.
As a result, the reception sensitivity improved for all frequencies.
In this sample evaluation, the reception sensitivity improved by up to 5 dB, and the drop in reception sensitivity when power was supplied wirelessly was zero.
Reception Sensitivity for Voice Communication
Simple Equivalent Circuit of Receiver Module
Noise suppression measures using ferrite beads enable a significant improvement in reception sensitivity.
Summary of Noise Suppression Measures for Receiver Modules
- Noise Issues for Wireless Receiver Modules
- Noise generated by the charger flows from the transmitter coil to the receiver coil, and the insertion of a wireless circuit reduces the reception sensitivity of the wireless circuit.
- Noise Suppression Measures
- As shown below, ferrite beads are used at the receiver circuit to implement noise suppression measures and enable improvement of the reception sensitivity of the wireless circuit.
- (1) Installation of ferrite beads at the base of the receiver coil (Recommended: BLM15PD800SN1)
- (2) Installation of a ferrite bead on the power line connected to the smartphone (Recommended: BLM15PD800SN1)
- (3) Installation of a ferrite bead on the ground line connected to the smartphone (Recommended: BLM15PD800SN1)
Noise Suppression Measures for Wireless Transmitter Modules
Following the receiver module, next, we will examine the noise that must be suppressed in the transmitter module. In the transmitter module, the inverter used for supplying AC to the transmitter coil is a source of noise. Noise emitted from the power supply cable becomes an issue as externally-emitted noise, and noise that flows into the transmitter coil side is emitted externally and also reduces the reception sensitivity of the smartphone unit.
Consequently, we recommend noise suppression measures for preventing noise emissions and for preventing drops in reception sensitivity.
First, we implemented the measures for preventing emission noise.
This consists of noise that leaks to the power supply cable side from the inverter and noise that leaks to the transmitter side, and so noise suppression circuits are installed for each location. (Suppression measures (1) and (2))
Emission noise consists primarily of noise transmitted in common mode, and so a common mode choke coil is used. Also, because the transmitter coil side contains a large amount of high-frequency noise, a line bypass capacitor is also used.