In many cases of internal headphone interference, unwanted radio waves within the device are superimposed on top of the signals required for communication, which creates noise and causes the audio to skip.
Here we used commercial products to measure the minimum reception level of a 2.4 GHz signal to verify differences in the likelihood of audio skipping due to design countermeasures to prevent interference within Bluetooth devices.
In terms of internal device interference, large graph values indicate that communication is possible even with a weak signal and the audio is unlikely to skip.
We were able to verify various levels depending on the product, but why are they so different?
We verified the reason why such differences occur using Product A, where audio skipping was frequently observed, and Product D, where that problem was not often observed.
In order to know the differences between the minimum reception levels of Product A and Product D, we observed the noise spectrum received by the antenna. Signals flow through a Bluetooth antenna to communicate, but communication failures occur if noise gets into the signal flow.
The left side of Figure 3 shows Product D, which has good reception sensitivity, and the right side shows Product A, which has poor sensitivity.
The red areas on the graph show the noise level when the power supply is turned off, and the blue areas show the noise level during pairing.
Bluetooth uses frequency hopping, so the communication signals appear as a narrowband spectrum. Due to high sensitivity, the communication signals were only verified on Product D, and no other spectrum appeared.
In contrast, a spectrum with a frequency band of around several MHz was verified on Product A with its poor sensitivity. (Red colored marks)
Because Bluetooth uses hopping during communication, when this type of noise spectrum occurs across all communication frequency bands, noise mixes with the communication signal and degrades the sensitivity.
To investigate the cause of the wideband noise shown by the red marks, we measured the magnetic field distribution on the board surface of Product D. (Figure 4)
Because the noise source differs in actual noise suppression depending on the setting and situation, it is very important to identify the circuit location for effective noise reduction in advance.
The right side of Figure 4 shows the results for the magnetic field distribution strength when the frequency is fixed to 2.4 GHz. The red area represents a strong magnetic field, which indicates that the Bluetooth RF-IC with a particularly high magnetic field strength in the DC-DC converter circuit area, would be an effective location for noise suppression.
This noise is switching noise which occurs when generating power internally, and we can presume that the higher harmonics of the switching frequencies are occurring in the 2.4 GHz band.
Here we will introduce some countermeasures to deal with this problem.
Figure 5 shows the measurement environment used to take measurements and the result of measuring noise coupled with the Bluetooth antenna.
An extremely high level of noise was observed, which requires noise suppression to reduce the level.
When implementing Bluetooth noise suppression, there are two key implementation areas.
The first area is the power supply line, and the second is the clock line.
Because the power supply line generates higher harmonics due to switching, and the higher harmonics of the clock signal extends into the 2.4 GHz band, noise is generated in the Bluetooth signal. Filtering is an effective way to suppress noise conduction.
At Murata, we have commercialized two types of filters which are designed to remove noise in the 2.4 GHz band.
The first type includes the BLF02RD and LQZ02HQ filters for power supply lines, and the second type consists of the LQZ02HQ series for clock lines.
The characteristics of each filter type are introduced below.
Table 1 and Figure 6 show the representative electrical specifications and insertion loss frequency characteristics of the BLF02RD and LQZ02HQ filters used for noise suppression in this instance.
In many cases, the power supply line and clock line are major sources of noise and using the appropriate filter in those circuit areas is an effective solution.